Which sounds better for affordability?

  1. Tear down an older, modest home and replace it with a “McMansion,” or…
  2. Take that same house, preserve it, and add an in-law apartment and backyard cottage?

Seattle residents and policymakers have spoken: They’d prefer more modest home choices instead of the massive, flashy houses popping up across the city.

To act on that preference, Seattle is on the verge of adopting a set of rule fixes for accessory dwelling units (ADUs) that includes a startlingly radical move: a size limit on all new houses built on the three quarters of the city’s residential land set aside for single, detached homes.

But here’s the key: that limit wouldn’t apply to ADUs. So builders could fit more on a lot by constructing ADUs in addition to a house. The result—smaller, less expensive homes and more of them—would be a win-win for affordability. The size cap would also curtail teardowns, promoting community stability and sustainable (re)use of existing homes.

What might this new set of rules yield on the ground? We visualized the possibilities. Check out the illustrations below, or download a pdf version of our explainer here.

Seattle’s inventive new approach to limiting the size of new houses

Today, Seattle regulates house size by height, lot coverage, and setbacks from the property lines. The code allows up to a 5,250 square foot (sf), three-story house on a 5,000 sf lot (50 feet wide by 100 feet deep is the standard Seattle lot size). In recent years, the typical size of new houses has been smaller than that—around 3,500 sf—though that’s still far larger than the city’s ~2,000 sf average house size.

Seattle’s proposed ADU reform would set a maximum floor area of 2,500 sf for houses built on lots of 5,000 sf or less. Houses on lots larger than 5,000 sf would be capped by a metric called floor-area-ratio, or FAR, which is the floor area of the house divided by the area of its lot. Houses on lots larger than 5,000 sf could not exceed an FAR of 0.5. For example, the maximum house size on a 6,000 sf lot would be 3,000 sf. This keeps the house size limit in scale with the lot size.

The plan would permit two ADUs per lot, up to 1,000 sf each. So a builder could construct a total of 4,500 sf of floor space on a 5,000 sf lot, though it would have to be three legally distinct homes: a house and two ADUs. That 4,500 sf maximum is less than the 5,250 sf allowed under existing rules, but it’s still more than the current 3,500 sf average for new detached houses.

Tipping the scales from giant McModerns to modest granny flats

Because of the current city rules they must follow, builders tend to construct the largest house they can fit on a lot: they would be throwing money away not to. In contrast, under Seattle’s proposed changes, builders would throw money away not to build ADUs along with a new house. Or in some cases, the way to avoid throwing money away would be to keep the existing house instead of tearing it down, and add ADUs to the lot.

  • Our work is made possible by the generosity of people like you!

    Thanks to Jeanette Henderson & Andrew Behm for supporting a sustainable Northwest.

  • Seattle’s relaxed ADU rules and house size cap would enable a variety of ways to add modest homes to low-density residential neighborhoods, as we illustrate below. This incremental, low-impact infill stands in marked contrast to the one-(giant)-size-fits-all McMansion development that’s happening under the status quo—homes that are also one-(giant)-price-tag fits all.

    Download (PDF, 1.37MB)

    Current status of Seattle’s ADU legislation

    Since Sightline’s last write up, policymakers added a new requirement that the second ADU must either meet a green building standard, or be reserved for “income-eligible households.” Most owners would likely opt for the green certification. Either way, though, imposing these extra mandates will make it more financially challenging to construct ADUs—counterproductive policy for a housing type that is relatively green and affordable to begin with.

    Councilmembers have also introduced two amendments that would weaken the reform package:

    • Require a year of ownership before permitting a second ADU: This would prevent builders from constructing a new house with two ADUs at the same time—a process that has helped Vancouver, BC, boost backyard cottage production to six times the rate in Seattle.
    • Ban short term rentals outright in all ADUs: This would stymie ADU construction by eliminating a flexible way to earn income from them. Most homeowners have to take out loans to build ADUs and may be dissuaded from doing so if future income from a short term rental is off the table.

    The amendments for an ownership requirement and a short-term rental ban single out ADUs for restrictions that don’t apply to any other housing type in Seattle. It is unfair and counterproductive to load extra burdens on ADUs specificially—especially given that they are such low-impact homes and just the types of smaller, more affordable, close-in homes the city’s renters need.

    Councilmembers also introduced two good amendments. The first would grant two-story cottages the increase in backyard lot coverage from 40 to 60 percent that in the original bill applies only to one-story cottages. The second would grant an extra 25 sf for ADUs to provide bike storage.

    How Councilmembers could make Seattle’s ADU reform even better

    If adopted as proposed, Seattle’s ADU policy would rank among the best in North America. But there’s still room for improving the legislation:

    • Allow the option of two backyard cottages, attached to each other, or detached: The same size limits that apply to a single cottage would apply to two cumulatively, so the built structure couldn’t be any bigger.
    • Allow ADUs inside the main house to exceed 1,000 sf:  Exempt the first 1,000 sf of ADU from the total house size limit, but allow the owner to divide the area inside the house however they like.
    • Eliminate the minimum lot size requirement: Lot coverage limits and setback requirements preclude cottages on most smaller lots anyway, but some small lots—ones with very small houses, for example—could easily accommodate a backyard cottage.
    • Remove occupancy limits based on legal relationship status: Over recent decades family structures have evolved and tend to be less conventional. Barring additional residents because they are not legally related is discriminatory.

    What happens next?

    Seattle’s ADU reform legislation will have its second public hearing at the City Council’s Sustainability and Transportation Committee, today at 2 pm. Then, the package could move on for consideration by the full Seattle City Council as early as July 1. There’s little doubt at this point that Council will pass good ADU legislation—the only remaining question is whether they will avoid undermining it and pass great legislation.


    Sightline thanks Courtney Ferris for creating the exquisite illustrations featured in this article.