Jane Jacob's Density

Great  blog post here: "Squeezing Jane Jacobs Density". The comments are also worth a read. 

I'd like to do a bit of expository thinking on this bit of text:

....your lower units start losing access to sunlight at 4+ story heights. Even Jacobs notes the disadvantage of coverage that is too high in Chapter 11, discussing the North End in particular. The block on the left side of the North End photo at the top, for example, had 72% building coverage in 1960 - way too high for comfort for her (actually, she called it "intolerable"). That is why it was 123 DUA in that 4-7 story height range. 

For most 1-5 acre lots, 60% lot coverage for 6+ story multifamily development is a sober number not to surpass for your development. If you want to hit 150 DUA with this, you will need to go to at least 8 stories to secure adequately sized multi-bedroom units.  That's what 150 DUA looks like at a bare minimum with underground parking.  If we elect to squeeze 150 DUA into 6 stories instead, we are going to be building too many one bedroom units - exactly what we shouldn't be building, according to Jacobs, if we want to promote diversity! To use her terms, that would "standardize" your development to stamp out diversity.

Two topics it brings to my attention: First is the matter of density, building height, and lot coverage. The second is the difference between lot coverage and block coverage.

Density is a bit 'squishy'; it's a zonal measure of intensity, or the number of of things per areal unit. But what thing is used as the metric generates very different results. Population, employment, dwelling units, and square feet/square meters are all used, and the ratio between them is rarely constant. Which is troublesome, as zoning regulates the built environment using all of the above.

I live in a 1.5-story four-plex, which has four units, and 2-3 people per (2 bedroom) unit. So about 10 people. Total square footage (including the shed) is about 3000 SF. My mom's single family suburban houses is a 2.5 story single family detached, with one person in it, and is about 3000 SF. Very different styles of housing. I'd hazard her lot coverage is lower than mine, something like 50% vs. 70%.

My mom could probably triple the number of dwelling units on the lot by refinishing the basement, and add another unit by retrofitting the top floor. (I lived in a large old house that bad been so converted). Parking would be a non-issue, with the garage, RV-pad, driveway, and on-street parking.