This chapter focuses on the fourth of Jane’s generators of diversity: “A sufficiently dense concentration of people, for whatever purpose they may be there. This includes people there because of residence.”
She recommends using a Goldilocks Principle for density: Not too low, not too high, but just right.
You want residential density to be high enough to fill the neighbourhood with people who use the streets, parks, schools, offices, grocery stores, bookstores, community centres, theatres, comic book stores, bike shops, coffee houses, music venues, etc., particularly in the evenings and weekends. However, this concentration of residences must also be low enough that it can be diverse itself, including many different types of housing:
“City dwellings have to be intensive in their use of the land for reasons that go much deeper than the cost of land. On the other hand, this does not mean that everyone can or should be put in elevator apartment houses to live — or into any other one or two types of dwellings. That kind of solution kills diversity by obstructing it from another direction.”
So we need concentration to stimulate neighbourhood diversity, but if we only build tiny bachelor-size condos, we squelch diversity in another way.
Then she starts throwing some numbers around. Let’s be frank:
I struggle to visualize the numbers Jane uses when she talks about density.
Here we go:
“I should judge that numerically the escape from ‘in-between’ densities probably lies somewhere around the figure of 100 dwellings to an acre” (italics in original).
At this point, I felt a bit like this guy:
Does my neighbourhood meet that criteria? How can I tell? How many net acres are in my neighbourhood? And what about units? I can count balconies on the apartment tower down the road, but what about the rest? Does Toronto publish such data?
She mentions specific districts as examples: What did they look like in 1962? Would 2012’s Google Street View be illuminating at all? (I suspect not.)
So, like all good geeks, I started researching.
What is the density of the street you live on? The answer is not any one number but several, depending on how you measure density and how broad an area you include in your calculation. Density can be expressed in different ways — persons per square mile, units per acre, or floor area ratio. Residential density is typically expressed in housing units per acre and measured as net or gross.
But do we need to get technical about it? We all know what high density feels like, right? Let’s test your density-meter:
Which of these has a higher density?
Trick question! They have about the same density. Surprised? Let’s zoom out and get some more details:
Specifically, the Detroit development puts 325 units on 9.6 acres (33.9 units/acre) and the Boulder development puts 191 units on 5.8 acres (33 units/acre). The Detroit development has low ground coverage, while the Boulder development uses more of its land.
Get all the technical details about density levels, along with lots of great examples of density in action, at the Lincoln Institute of Land Policy’s Visualizing Density site.
Improve your Density-Meter
Try the What Does Density Look Like? quiz:
The Western Australian Planning Commission has a great document which I found very useful: Residential Density and Housing Examples (PDF). It illustrates different levels of density in Perth, Australia. They provide examples of housing ranging from 10 units per net acre all the way up to 190 units per net acre.
How does Jane measure density?
She focuses on districts, not individual lots.
This is important to bear in mind, because it enables variety. Since we’re measuring at the district level, we can mix intense developments like condo towers and apartment buildings with houses and low-rise developments and reach a fairly high density for the neighbourhood overall. Densities become too high when they “begin to repress diversity instead of to stimulate it.” Variety is vital!
No one way is a good way to house a city neighbourhood; no mere two or three ways are good. The more variations there can be, the better… It is not easy to reconcile high densities with great variety in buildings, yet it must be attempted.
She measures units per net acre (or residential acre).
If you measure by net acre, you only count the land zoned for housing. In a district with a lot of short blocks, more land will be taken up by streets and sidewalks, and that land isn’t counted and therefore doesn’t lower your density measurement. However, if you build a huge yard, that under-used land will get counted and lower your density measurement. This measure focuses on how well you’re using the land available for housing.
If you measure by gross acre, you count all the land, whether it can be used for housing or not. This measure focuses on how much housing is in the area in general.
No one else seems to measure density this way.
Perhaps I’ve been looking in the wrong places, but I can’t find anyone else who measures density by units per net acre at the district level. I’m not even sure how she managed it in the first place, since she doesn’t cite her sources. Any suggestions?
Toronto measures density by gross floor area. If you know how to translate that into units per acre, I’m all ears. I suspect it’s not possible, but I’d love to see if it is!
I’m still not entirely sure what Jane’s neighbourhood district with 100 dwellings per net acre looks like. However, I do feel more confident on the following points:
- Measuring at the district level asks us to take the context into consideration when building on a specific lot. Is a tower appropriate here? It depends on the neighbourhood. It could be just what’s missing, or it could be squelching diversity.
- Thinking in terms of net acres also informs how we treat non-residential land. If we have high ground coverage and high density on our residential land, we must consider: Are our streets wide and welcoming? Do they leave us room to breathe, or are we trapped in a glass canyon?
- One thing we haven’t talked about is unit size. If I build a 10-storey building with one unit per floor, that’s pretty low density but would allow for large family units. If I pack my building with tiny bachelor units, then we have more units per acre. Does anyone control for this? Perhaps we could measure by number of bedrooms?
- Jane’s lower limit for residential density is 100 units / acre. What’s her upper limit? She doesn’t offer one. She knew building technology was changing rapidly in 1962 and would continue to change, so she stuck with the Goldilocks Principle: high enough to enable diverse primary and secondary uses, and low enough that you can still have a mix of housing types. We’re able to build much higher now, so we can put more people in a neighbourhood while maintaining a diversity of types than was possible when she wrote Death & Life.
- Evisioning infill: Urban Advantage has some intriguing illustrations of how we can infill our spaces to add more density.
- Did you know that Toronto currently has the most high rise construction in North America? See Page 15, ‘Economic Dashboard’ (PDF), City of Toronto.
- The vast majority of 132+ highrises built in Toronto in 2011 were condos. As reported in The Toronto Star in October 2011, “you could probably count on your hand” the number of rental high rises and office buildings going up in the city. Are we threatening our balance of primary mixed uses by building so many condos and so few buildings of other types?