HB 1110 · WA Middle-Housing Law
Duplex, triplex, fourplex builder for Seattle and the Eastside.
HB 1110, the 2023 Washington middle-housing law, requires cities to allow two to four units per residential lot. We design, permit, and build duplexes, triplexes, fourplexes, courtyard apartments, and townhomes on lots that previously allowed only a single-family home.
Written by Aaron Elisha, founder of MNBE Construction & Development. WA license MNBECCD770R9. Updated 2026.
What HB 1110 actually does.
HB 1110 is Washington's 2023 middle-housing law. It preempts city zoning to require that lots in residential zones inside cities of 25,000 or more allow at least two housing units. Lots within a quarter mile of major transit must allow at least four. Codified in RCW 36.70A.635.
In practice, HB 1110 opens a path for duplex, triplex, fourplex, courtyard apartments, townhomes, and stacked flats in zones that previously allowed only single-family homes. Cities had until June 2025 to update their codes (or July 2025 in some cases). Most Seattle and Eastside cities have updated. Cities that missed the deadline must apply the state default rules directly until they update.
What HB 1110 forces cities to allow.
2 units per lot, minimum
Cities of 25,000+ must allow at least two housing units on every residential lot. Cities of 75,000+ must allow at least four.
4 units near major transit
Lots within ¼ mile of frequent transit must allow at least four units regardless of city size, in qualifying zones.
Allowed building types
Duplex, triplex, fourplex, townhomes, courtyard apartments, stacked flats, and cottage housing all qualify under specific lot conditions.
Lot-size minimums capped
Cities cannot require larger lots for middle housing than the smallest single-family minimum in that zone.
Setback floors capped
Setback rules cannot be more restrictive than the standard for single-family in that zone.
Parking minimums eliminated
Off-street parking cannot be required for units within ½ mile of major transit. Reduced minimums elsewhere.
What HB 1110 does not do.
Building code. HB 1110 is a zoning law. The Washington State Energy Code, fire code, and the International Residential Code still apply. Multi-unit buildings must meet code-required fire separations, egress paths, and energy compliance.
Critical-area review. Steep slopes, wetland buffers, tree retention, and stream corridors are not affected. Lots with critical-area triggers go through the same review as before.
Height and lot coverage. Cities can still enforce height limits and total lot coverage caps. The law caps zoning rules but does not abolish reasonable building-envelope rules.
Design review. Some cities apply design review boards to multi-unit projects in specific overlays. HB 1110 forbids treating middle-housing more harshly than equivalent single-family in design review, but the review itself can still happen.
Utility capacity. Adding multiple dwelling units can trigger utility capacity charges and stormwater management requirements that have nothing to do with zoning. Engineering happens during design.
"HB 1110 and HB 1337 together will change what gets built in Washington over the next decade. Two ADUs and a primary plus middle housing on what used to be single-family lots. We are building for the new code, not the old one."
Aaron Elisha · Founder, MNBE Construction & Development
Related Laws + Services
Where middle housing fits.
Sister Law
HB 1337 Explained
The 2023 ADU law. HB 1337 + HB 1110 together rewrite what residential lots allow.
Read the guide →
Companion
Infill Construction
Most middle-housing builds happen on infill lots inside existing neighborhoods.
See infill →
Step One
Site Feasibility
Confirm what HB 1110 specifically allows on your lot before design begins.
See feasibility →Frequently Asked
HB 1110 questions Seattle homeowners ask before they sign.
Six practical answers from a builder who runs middle-housing projects under the new law. If yours is not here, call us.
Still have questions?
We answer the phone Monday through Saturday. Two minutes on the call usually gets you further than an hour online.
HB 1110 is the Washington state middle-housing law passed in 2023. It requires cities of 25,000 or more to allow at least two units per residential lot statewide, and four units within a quarter mile of major transit. Some cities allow more. The law preempts local zoning where local code conflicts. Codified in RCW 36.70A.635. Distinct from HB 1337 (which expanded ADU rights) but complementary.
HB 1337 expanded ADU rights: two ADUs per lot, owner-occupancy bans illegal, parking minimums eliminated near transit. HB 1110 expanded middle-housing rights: duplex, triplex, fourplex, townhouses, and courtyard apartments on lots that previously allowed only single-family. HB 1337 covers accessory dwellings. HB 1110 covers primary residences in non-single-family configurations. Both apply across Seattle and the Eastside. Both took effect 2023, with city compliance deadlines staggered through 2025.
Almost certainly yes if you're in a city of 25,000 or more. HB 1110 mandates that cities allow at least two units per residential lot. Some cities (Seattle, Bellevue, Kirkland) allow up to four units within a quarter mile of major transit. The lot needs to meet minimum size for the unit count, but minimum lot sizes were also capped by HB 1110. We confirm specifically what your lot allows during the feasibility step.
Similar to a custom home but with multi-unit considerations: separate dwelling unit egress, shared-wall fire ratings, separate utilities, parking analysis (often zero required near transit), and unit-by-unit address assignment. Design phase is typically 4 to 6 months for a fourplex. Permits run 4 to 8 months at Seattle DCI. Construction is 12 to 18 months. The longest pole is design and engineering coordination across multiple units sharing infrastructure.
Sometimes. HB 1110 does not directly enable separate sale, but related Washington condominium law allows multi-unit buildings to be platted as condos and sold individually under specific conditions. The structuring decision happens during design with appropriate legal coordination. We have built fourplexes intended for single-owner rental and fourplexes intended for individual condo sale. The construction is similar; the legal structure is different.
Yes. Building additional units on the lot increases the assessed value, which raises the property tax bill. The increase is usually modest compared to the rental income or sale value the additional units generate. Specific math depends on your county and the building. We walk through the projected tax math during the consultation.
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