Wednesday, March 8, 2017

A catalog of density (Québec/Canada version)

So, I've recently discovered an interesting tool to visualize census data for Canada, www.censusmapper.ca. Among its various data, it allows to do visualization of population density bloc by bloc (rather, census tract by census tract, but most of them are bloc-sized). This made me want to do a kind of encyclopaedia of density, for typical forms of development in Québec, and maybe a bit from Canada too. 

Why Québec? Because I live here and Québec has a special focus on low-rise developments lacking in a lot of North America. I may do one for American developments one day, with city-data providing a similar tool to explore population density bloc by bloc in the US.

Before going on, I'd like to point out a few things.

First, there is the question "is there a density threshold for walkability?". This depends on a lot of factors, and my own thinking on the subject has led me to doubt that residential density is even the most important factor. Overall, though, I tend to consider that local walkability is highly dependent on access to a modern supermarket, the most important and frequently visited store for most people, of which there tends to be about 1 per 8 000 to 12 000 people in North America and a 10-minute "walkshed" is about 1 kilometer big. So a walkable neighborhood, supposing a good design that maximizes walking speed, should have 8 000 to 10 000 people per square kilometer. Which comes down to 80 to 100 people per hectare.

Around here, people also suppose that the minimum density to support high-frequency transit is about 35 dwelling units per hectare, which would be about 60 to 80 people per hectare.

Oh, and a quick summary for units.

4 people per acre = 10 people per hectare = 1 000 people per square kilometer = 2 560 people per square mile

I'll use people/units per hectare in the following.

OK, so let's get started.

Traditional Québec developments

Traditional village mix

50 to 70 people per hectare
30 to 40 dwelling units per hectare

This is an old type of chaotic developments common to old villages, with small detached single-family homes living alongside semi-detached homes, duplexes and other low-rise multifamily buildings. What characterizes it is the lack of uniformity, neither building line, nor architectural style, nor building function are uniform. This is the orderly chaos of developments pre-zoning.

Old single-family suburban areas

 40 to 50 people per hectare
15 to 20 dwelling units per hectare

This is a form often seen in older suburbs, a transitionary period between the chaos of early villages and the strict planning of today's cities. Lots tend to be about 15m x 30m (50' x 100'), front yards are more common, rear alleys are extremely rare.

Old suburban areas, mix of houses and low-rise apartments

 55 to 70 people per hectare
25 to 35 dwelling units per hectare

This is present in some older suburbs that were built before strict single-use zoning came about and that had a mix of homes and low-rise apartments. As a result, zoning in these areas often allow both single-family and multi-family uses, even today allowing for the replacement of old houses by small condo buildings. In a way, this is very similar to the old village mix, simply with a more orderly design, with a clear building line, for instance, with front yards and more regular spacing between buildings. Density is consequently very similar between the two. Still the density here is not quite up to par with highly walkable cities, it's a bit too low but it's pretty close and it can justify adequate transit options. When they were built, people tolerated smaller stores and more crowding per unit, so these were highly walkable then.

Duplex developments

  55 to 60 people per hectare
30 to 35 dwelling units per hectare

The duplex is perhaps one of the most common types of buildings in traditional Québec towns and villages, especially the squarish duplex with exterior stairways, balcony and porch. Uniform streets of these are relatively rare, but they exist, like this neighborhood in Trois-Rivières. The density here is still at or a bit below the threshold for minimum density. Traditional duplexes allowed people who couldn't afford single-family houses to afford housing by building a second story and renting it out, creating revenue that allows the owner to afford his own unit. Duplexes also allow for more air and light for every unit, with side windows made possible by each building being built apart from one another and separated by a few meters. This also allows for 1 or 2 parking spots to be built per unit, so even if you're in a car-centric city, this form of development is viable, all without violating some people's low-rise sensibilities.

Standalone triplexes

 60 to 90 people per hectare
50 to 70 dwelling units per hectare
Standalone triplexes aren't rare in older towns, but it's rare to have blocs full of them. Shawinigan, a small town north of Trois-Rivières is one of the rare cases I know of where this is a dominant typology of its older neighborhoods. The population density could be higher if it weren't for the fact that this city is in demographic decline and so housing is cheap, allowing singles to rent big apartments. I'm not kidding, I checked the rental ads for the town, you can get a 3-BR apartment in one of the buildings on the picture for 400$ a month. Anyway, if properly occupied, this typology would easily be favorable to walkable cities and good transit. Also, contrast this image with the previous one to see the impact street trees have on the beauty of a street.

Attached duplexes


  120 to 150 people per hectare
60 to 75 dwelling units per hectare

This is mainly seen in major cities that had streetcar suburbs. Montréal has neighborhoods with this typology. It's a bit similar to the previous duplexes, but instead of being squarish detached structures, these are deeper than wide and built wall to wall. Otherwise, the concept is the same. Since side spacing is eliminated, it allows for essentially a doubling of density. We are well in the territory of the walkable city here, and indeed neighborhoods with this typology in Montréal are associated with strong neighborhood stores, high walk scores and low (30 to 50%) car commuting mode shares.

Attached triplexes


   190 to 220 people per hectare
90 to 120 dwelling units per hectare

This is like the previous case, but instead of being limited to 2 stories, you have 3. Obviously, this results in a 50% boost to density and creates very high density. This doesn't provide more parking than the attached duplexes of the previous example, while density is boosted. So it won't satisfy suburban parking requirements and it will yield lower car mode shares, simply because it's next to impossible to maintain universal car ownership due to the lack of parking spot. Many Montréal neighborhoods include a mix of 2-story and 3-story attached buildings, and have density between these two examples of uniform duplex and triplex developments.

European-style blocs

 200 to 280 people per hectare
120 to 150 dwelling units per hectare

Yes, these do exist in North America. Very rare, but you can find them in the Vieux-Québec area of Québec City. Tightly packed 2- and 3-story buildings with high lot coverage. Very high density, very few parking spots and narrow streets. Nice to visit, more debatable how nice to live in considering the proximity of buildings both in front and in back.

Modern and transitional developments in Québec

These are developments made since the advent of mass motorization, often, but not always, with parking. These developments often incorporate modern construction techniques, whereas the traditional developments didn't.

Usual bungalow developments


 25 to 30 people per hectare
10 to 13 dwelling units per hectare

This is the usual sprawl-type development in suburbs all around Québec. Generally wide (20m x 35m) plots with wide 1-story houses (with livable basements) and driveways. Density half to a third that of traditional villages and even 30 to 50% less than earlier single-family suburbs. Far from density thresholds for walkability and for transit services.

3-story condo or apartment buildings

 60 to 80 people per hectare
45 to 60 dwelling units per hectare

This is a type of low-rise apartments often built in suburbs. Some are apartments for poorer residents, others are more upscale for the middle class and sold as condos. They offer similar density to tradition isolated duplex neighborhoods or traditional village mixes, but offer a lot more parking. They also have bigger setbacks and thus maintain air and light in a way that is tolerable for the suburban-minded. As they tend to have little or no underground parking, instead opting for parking lots, they are quite cheap to build.

Dense low-rise suburban apartment or condo complexes 


100 to 120 people per hectare
80 to 90 dwelling units per hectare

This is more or less the maximum density you can get while respecting suburban parking requirements without getting into mid-rise or high-rise territory and without resorting to massive underground parking to keep costs down. This density is sufficient to provide for walkable neighborhoods and to sustain transit lines. However, the quantity of parking lots they generate reduces the desire to walk and makes it easy for people to opt to own and use cars.

Attached duplexes built over garages

140 to 160 people per hectare
55 to 65 dwelling units per hectare

This is essentially the traditional attached duplex neighborhood adapted to the car era, with slip-under, front-loading garages to satisfy parking requirements. The result is that they retain the old duplexes' density, but at a cost of comfort to walk and of street design. The massive driveways mean frequent curb cuts making walking less comfortable and reduces trees and grass to a minimum, making the area less hospitable to human beings. The number of units per hectare here isn't all that high, but the location of these buildings near Montréal, the size of the units, the private door and the back yard all result in a high number of occupants per unit, and thus higher population density than the isolated low-rise apartments in the two previous types.

Modern townhouses

 100 to 130 people per hectare
40 to 45 dwelling units per hectare
This is a rare form of modern townhouses present in a few areas. Its modernity is not just in its architectural style, but in the parking present below the back yard's porch, with a driveway sloping down between each group of townhouses. This is an innovative attempt at reconciliation of the car-centric development but with an urban face, limiting curb cuts and preserving front and back yards while still offering at least one parking spot in a garage per house. However, it's not particularly affordable and its design over 4 stories (basement, 1st floor, 2nd floor and 3rd floor) means a lot of stair-climbing for every member of the family. No need for a Stairmaster with a house like that. 

Attached modern walk-up apartments

330 to 380 people per hectare
140 to 160 dwelling units per hectare

I wasn't sure whether I should have included this in traditional or modern developments. On one hand, its architecture style is reminiscent of the duplexes built over garages and there are some driveways and garages present, at the same time, it clearly offers way less parking than most other modern developments. Anyway, this is, as far as I'm aware, the densest low-rise area in Montréal, located in Parc-Extension. These are effectively 4-story buildings as the basement has units too. It's also a particularly poor area, populated by recent immigrants for the most part, so it has higher than average number of occupants per unit.

Modern planned mid-high-rise condos


350 to 500 people per hectare
250 to 300 dwelling units per hectare

This is a form of planned condo development on old disaffected industrial grounds. The planning is very evident and show bloc-sized planning, with huge garages in the interior of the blocs being covered by parks for the enjoyment of the residents, though the park is not visible from the street as it is elevated a couple of stories up. The high cost per square foot and deep building results in a very high amounts of units, as most of them are studios or 1-bedroom apartments. It's good density but that doesn't help life on the street as much as previous eras duplexes and triplexes.

High-rise compact developments, the top of Montréal density

800 to 1 200 people per hectare
500 to 800 dwelling units per hectare

I couldn't avoid mentioning the highest density blocs of Montréal that I know. These are high-rise apartments located near Concordia university in downtown Montréal. These are not only high, they have pretty high lot coverage, for a very high population density. This is the kind of development you can barely imagine anywhere else but there, to satisfy student's requirements for housing. The use of the ground floor for retail and restaurants also helps attract and retain people to the area, unlike the pretty sterile modern condos shown in the previous type.

So here you go, an overview of densities present in Québec, from traditional villages with isolated buildings to modern developments. This can provide a good idea of the possible ways to achieve a density that is sufficient for walkability and transit use, and how high-rises are not necessary to achieve sufficient density.

Select typologies from the RoC (Rest of Canada)

So this was a description of some housing typologies found in Québec, both traditional and modern. But Canadian cities can be a bit different, they're more like American cities in some ways, often with a focus on single-family housing and bigger tenement buildings.

Traditional urban homes - low density (Hamilton)

45 to 60 people per hectare
20 to 25 dwelling units per hectare

For anyone who knows American small towns, this is nothing new. These are detached homes built close to one another but not sharing walls, on narrow but deep lots, a typology of urban housing extremely common in anglo North America. The density isn't quite high enough for walkability today, but once upon a time, you could have expected many of these homes to have taken in boarders. Today, this is extremely rare and families are smaller, so population density is probably much lower than 100 years ago.

Traditional urban homes - high density (Hamilton)


80 to 100 people per hectare
35 to 45 dwelling units per hectare

This is just a denser form of the previous, with even narrower lots and homes nearly touching, with barely enough space for a man to walk between them. The density here is similar to Japanese single-family house neighborhoods and is actually sufficient for a walkable neighborhood and adequate transit services.

Toronto traditional urban homes and semi-detached

 100 to 130 people per hectare
45 to 55 dwelling units per hectare

This is the traditional form of development in Toronto, the equivalent of Montréal's duplex neighborhoods, except instead of relatively wide duplexes on shallow lots, these are detached and semi-detached homes that are very narrow on very deep lots.

Modern snout houses (Mississauga)


 60 to 100 people per hectare
15 to 30 dwelling units per hectare

Just like Montréal has duplexes built over garages which are an adaptation of its traditional duplex type of housing, anglo-Canada has a modern adaptation of the old narrow and deep urban home... the snout house. Thus named because of the protruding garage doors that look like a snout and reduces front-facing windows to a minimum. Note that the population density here is uncommonly high considering the number of dwelling units. We're approaching 4 people per household on average, versus 2,5 on average in Québec single-family housing. I don't know why that is, but that ratio is high all across Mississauga, at least, in that area. Is it an ethnocultural community that has unusually high fertility rates? Is the price of housing pushing people to live with their parents and children longer? I don't know.

Traditional low-rise apartments (Hamilton)

110 to 130 people per hectare
70 to 90 dwelling units per hectare

This is a relatively rare case of an old type of density in an old Anglo-Canadian city (Hamilton). As the lots are narrow and deep and buildings are detached, you can see some areas where, among the old urban homes, you can spot narrow and long apartment building 3 to 6 stories high. However, this process seems to have been interrupted by the arrival of modern zoning practices and parking requirements. It's interesting to note the difference between low-rise apartments in Québec and in Anglo-Canada, with Québec having small low-rise apartments built wall to wall, often owned by one of the occupants, without the need of a lot of capital to own it, and Anglo-Canada having bigger apartment buildings that were probably owned by richer owners who resided somewhere else.

Urban high-rises

250 to 400 people per hectare
200 to 300 dwelling units per hectare

I know of nothing else that can showcase the lack of middle housing in Anglo-Canada than this. High-rise apartment buildings carved out of old neighborhoods of urban homes, resulting in 2-story urban homes being next to 20+-story apartment buildings with underground parking. This is probably the stuff of nightmare of most NIMBYs... and even of a ton of urbanists, I suppose. I would be curious to know the value of the remaining homes next to the high-rises, to see if their value is higher or lower than similar homes a bit further away, just to test the theory that high constructions depress the value of neighboring low-rise single-family properties. Anyway, very high density nonetheless, almost Paris-level density in some areas.

Vancouverism (do I need to say where?)

 500 to 800 people per hectare
350 to 500 dwelling units per hectare

Vancouverism is the name used to describe the style of development that forms the basic pattern of Vancouver's downtown. Funnily enough, it's not unlike the previous chaotic development in Hamilton in that it combines low-rise and high-rise developments on the same bloc. The difference is that Vancouverism is more ordered than what we see in Hamilton's central neighborhood, building lines are more respected, the placement of the skyscrapers resembles a checkerboard, to prevent buildings from cutting light and air access to one another and to avoid a trench feeling for people on the street. The density level is probably among the highest in the Western world, equaling or topping the dense neighborhoods of Barcelona and Paris. This is Manhattan Upper East Side levels of density. The main issue of such development is cost, as skyscrapers are not affordable to build at all. Vancouver's out of control housing market makes such development possible, but that may not be the case everywhere.

Conclusion

OK, so this was just a catalog of density of different development patterns seen in Québec and the rest of Canada. I don't claim this list to be exhaustive. I think it may be useful to have such a catalog to know what different levels of density look like and to lay to rest some ideas. Thus, both the idea that you HAVE to go very high to achieve walkability-supporting density (when even 2-story developments with detached buildings can approach that threshold) and that there is no point to go very high because lower lot coverage means you won't get more floor space are wrong.

Saturday, December 31, 2016

Building affordable housing: what can work in urban areas

OK, I've been away long, just busy and without much motivation lately, with this follow-up hanging on me, preventing me from starting other posts. Anyway, if anyone still follows my blog, here is this article I wanted to finish before 2016 is over...

...so in the last article, I pointed out that the economics of building new housing is not really favorable to affordable housing. If you only had construction costs to contend with, it would be possible to have reasonably large affordable housing, however, in places where land prices tend to be high, these impose a premium that make it really hard to build affordable housing of decent size.

Furthermore, the post-WWII idea of affordable housing, the starter home for young families, is based on a "grow home" model, the idea that houses be sold with the basement or attic unfinished, with the expectation that, as the family grows, so will the house, thus keeping the initial price low but allowing for expansions further on. This worked well in greenfield developments, but the problem is that, once "grown" the house cannot be ungrown for the next generation. So a "grow home" is only affordable once, then it gets progressively more expensive.

All of that leads to a simple question: is it even possible to have affordable housing in an urban area, where land is expensive and greenfield development impossible?

So let's see a few approaches that can make economic sense...

Very high increase of density, without going too high

Replacing existing buildings by higher density ones is the traditional approach to increasing density. However, this isn't a silver bullet. There are two main pitfalls to that approach.

The first pitfall is if the density of the proposed building isn't high enough. That's because developers who want to redevelop a property have to compete to buy that property with people who would want to buy it to live in it.
Two major types of buyers competing for a given property
That means that the minimal price a developer could pay is the market value of the building, because if he's not willing to pay that much, a potential resident will just outbid him and buy the house to live in. And usually, if a developer wants to build something, it's because the area is reasonably desirable and the market value isn't too bad. That means that the land price the developer has to pay is AT LEAST equal to the market value of the current building which is usually AT LEAST equal to the construction cost, to which you have to add destruction costs (not that high usually) and the construction cost of the new building.

So let's say we take this house...

Which could be worth 270 000$ in market value.

And we want to replace it with this triplex:
So let's estimate things...

The current house's market value is 270 000$, let's say 30 000$ for removing the current house and 300 000$ for building the triplex. That's 600 000$, let's add another 10% for profit margin, that's about 660 000$, while the building has about 2 900 square feet of livable space. That's about 227$ per square foot, or 220 000$ per 969-sf 2-BR apartments.

That's not necessarily very affordable, especially when you consider that this replaces a 3-BR 1 260-sf house with another 1260-sf of basement that sells for 270 000$.

Now, if you could replace the house with an 8-plex like this one...
Then we add the value of the house (270 000$) and the destruction cost (30 000$) and the construction cost (1 030 000$), that comes up to 1 330 000$, plus 10% of profits, that's about 1 500 000$. That comes out to about 150 $ per square foot, or a bit under 200 000$ for a 1200-sf, 3-BR apartment. That's more reasonable.

Of course, that's ignoring factors like parking minimums, regulatory costs of rezoning and the like.

The saving grace would be a house in disrepair, a "fixer-upper" which market value would be lower than similarly-sized houses. Thus allowing for a lesser increase of density to have affordable housing as the effective land price is significantly lower.

The other pitfall is when the actual building is high enough density that the only way to add density in any significant manner is to go for a mid-rise or high-rise (4+ stories) building, which is significantly more expensive to build as they require concrete, elevators, water sprinklers, and other elements. So if the cost of construction of the new building alone is 200$ per square foot, that imposes a high threshold for housing prices.

So another approach might be needed.

Subdividing

So building a new building in replacement of an existing building is not a great proposal economically speaking. But what if you used the second of the three Rs? Reuse. After all, if you have a perfectly suitable building, maybe you don't need to destroy it to increase density. How? Well, by subdividing the existing building. Simply rearrange the interior of a large home to create many units out of just one.

For example, here is a house that could easily be subdivided into a duplex:
A raised ranch in Québec
This building has a door that opens on a small lobby which has 2 sets of stairs, one that goes up to the upper story, the other that does down to a half-buried floor.

If you put a wall in the middle of the lobby and add another door to the outside, you separate the floors, possibly creating two 2-BR or 3-BR units. So if you build a raised ranch with an unfinished basement, you can finish the basement, maybe for 70 000 or 80 000$, then convert the house into a duplex.

What if the house is too small to be subdivided into reasonably big units? Well, buildings can have additions built to them. You can build additions next to them, or even add another story in some cases. This seems to have been done relatively frequently in older American towns, like these examples in Lowell, Massachussetts:

As you can see, you have these long elongated buildings, often with dissimilar roofs and the front part of the building looks like a self-standing house, which is probably how they started. Additions aren't that cheap to add, as they are more complicated to build than a new building and so require more labor. I've seen estimates that they can cost 200$ per square foot to build in the case of an additional story. But the advantage of such additions is that the land cost is nil, unlike new constructions, since it's an addition, not a replacement.

Additions are also not a magic bullet, they are limited by zoning, like everything else, and the structure of the building must be able to accommodate the addition.

New building on the same lot

This is a reasonably frequent avenue used in many cities like Vancouver, taking the form of the laneway house for example. A laneway house is a possibility afforded by a lane behind rows of houses, as a second house can be built on the same lot, just facing the rear alley rather than the street. Like additions, the advantage of this approach is a nil land cost, since the owner can build it without replacing the existing building. In a way, it can be seen as a particular type of subdividing, where one subdivides the lot rather than the building.

Boarding houses

Boarding houses are a practice common in traditional American cities but that is now rarely seen. Basically, once upon a time, when an homeowner had empty bedrooms in their houses, for example, because their children were grown up and had moved out, they would advertise these bedrooms to rent. And thus, single people looking for affordable housing could rent out these bedrooms, often with meals being offered as part of the renting contract. That way, the occupancy of single-family houses could remain high, the "empty nesters" phenomenon didn't occur much and affordable housing options were available to singles, and maybe even some couples. This also solved an income issue for older people who were retired before the institution of government pensions... Hmm, interesting hypothesis: one of the reasons for the disappearance of boarding houses may be that government pensions reduced the need for older homeowners to find new income sources.

Famous boarding house occupants are Sherlock Holmes and John Watson, who rented out bedrooms in Ms Hudson's personal house.

What potential does renting out empty bedrooms have?

Well, going by the Canadian census, I estimate there are as many as 11 million unoccupied bedrooms in occupied dwellings in Canada. Enough to absorb a 30% increase in the population. Of course, most of these bedrooms may not be in desirable locations, but even in the city of Vancouver (NOT including the suburbs), there are probably around 82 000 unoccupied bedrooms in occupied dwellings, enough to increase the population by nearly 14%. In the city of Toronto, there are 340 000 unoccupied bedrooms, again, enough to absorb a 14% rise in population without building a single new home.

In older suburbs, the amount of unoccupied bedrooms is amazing. In my childhood's suburb of Boucherville, there are at least 47 420 bedrooms:

Meanwhile, there are 40 753 people living in the city, and 10 890 couples, married or common law. If we suppose couples sleep in the same bedroom, that means at most there are 29 863 occupied bedrooms, leaving 17 557 bedrooms unoccupied in occupied dwellings. That's enough to house a population increase of 43%, with a similar rise in density.

Filtering

Filtering is a phenomenon where the construction industry in a metropolitan region is able to build sufficient amounts of housing to outpace the growth in population and households. Basically, supply grows faster than demand. In such a situation, even if newly built units are not affordable on their own, they may lead to a decline in market value of older units that are not as desirable, therefore their owners who are renting them out or trying to sell them have to lower their asking prices to find a buyer or a renter.

This is not that common in the major cities of Western countries, because though fertility rates have fallen, governments have compensated that fall with increases in mass immigration, and immigrants tend to gather in the major metropolitan areas. This factor adds up on the rural-to-urban migration and makes it often unlikely to see demand growing slower than supply, as the local industry fails to keep up with the population growth. The temporal reality of supply and demand and industry output must not be neglected.

However, it does happen in Japan, as these graphs I made from condo (mansshon in Japanese real estate parlance) prices per square foot in central areas of Sapporo and Tokyo reveal. Data from suumo.jp:

When density in a city is high enough, filtering is the only approach that works, because subdividing would result in too small units for the needs of people and the current density makes it impossible to economically replace current buildings with walk-up low-rise buildings.

An alternative to the "grow home": the adaptable home

Finally, let's look at my own counter-proposal to the "grow home" model, that I've already derided. A reminder, the "grow home model" is the following:

First, a cheap home is built with an unfinished basement and/or attic, which is bought by a young couple.

As the family grows, the attic or basement is finished, or the house likewise improved, to accommodate the needs of the family at its peak (teenage or young adult children still in the home).
After the children leave, the parents still occupy the fully improved, now very valuable home.

When the parents have enough, they sell the house, at its current market value... likely twice what it was worth when it was built, because of all the improvement and additional living space.

So the house starts cheap, but doesn't remain cheap much longer... what to do?

Well, let me present the alternative, the "adaptable" home.



The two first look like duplexes, they have two doors side-by-side. The second looks like a triplex, with two doors on the first story, one going upstairs, the other opening on the 1st story unit. But here's the thing, the two side-by-side doors can in fact be easily converted into one single door, and the lobbies on the other side of them can be merged by removing the wall that separates them, thus integrating the stairway of the upper or lower unit into the other unit. This effectively merges the units together.

The building can thus be easily merged or subdivided as needed.

Raised ranches fit the bill, as does the triplex shown on this very page, since it has two adjacent doors that could be easily modified to merge both top units.

Imagine this but with one door instead of two on top of the stairs

So the life of these buildings can be the following.

First, they are built complete, all floors finished, and sold to a family that will occupy it. The family is small, so it first put the building into its duplex or triplex form: living in one unit and renting out the other(s).

As the family grows, it may lack space, in this case, the owner warns the renters to move out as they plan to merge two units to create one big 4- to 6-bedroom unit to accommodate their growing family.

As the children leave, the owners may decide the unit is now too big for them, they can then easily separate the units anew and rent out the now unoccupied unit.

This provides needed affordable housing to a city and a retirement supplementary income to the owner. Though the initial capital cost is higher, the revenue from renting out the other unit(s) more than makes up for the higher cost. In fact, if I compared a "grow home" scenario of a 200 000$ home which has its basement finished for 80 000$ 7 years afters its initial sale to an adaptable home that is built with a finished basement unit for 280 000$, renting it out for 7 years at a price of 600$ a month for a 2- or 3-bedroom basement unit, then converting into a single house, losing the revenue. The adaptable home is paid back 2 whole years before the grow home, thanks to the rent income.

In fact, an adaptable home can even start out as a grow home that ends up being subdivided later in the life of the family that occupies it.

The main issue here is purely regulatory. An adaptable home goes from single-family to multi-family and vice versa during its life, adapting to the needs of the owners. This would be illegal in most cities, and is just further proof of the idiocy of the current North American zoning practices.