walkable density with SFH

[mindstalk]

Various sources point to a minimum level of population density needed for walkability. A source I have lost said 10-20 dwelling units (du) per acre. This Australian model derived 25 du/hectare (2500 du/km2), which is the same as 10 du/acre, as a minimum, though 35 was notably better. At an assumption (as the paper used) of 2.6 people per du, 25 du/hectare is 6500 people/km2, 35 is 9100. My own personal experience, of places I have lived and looked up the densities of, is that nice walkability starts around 9000 people/m2, while 6000 tends to be doable but a bit anemic.

It is naively intuitive that we can get pretty high densities simply by stacking apartments, but how dense can we get while keeping single family homes (SFH)? I will make a couple conservative assumptions:

1. Half the land is residential lots (house + yard, if any); the rest is some mix of sidewalks, streets, and non-residential uses. If you were designing from scratch you could do better, with narrow streets and shophouses mixing different uses, but given existing US streets and the modern economy, those don't apply. (Even if we totally legalized shophouses, which we should, it's not like small businesses and offices would immediately infiltrate neighborhoods.)

2. The above mix applies uniformly, so I can analyze a model square kilometer (km2) as a stand-in for the whole city. In reality we would expect some areas to be more residential, thus achieving a higher local density (and thus walkability.)

3. One du per lot, with small-scale rentals being vanishingly insignificant.

4. American families like having 200 m2 houses, but can make do with around 100 m2.

So, in a model km2, we have 500,000 m2 of residential land. We want at least 2500 lots (25 du/ha). Divide, and get 200 m2 per lot, a bit over 2100 ft2, or 1/20 acre. Not exactly huge, but larger than the 1500 ft2 of Houston's reformed code, or the 120 m2 lots I virtually estimated (Google Maps) in Osaka. How big is it? Well, if you don't waste your space with front and side setbacks, pretty big. 10x10 2-story house in front, with living space of 200 m2 (minus stairs), and 100 m2 of back yard, or a square backyard of 33 x 33 feet. Certainly enough for some intensive gardening, or some toddling around; in a city like this, kids really running around would go to the park, or perhaps out on a safe street or sidewalk.

If you don't like being wall to wall, have a house 8x12 m. Still 192 m2 of house space, with some air gap on the sides. If you insist on a single-level house, e.g. for disability reasons, there's still room for an 8x15 = 120 m2 house, or bigger.

Lot size of 7x20 = 140 m2, about the size of traditional Kyoto machiya lots, gives 3571 lots/km2. House of 7x10 gives 140 m2 (still assuming two stories), 7x14 gives 196, and 42 m2 of back yard.

But my lost source said "10-20 du/acre", or 25-50 du/ha. Can we hit 50 via SFH? The obvious approach is even smaller lots, 100 m2, perhaps shaped as 5x20, or 6x16, or 7x14. (If you have existing deep 40 meter lots, you'd probably need flag lots, to subdivide into front and back lots.) You'll have to really choose between large house or a yard -- to get 200 m2 of 2-story house, you'd have to completely fill the lot. But you can, or go smaller.

If we weaken my assumption of only using 50% of the land, then that's another approach: 70% of the land being residential lots, and 140 m2 lots, gives 5000 lots/km2.

How high can we go while keeping SFH? I'd say a house of 4x15 meters, with no yard, just stretching from alley to alley. 120 m2 of house (more like 90 after hallways, still enough for a small 3BR), 60 m2 of lot, 8333 lots/km2. Breaking my first two assumptions, and assuming that locally 80% of the land can be lots, 8333 * 8/5 = 13,333 lots/km2, and a population of around 34,000 km2. Safe to call that an upper bound for dense SFH at this household size.

(Zoning can directly affect lot and house size, but not average household size, so I take the latter as an unchangeable given.)

Of course, even with small lots, not everyone wants to (or can) own a house all the time. If zoning doesn't get in the way, there's a lot of rental potential: third floor units (or even second floor), half-basement units, rear houses or ADUs. If on average half the landowners host another family (or the equivalent population in single tenants in even smaller units), the 10x20 lots, with 2500 lots/km2, can house 3750 du/km2.

And in reality, there is no reason for housing to be so uniform. Left alone, people would naturally build taller and live denser near high value locations like train stations, so walkability can be supported by a mix of SFH and multifamily/rental housing. But it's good to know that you can support it with pure SFH too... as long as you allow small lots.

Though it also means that bigger lots that don't support bigger households (via large household or various rental units) are kind of free-riding on higher density elsewhere, if the inhabitants enjoy walkability.

Comments

[mtbc]

Stacking apartments might become more palatable if it's relatively affordable to isolate them better from each other in terms of noise from beside or above, protection from water leaks from above, etc.