While buyers watch interest rates, the more lasting shift in Greater Seattle value is happening in city council chambers — a wholesale rewrite of what you're allowed to build.
The big picture: Washington legalized more homes
Over the last few years, Washington passed a pair of landmark housing laws that are now being implemented across local jurisdictions. Together, they represent the most significant change to what's buildable on residential land in generations — and they reshape value for buyers, sellers, and especially investors.
HB 1110 — the middle-housing law
HB 1110 requires many Washington cities to allow “middle housing” — duplexes, triplexes, and in larger cities up to four-to-six units — on lots that were previously zoned for a single house. More units are typically allowed near frequent transit or when some of the homes are income-restricted. The exact unit counts and rules vary by city size and by how each jurisdiction implements the law, so the specifics on any given lot depend on the local code.
The practical effect: a plain single-family lot can become a site for two, three, or more homes — dramatically changing its highest-and-best-use and its value.
HB 1337 — the ADU law
HB 1337 expanded the right to build accessory dwelling units (ADUs) and detached ADUs (DADUs — think backyard cottages). It limits the ability of cities to block them with owner-occupancy requirements, excessive parking mandates, or size caps. For many homeowners, that means the legal ability to add a rentable unit or multigenerational space where it wasn't feasible before.
“One Seattle” and the city's comprehensive plan
Seattle's own “One Seattle” planning effort layers on top of the state laws, expanding where multiplexes and denser housing are allowed across formerly single-family neighborhoods, with particular focus on areas near light rail and frequent transit. The combined effect is a city steadily adding housing capacity — and creating value-add opportunities where the old zoning capped what you could build.
What it means for each kind of client
- Investors: the headline opportunity. A well-located single-family lot may now pencil as a duplex or triplex, or as a single-family home plus an income ADU. Underwrite properties both as-is and at their new buildable potential.
- Homeowners: you may be able to add a DADU for rental income, aging parents, or adult kids — and boost your property's value and flexibility.
- Buyers: more housing supply over time can ease price pressure, and proximity to transit-upzoned corridors may matter more than ever for future value.
- Sellers: if your lot has development potential under the new rules, marketing that upside can expand your buyer pool to include builders and investors.
The fine print that matters
Implementation is local and still rolling out. Unit limits, design standards, permitting timelines, and parking rules differ by city and even by neighborhood, and they continue to evolve as jurisdictions adopt their codes. Construction and permitting costs remain elevated, so a lot's theoretical potential doesn't always pencil in practice. Before you pay a premium for “development potential,” the buildable program needs to be verified against the current local code and a real cost estimate.
This is exactly the kind of analysis we run — reading a property both as it stands today and as the new zoning allows it to become. If you're weighing a lot for its middle-housing or ADU upside anywhere in Greater Seattle, let's underwrite it together before you offer.
Have questions about what this means for your own situation? Let's talk it through.
Book a consult More for investors