FAR and density

[mindstalk]

Another exercise, regarding Floor Area Ratio.

Imagine a model km2, 36% of which is road (not unusual for the US), so 64% is buildable lot -- 640,000 m2. Say half is zoned for residential. (I don't know why so low, but I recall Seattle being about that.) Americans apparently have 80 m2 of housing per capita, which seems high to me, but let's use it.

Say the FAR is 1.0 -- every lot is filled with a one-story building, or a 2-story takes half the lot, or a 3 story takes 1/3 the lot. 320,000 m2, 80 m2/person, so 4000 people/km2. Not particularly dense -- Boston is 5500, Chicago used to be and is still around 4500 -- but not terrible.

Of course, residential often clumps, so if we imagine a mostly residential zone, then even with grocery stores and schools, we would approach doubling the density locally -- 8000 people/km2. Which is significant for supporting small businesses in a walkable neighborhood. One supermarket per km2 or so, a 10 minute walk away for everyone.

If the FAR is 2.0 -- two story terraced housing filling the lot, 3-story on 70% of the lot, 4-story on 50% -- then double both numbers. 8000/km2 for the city, now denser than San Francisco, 16,000/km2 for the residential clump. A real city!

OTOH, if the FAR is 0.5 -- a one story building on half the lot, or 2-story on 1/4 the lot -- then the city is at 2000/km2. Like most of Silicon Valley, as it happens. Also modern Detroit.

Emotionally I would say you need a FAR of 1 to even qualify as a city, and really more like 2. One story buildings that don't even fill their lot isn't a city, it's a village.

What about parking? The whole point of a walkable city is that not everyone needs a car! But part of that 36% land use for roads is going to be curbside parking, so lots of spaces there -- not enough for everyone to have a car, but maybe enough for everyone who needs a car, especially if you price them right. You can also have garages a la Japan to meet further demand.

If you didn't build specifically for cars, then your roads are probably 15-25% of the land, not 36%. Which allows even more low-rise density.

Comments

[elusiveat]

Thanks for sharing all of these calculations with us.

Quick reaction is that I think I'd rather use human density, not FAR to determine what is a city. I love the idea of small footprint homes interspersed with lush green space.

Row houses feel like a nice way of saving space without losing amenities...

[mindstalk]

Thanks for letting me know someone likes them! [sweatdrop]

Fair point; I myself would default to people first, with 4-5k as a bare minimum. And if you halve the space used to say 40 m2/person -- I think read somewhere that UK *houses* average around 80 m2, for a family -- obviously you can double the populations given.

That said, you do need the FAR too. FAR of 0.5 at 40/m2 per person still gives you 4000 people/km2, barely urban IMO. And I feel that if your land isn't valuable enough that you're building at least 2 stories as a matter of course, you can't really be a healthy city.

Row houses: small side setbacks can easily be just wasted space, or no more than an open walkway to the back, so yeah row houses save that space. Supposedly modern codes require fireproof 'party walls', so the fire control excuse doesn't work.

But there's maybe a bigger point to them that's tangential to just density, which I was going to mention in another post: granularity. Small lots mean less money need to get one, and more landowners making more diverse decisions, and smaller buildings mean less disruption if you have to tear something down. A block of small lots will probably be more diverse and responsive than a block that's one big development, even if it's physically the same at some point at time.

But side setbacks and murder small lots. Or at least give an incentive to combine multiple adjacent lots to build one bigger building, so less space is lost to side setbacks. Front and back setbacks, while likely wasting space, at least don't waste any *more* space for narrow vs. wider buildings. (Usually lot depth is constant on a block, and 'small' means narrower.) (Though if you're a city deciding how to sell off a huge block of land, you should maybe consider making sure the lots aren't super deep, too, see previous townhouse post.)

Without side setbacks, small (narrow) lots can remain competitive, and also you get terraced building.

[elusiveat]

Small lots mean less money need to get one, and more landowners making more diverse decisions, and smaller buildings mean less disruption if you have to tear something down.

Are you familiar with the sad situation with the old rowhouses in Baltimore? They've effectively bundled these two issues together: they were originally a way that a low income family could actually own a home with a small yard, and were evidently a wonderful thing when they were new, but the market price for a unit is *so* low that over the decades unit-owners have moved away and it hasn't been worth the effort of finding a buyer, so they just abandoned the unit. Baltimore is now gradually tearing down the neglected buildings, but it's slow going because there are lots of rows that still have a small number of units occupied, and that's a difficult situation to navigate.

wasting space

Have you written anything about what you see as constituting "wasted" space? I know you wrote a thing a while back about the value of trees in cities, and I think that there's similar value to front gardens as well. Back yards are a different issue because they cordon off large spaces in ways that don't have a public benefit. Off the top of my head, maybe a nice alternative would be to have homes back onto public parks.

I live in Somerville, MA, which is one of the denser towns in the U.S., and feels plenty vibrant to me (I also like Chicago and Berkeley). The standard small lot here is 1/8 acre lots with a two-family house filling most of the lot and a tiny yard. We currently live in one of these, and according to Zillow one unit is 1,234 square feet = 115 square meters for a family of three (40 per person, though one of the people is a small child). I'm reasonably satisfied, but I wish we could replace our driveway with grass. We rent, though, so that's not going to happen.

I would be happier still if we also had flatmates and slightly more common space inside the house (multiple families / individuals sharing a unit tends to get erased from conversations about density). Having a larger house with more people per square foot and a larger yard would be lovely.

[elusiveat]

Quick additional thought on Somerville: one of our neighbors is a state representative and has been pushing back against zoning changes to further increase density specifically because the city *already* has so much impervious surface that there are drainage problems.

[mindstalk]

*skeptical*

Japan has more rain and more density and somehow drains itself.

Allowing buildings to be replaced with taller buildings doesn't increase impervious surface, though allowing ADUs might. But a lot of 'yards' are part garages or parking spaces already.

[mindstalk]

Not particularly familiar. I think the issue in Baltimore is collapsing economy and population -- 25% vacancy -- not row houses, though maybe row house make demolition more difficult? I would think you can take out an empty row house from between its neighbors but I don't know construction details. I suppose zoning could require separate load-bearing walls, or setbacks of a couple inches, without losing much.

Note that row houses in SF or Georgetown, DC are *really valuable* now.

wasted space: I don't think I have yet, though https://www.strongtowns.org/journal/2019/11/19/visualizing-place-vs-non-place is related.

But in this context... a side setback can be too small and shaded to really enjoy. A foot or 3-foot wide strip of grass is pretty meh. In Somerville, many sides are just a driveway to parking in the back. None of these are of particularly high value.

I can imagine wanting to maintain walkways to the back, though obviously many places see no such need. But you could allow covered walkways, so you wouldn't lose the building room above the path, for however many stories there were.

As for yards, they can be nice, but should they be mandatory? And there's no guarantee you'd get a front *garden* unless your laws are particularly demanding and intrusive. Likely you'd get a lot of lawns -- hell, in the US you often have to fight to put in a garden instead of a lawn!

There's also, if you go that route, how much? Berkeley setbacks seemed pretty small, like a foot or three, and many people were pretty creative about planting that space with subtropical plants. So it was nice without losing too much space. But many US cities require stuff like 20 foot setbacks, that's huge.

And if you allow mixed use as I favor... businesses prefer to be right up to the sidewalk, for easy appeal and access. If passers-buy have to walk 5-10 seconds just to get to your door, fewer will pop in just to check.

In Japan, when houses *did* have front yards (most didn't), they were often walled off, so you don't see much anyway. OTOH there were lots of potted plants lining the streets, providing greenery anyway.

Somerville: I lived in Porter and then Powderhouse Squares, so I have a fair bit of experience with part of Somerville. Mostly of triple deckers on what I thought were rather smaller than 1/8 acres, though the Powderhouse rental was the second floor of a 2-story.

1/8 acre is 500 m2. Ceding 20% to yards, that's 400 m2, 1200 m2 if 3-story, and 15 people at 80 m2/person, 30 at 40. 5-10 families. Are you sure they're filling most of the lot? :) It's really hard to judge area proportions when there are reductions in both length and width.

The 20% ceded to yards could house another 4-7 people if it were building. That's the thing about mandating yards: really high opportunity cost when values increase. Is a yard worth 2 families' worth of housing?

[elusiveat]

Are you sure they're filling most of the lot?

Might be failing to account for driveways, plus it's hard to estimate area when it's spread in a fringe around the house. With our current place, I think it's the combination of forgetting to add front yard and back yard together and oh yeah driveway.

Anyway, I think Somerville is plenty dense, and I wouldn't want to make do with less greenspace.

Did you know that planners in New Hampshire think of 1/4 acre lots as "cluster zoning"? True facts.

[elusiveat]

Addendum. My highly scientific guestimate of our actual lot size on Google Maps puts the area of the lot at something more like 2400 square feet = 222 square meters. So that was probably throwing me off as well.

[mindstalk]

> Did you know that planners in New Hampshire think of 1/4 acre lots as "cluster zoning"? True facts.

*shudder*

I have no objection to people wanting to live with space, I object to their forcing it on everyone else. And banning apartments.

> 222 square meters

Yeah, if 1/8 acres are standard somewhere in Somerville, it's not up against the Cambridge border where I saw.

[elusiveat]

I have no objection to people wanting to live with space, I object to their forcing it on everyone else. And banning apartments.

In this case it's actually the opposite. This is a big pro-density push. Historically, standard lots in most NH towns have been 2 acres.

Yeah, if 1/8 acres are standard somewhere in Somerville, it's not up against the Cambridge border where I saw.

Part of my overestimation of the lot area was because of an argument I had with a New Hampshire colleague who didn't believe you could fit a house on less than 1/8 acre. I remembered looking it up on Google maps, but I'm not sure what the actual lot size was on the houses I was checking. 1/8 acre was what stuck wth me.

[mindstalk]

(I'm re-reading old botec posts)

"who didn't believe you could fit a house on less than 1/8 acre"

Lol/cry.

How does one even think that? Just do the math! 1200 sqft home in two stories needs like 600 sqft of ground for the house, which certainly doesn't need a full 5000 sqft (1/8 acre) lot...

[elusiveat]

I imagine they have some preconceived notions about a house including a two-car garage and a lawn, but I'm not sure.

[mindstalk]

*Still* wouldn't need 5000 sqft, especially if you build the house over the garage!

[mindstalk]

> plenty dense

Housing prices suggest otherwise.

Natural city development would be high density in the core, less so further out. As the city grows, the density rings also move out.

Zoning a density limit is like trying to order back the tide. The demand is still coming; if density doesn't rise, prices will. As they have. Somerville is unaffordable. Is greenspace more important than that? (And again, what greenspace? People's side lots and back yards?)

Of course, Somerville can't solve the Boston housing crisis on its own. But everything from Boston itself to the outer suburbs is full of people saying they're "plenty dense" as they are.

Relatedly, the City of Somerville said that almost every building in the city is in some violation of zoning, *before* counting parking requirements. This is what really horks me about defenses based on "preserving neighborhood character": it's typically used in support of laws that would prevent those neighborhoods from being built today!

[elusiveat]

I am confused. I thought we were talking about the vibrancy of living in a place.

[elusiveat]

Throughout this conversation you keep referring to mandates, legal restrictions, and so on. That's not the way I've been approaching the conversation at all. I'm just thinking about what a community could and should be, from my own perspective.

I think mixed use is fantastic. I think cities should be walkable. I think vibrant cities combine residential space, greenery, and commercial space. I think it's really unfortunate that as demand for urban living goes up the lower income people who keep the city going are driven out to the suburbs, so that vehicle miles traveled aren't reduced at all, just who is doing the driving is changed. I don't know the best way to get the things I value, but I do think it's useful to identify those things.

I never said that there should be a mandate that people have a front garden (how did that idea even come into the conversation???), and at the moment I'm not concerned with the fact that lots of U.S. cities mandate against a front garden because most U.S. cities also aren't *at all* interested in the kinds of goals that I think would make cities more pleasant, sustainable, and livable, so the fact that they want to cover everything with grass monocultures of uniform height is kind of beside the point.

[elusiveat]

And when I say "plenty dense" what I mean is "plenty dense to be liveable without a car." Lots of people here have cars anyway. I think most of them shouldn't.

[elusiveat]

If we're talking about supply and demand, my gripe is that high density housing construction is driven by real estate speculators who want to get rich by filling apartments and condos with rich people without much regard to whether there exist enough rich people to fill those spaces. There's something really wrong if you vastly increase the density but housing prices don't go down. Also not at all okay with the fact that these luxury structures are not built to last. When they start falling apart *then* housing prices will go down. That's not good planning.

[mindstalk]

"There's something really wrong if you vastly increase the density but housing prices don't go down"

That hasn't happened, though. A handful of high-rises don't vastly increase density. Though they *have* been shown to have some impact on prices in a very local area -- some study of Miami neighborhoods with and without new construction, and housing prices in the former growing slower than in the latter.

And of course, if you increase supply but increase demand even faster, prices aren't going to go down. Everything I've seen about the big booming metros like Boston, SF, Seattle, or LA has been "adding 3-6 times as many jobs as housing units, for years and years".


"my gripe is that high density housing construction is driven by real estate speculators who want to get rich by filling apartments and condos with rich people without much regard"

Well a few things:

* I haven't heard that they've had much trouble filling spaces overall. Vacancy rates are still painfully low.

* A Boston housing activist I know has looked at developer profit rates, and they're not that "get rich" as far as building goes. The real riches go to whoever was lucky enough to be owning land early enough.

* Likewise, lots of units are expensive not because "we want to target rich people" but because "being allowed to build a limited number of units, with required parking, makes the units expensive just to break even."

* The difficulty of building caused by zoning and local opposition biases things *toward* big developers, who have the legal budgets to fight with, building big projects that are worth fighting for. 1910s style "developers", down to the small scale of some homeowner who wants to expand or replace their building, are shut out. But the point of my recent posts has been that you can get rather high density just by allowing small landowners to build smallish (but bigger) buildings on small (but fully used) lots.

Though to be fair it is harder in Somerville, where to a degree that already happened. It's hardly full IMO, but incremental re-development, like replacing a 3-story with a 5-story, isn't that cheap (you have to pay for building the 5-story, *and* for acquiring and demolishing the 3-story, all for a net of +2 stories, so the cost per new story is pretty high. It's different if you replace a single-family house with like 6 apartments. But that probably won't happen much in the outer suburbs without a state override of city zoning laws, a la Oregon.)