What HB 1110 Actually Requires Washington Planners to Understand About Their Zoning

Washington’s middle housing law has been on the books since 2023. Most planners in the state know the headline: cities must allow duplexes, triplexes, and other “missing middle” housing types in residential zones that were previously single-family only. What’s less clear, and what’s generating a lot of the real work right now, is the parcel-level question underneath that headline.

Knowing your city is required to allow four units per lot doesn’t tell you which lots can actually support four units. That’s a different analysis entirely, and it’s the one that tends to consume staff time.

The Three-Tier Framework and What It Means in Practice

HB 1110 applies differently depending on a city’s size. The law established three tiers:

Tier 1 cities (75,000+ population) must allow at least four units per residential lot, and up to six units on lots within a quarter-mile of a major transit stop or where at least two units are affordable.

Tier 2 cities (25,000 to 75,000 population) must allow at least two units per lot, and four units near transit or with one affordable unit.

Tier 3 cities (under 25,000, within a contiguous urban growth area of a county over 275,000) must allow at least two units per residential lot.

The compliance deadline is tied to each city’s GMA Comprehensive Plan periodic update, specifically, six months after that update is due. That’s why cities across eastern Washington are hitting their deadlines now, on a staggered timeline from the Puget Sound cities that came first.

But the tier designation and the unit allowance are only the starting point. The harder question is what those allowances look like on the ground, parcel by parcel, once you account for existing setbacks, lot coverage limits, height restrictions, and overlay zones. A lot that technically allows four units under the new rules may only physically support two once the development standards are applied. That math doesn’t do itself.

 Where the Work Actually Lives

Here’s what we hear from planners in Oregon who’ve gone through equivalent processes under DLCD’s housing capacity analysis requirements and it maps closely to what Washington planners are navigating under HB 1110:

• The code adoption is step one. The analysis is step two.

• Updating your zoning code to allow middle housing is a discrete task, and most cities are through it or close. But demonstrating actual housing capacity - showing that your zoning, as written and applied, can accommodate the housing your community needs - requires parcel-level data. You need to know what’s buildable, not just what’s allowed in theory.

• The setbacks, lot coverage, and height limits are where complexity lives.

• A duplex is allowed. But on a 5,000-square-foot lot with a 20-foot front setback, 5-foot side setbacks, 40% lot coverage limit, and a 30-foot height maximum - what does that actually yield? That calculation, repeated across every residential parcel in your city, is the core of a housing capacity analysis. It’s also what takes weeks when done manually.

• Public inquiries follow code changes.

• *Once middle housing is allowed, residents and developers start asking questions your counter staff has never fielded before. What can I build on my lot? Can I add a duplex? What are the setbacks for a triplex? These aren’t questions a zoning map answers. They require the kind of parcel-level detail that typically means a call to the planning counter or a tool that can answer them without one.

What Oregon Experience Suggests

UrbanForm works with jurisdictions across Oregon that have been through housing capacity analysis under DLCD’s framework,  a process with meaningful parallels to what Washington cities are doing now under HB 1110 and their GMA housing elements.

A few things we’ve learned from that work:

• The analysis that feels like a compliance obligation turns out to be genuinely useful.

• Cities that build deterministic, parcel-level zoning data for their housing capacity analysis end up with something they use long after the report is filed. Planners reference it daily. It feeds public-facing tools. It makes permit counter conversations faster.

• The first question is usually “what can I build here?”

• Whether it comes from a resident, a developer, or a planning commissioner, that question keeps coming up. Having a tool that can answer it from the adopted code - not a lookup table, not a rough estimate , changes how a planning department operates.

• Accuracy matters more than speed.

• The value of parcel-level zoning data is only as good as its connection to the actual adopted code. When numbers are wrong, they create problems downstream in permit applications, in HCA reports, in public trust. The data needs to be traceable back to the ordinance.

A Note on Where We Are

UrbanForm is built for exactly this kind of work - deterministic, parcel-level zoning analysis built directly from municipal code, traceable to the ordinance sections behind every number. We currently work with 18 Oregon jurisdictions who use it for housing capacity analysis, permit counter support, and public-facing zoning portals.

We’re expanding into Washington now, and we’re starting by working closely with a small number of jurisdictions to build their data the right way, from the ground up, from their actual code. If you’re a Washington planner navigating HB 1110 compliance and housing capacity work, we’d genuinely like to understand what that process looks like for your city.