Seattle Council Sets the Stage for a Potential Multiplex Boom

As the Seattle City Council amended Mayor Bruce Harrell’s proposed update to the City’s Comprehensive Plan in September, they approved a series of height and density bonuses for stacked flats that could unleash an uncommon type of development in the city’s former single family zones: stacked flats. Builders able to stack incentives could build 12-unit multiplex apartments, in some cases, where formerly just one primary single family home and two accessory dwelling units were allowed.

Single family zones, officially renamed “Neighborhood Residential” (NR) zones in 2023, make up about three fourths of Seattle’s residential land, and the city council largely avoided making huge changes to the mayor’s map as it rubberstamped the 20-year growth plan. But while the council shied away from adding new growth centers, the updated NR zoning could be consequential — particularly if builders are able to thread the needle and successfully pilot and scale up a stacked multiplex model across single family lots.

As transmitted to city council, Mayor Harrell’s proposal topped out at three-story stacked flats on less than half the lot (or four stories on 60% of the lot, if affordable homes were included) and only on unusually large lots near transit. Housing advocates warned that developers wouldn’t actually build that housing and instead opt for lower density townhomes, due to limited capacity offered and barriers that would remain to stacked flats baked into the system, such as financing structures and state building codes.

Among the fans of the policy revision is Matt Hutchins, who is principal at CAST Architecture and serves on the Seattle Planning Commission. Hutchins has long advocated for stacked flats and pushed back on tight restrictions in Neighborhood Residential zones, popularizing the concept of the “Seattle Sixplex” during the lengthy process for the growth plan.

https://www.theurbanist.org/2025/10/03/seattle-council-sets-the-stage-for-a-potential-multiplex-boom/