Fitting more housing into the city

Seattle is suffering from a deep housing affordability crisis, with more and more people being priced out of living there. At the same time, it’s been deeply resistant to changes in zoning that would allow enough new housing to be built. One of the examples of this is that it has a program called “Encouraging Backyard Cottages”, and has gone through at least two rounds of legal reforms to support that, but figuring out whether one can be built on any given lot still involves going through a long checklist about the exact dimensions of the site and intricacies of zoning.

In theory, DADUs (Detached Accessory Dwelling Units, the much less appealing legal term for “backyard cottages”) allow a lot of small, affordable housing to be added to single-family zoned neighbourhoods and spread out enough to not feel like a radical change in the streetscape. In practice, the complicatedness of the process adds enough of a barrier that relatively few have been built so far. Hatchback Cottages has a plan to solve this with a set of ready-to-build designs and a package of support to help people through the process.

Even with their expertise, assessing a site under the arcane rules is a time consuming process. But computers are good at applying lots of rules and calculating all the measurements, so Hatchback contracted me to run a GIS analysis assessing every residential lot in Seattle for suitability.

A map of one city block with individual lots shaded green if they could have a DADU and red if they couldn't.  Existing building outlines and the area in which a DADU might fit are also shown.

Fortunately for us, Seattle and King County publish very comprehensive and regularly updated open data about zoning and development, so I had a lot to work with. The analysis takes into account existing building footprints, lot characteristics and potential complicating factors like steep slopes and landslide hazard areas. It will never be a complete replacement for a knowledgeable human looking at the site, but by ruling out all the sites that definitely won’t work it saves my client a lot of time. Now the experts can solely focus on sites that have a relatively good chance of working out.