Right after "should I build an ADU?" comes the question that shapes everything else: how big can it actually be? Square footage drives your budget, height decides whether you get a second story, and setbacks determine where on the lot the unit can even sit. Get these numbers wrong and you design a unit your city won't permit; get them right and you can plan a project that pencils out from day one.
The good news for Washington homeowners is that state law now sets clear floors that cities can't dip below. This is a general explainer, not a substitute for your city's code or a site-specific review. Dimensional standards are set and enforced locally, so your city or county permit office is always the authoritative source for your parcel. With that caveat, here's the framework.
State Law Sets the Floor
The dimensional rules for ADUs changed dramatically with HB 1337, Washington's 2023 ADU law. Before it, every city set its own size caps, height limits, and setbacks, and many used those standards to make ADUs impractical. HB 1337 flipped that by telling cities what they must allow. A city can be more generous than the state minimums, but it cannot be more restrictive on the items the law addresses.
Three of those items decide how big your unit can be: gross floor area, height, and setbacks.
Square Footage: At Least 1,000 Square Feet
Under HB 1337, a city cannot set a maximum gross floor area for an ADU that is less than 1,000 square feet. In plain terms, your city has to allow you to build an ADU of at least 1,000 square feet, and many allow larger. That's enough for a genuine one- or two-bedroom home, not just a studio.
A few practical notes:
- 1,000 sq ft is the floor cities must permit, not a hard ceiling on every lot. Some jurisdictions allow larger ADUs, and the buildable size is often limited more by your lot's setbacks and lot coverage rules than by a flat square-footage cap.
- The type of ADU shapes realistic size. A from-scratch detached DADU can usually be built out to the full allowance, while a garage conversion or basement conversion is bounded by the existing structure's footprint.
- Bigger isn't automatically better. Square footage is the single biggest driver of what an ADU costs to build. Size the unit to its purpose, whether that's rental income or housing family, rather than maxing out on principle.
Height: Cities Must Allow at Least 24 Feet
HB 1337 also prevents cities from imposing a roof height limit on ADUs of less than 24 feet. Twenty-four feet is generally enough for a two-story unit, which matters on tight lots where you can't spread out horizontally and need to build up instead. Going vertical lets you fit a larger, more livable floor plan on a smaller footprint while leaving yard space intact.
As with square footage, 24 feet is the minimum a city must allow; local code may permit more. Design details like roof pitch and how height is measured from grade are defined in your local code, so confirm the specifics before your architect commits to a two-story plan.
Setbacks: No Stricter Than Your Main House
Setbacks are the required distances between a structure and your property lines, and they often constrain an ADU more than any square-footage cap. Here the law is straightforward: a city cannot require setbacks for an ADU that are greater than those required for the principal dwelling on the lot. Your backyard cottage doesn't have to sit farther from the fence than your main house does.
Some cities go further and allow reduced rear and side setbacks for detached units specifically, which can be the difference between fitting a DADU on a narrow lot and not. Because setbacks, combined with maximum lot coverage rules, define the actual buildable envelope on your property, this is the number most worth confirming early. It's a smart thing to raise when you're interviewing builders, since an experienced local builder can eyeball whether your lot has room before you spend on design.
Parking Usually Isn't a Constraint Anymore
Parking isn't a dimension, but it used to eat into buildable space. HB 1337 bars cities from requiring off-street parking for an ADU when the lot is within a half mile walking distance of a major transit stop, and cities generally can't use parking mandates to block ADUs the way some did before. That frees up yard area that used to be lost to a required stall, which in practice can mean room for a larger unit or a better site layout.
How to Find Your Exact Numbers
The state floors tell you what's possible; your local code tells you what applies to your parcel. To get real numbers:
- Look up your city or county ADU/DADU code. Most Puget Sound jurisdictions publish an ADU handout or code summary with size, height, setback, and lot-coverage standards in one place.
- Call your local permit office. They can confirm how height is measured, what setbacks apply to your zone, and whether your lot has any overlay (critical areas, shoreline) that changes the envelope. Our Seattle permit requirements guide shows the kind of detail to expect.
- Get a site assessment from a builder. A builder who works your area can translate the code into what actually fits on your lot, factoring in slope, existing structures, and utility connections. That's also where you'll learn how size and height choices flow into timeline and cost.
The bottom line: statewide, you can count on being allowed to build an ADU of at least 1,000 square feet and up to at least 24 feet tall, with setbacks no stricter than your main house. Many cities allow more. Confirm your local numbers before you design, and you'll avoid the most common and expensive early mistake, drawing a unit your jurisdiction won't approve.
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