Floor Area Ratio (FAR)
Under proposed legislation, Seattle’s lowest density residential zones are going to have a new way to measure and control the size of houses, called Floor Area Ratio (FAR). It is used everywhere else in the code, but the time has come to apply it to SF zones.
So what is it, how does it regulate building size, and how is it going to apply to Single Family zoned streets?
Imagine the green board below is a typical Seattle single family detached lot, and each little block is one story tall part of a house. Floor Area Ratio is a the amount of allowable square footage of structures relative the lot area.
Today, houses can take up 35% of the lot, up to the 30’ height limit. Of course most older houses are no where near this bulk, but many newly built ones max out these threshold, giving rise to the dreaded McMansion. The FAR of today’s larger homes could be as high as 1.05 (three stories X 35% lot coverage).
Under the ADU reform, the size of newly constructed houses would be limited to a Floor Area Ratio of .5, meaning the amount of square footage couldn’t exceed half the total lot size (there are precise definitions of how and what is considered Floor Area and a bevy of exceptions, but we’re using blocks to illustrate this, so let’s keep this high level). Basically, if you had a 5000 square foot lot, you could build a 2500 square foot house.
That .5 FAR could designed like this:
Or this:
Or this:
Or any configuration that fits within the other development limits like setbacks, height limits, lot coverage. It is just regulating the relative size. Regardless any new house will be more in keeping with the traditional street scale than that McMansion.
Accessory Dwelling Units (ADUs) can be attached or detached from the primary residence.
Attached ADUs (in orange) must fit within the typical development limits like lot coverage etc.
Under the new ADU code, they don’t count toward FAR — meaning there is a an incentive to provide a convertible ground floor suite with basic kitchenette that could be used as an ADU (up to an extra 1000 square feet). You can get creative as to how to provide that space if you want to build a house larger than the .5 FAR.
The ADU could be separate from the primary house-a ‘backyard cottage’ or ‘DADU’ but it is governed by both the general development thresholds, and a second unique set of rules especially for DADUs — lower heights, different setbacks, coverage of rear setback area etc.
The reform would make their unique rules less restrictive and less expensive. DADUs could get a little taller, and a little bigger. They’re still subject to overall lot coverage, just as the house with no ADUs would be, but they aren’t subject to the FAR limit, just the DADU unit size (1000 square feet).
Finally, with some limitations a homeowner could have both an Attached and a Detached ADU. Again, Accessories don’t count toward the FAR limit, but would be controlled by every other setback, lot coverage, and height limit.
By reducing total FAR but giving ADUs a break, it limits the scrape off McMansions and bakes in new generally more affordable housing options for additional households within a more contextual, lower impact scale of house.
