Under Seattle’s current single family zoning rules, anyone can build up to a 5,250 square foot, 3 story home (a summary of the rules is on the city website here and an infographic on what that looks like on a 5,000 square foot lot is here). A straightforward way to open up land zoned single family to more affordable options would be to keep the same lot coverage and height limits, while allowing people to divide the structure into two or more homes as they see fit. These are the conditions under which many duplexes, triplexes, and stacked flats that still sit right alongside single family homes in Seattle were built, prior to nearly 100 years of downzoning (the Sightline Institute has the history here).
Rezone Seattle 