Updated September 2026. An earlier version of this post claimed an ADU adds a specific six figure amount to property value. That claim was not supported and has been removed.
The short answer
Washington has genuinely opened up accessory dwelling units, and the legal change is as big as people say. The resale math is not.
In Zonda's 2025 Cost vs. Value Report, an accessory dwelling unit is estimated to recoup 32.3% of its cost in the Seattle market, against 41.3% nationally. It is the second worst performing project on the entire Seattle list, ahead only of a primary suite addition.
So build an ADU for rental income, for family, or for a use you actually want. Do not build one expecting to get your money back when you sell.
What HB 1337 actually requires
House Bill 1337 passed in 2023 and applies to cities and counties planning under the Growth Management Act. Per the Washington State Legislature's bill summary and MRSC's guidance for local governments, the core requirements are:
- Two ADUs per lot must be allowed within urban growth areas, on lots that meet the minimum lot size for the principal dwelling. Two attached, two detached, or one of each.
- No owner-occupancy requirement. A local government may not require either the main house or the ADU to be owner occupied as a condition of approval.
- No floor area cap below 1,000 square feet, or 60% of the primary dwelling, whichever the local rule uses. See MRSC's summary.
- Limits on parking requirements near major transit stops.
Two things people get wrong. First, this applies to GMA urban growth areas, not literally every parcel in the state, so your specific city or county rules still matter. Second, "allowed" is not the same as "easy." Utility capacity, setbacks, tree regulations, critical areas and stormwater still govern what you can actually build on your lot.
What the permitting data shows
Seattle's own numbers tell a useful story about both the opportunity and the constraint. The City issued 987 permits for ADUs in 2023, a fourfold increase over 2018 levels, then saw applications fall by nearly 40% in 2024 as interest rates peaked.
That fall is the important part. Demand for ADUs is highly sensitive to the cost of money, because almost nobody pays cash for one. The same City announcement reports that ADUs rent for about 25% less than the median one bedroom apartment, and that roughly 12% of Seattle ADUs are occupied by family or friends completely rent free.
Read those two facts together. A meaningful share of ADUs are not income properties at all. They are housing for someone you love, which is a perfectly good reason to build one and a terrible one to model as a yield.
Seattle's Office of Planning and Community Development publishes an annual ADU report with current permitting detail, and it is worth reading before you commit to anything.
Why the resale number is so poor
An ADU is expensive per square foot because it is a complete dwelling. Foundation, envelope, a full kitchen and bath, separate utilities, egress, fire separation, and a permit process that is closer to building a small house than to remodeling a large room.
Meanwhile the appraisal may not treat it as proportional space. Depending on the property and the comparable sales available, an ADU can be valued closer to an amenity than to a second home. In a market without many ADU sales to compare against, an appraiser has little to work with.
We are deliberately not quoting a build cost range here. The credible per-square-foot numbers for this region come from contractors quoting their own work, and we are not going to launder a marketing estimate into a statistic. Get two or three real bids on your actual lot.
When an ADU is a good decision
- Housing someone. Aging parents, an adult child, a caregiver. The alternative is usually far more expensive than the build.
- Long horizon rental income. If you will hold the property long enough for rent to matter more than resale, run it as a rental investment with real vacancy, maintenance and management assumptions.
- You want the space yourself. A studio, an office, a guest unit. Same as any other purchase you make for use.
When it is not
- You are selling soon. At an estimated 32.3% cost recouped in this market, an ADU is close to the worst pre-sale project you could choose. If you are preparing to list, condition work is where the money goes.
- The lot fights you. Steep slope, no access for equipment, distant utilities, or critical areas can add cost faster than square footage does.
- The numbers only work at full occupancy. Model a vacant month or three.
- You are borrowing at a rate that assumes rent covers it. That is the assumption that broke for a lot of people in 2024.
How FLYP fits, and how it does not
FLYP plans and manages renovations, and the work is performed by Green State Restoration, a licensed and insured Washington general contractor that is the contractor of record. We serve Washington only, centered on the Seattle and Puget Sound area.
For a homeowner who is selling, the pay-at-closing structure funds the work and is repaid out of closing at a fixed price, with no interest during the renovation and none for six months after it is finished. But we would rarely put an ADU into a pre-sale scope, for the reason the table above makes obvious. That structure exists to fix condition before a sale, not to build a second dwelling on a deadline.
For a homeowner who is staying, an ADU is conventional paid remodeling that you fund yourself, and the financing conversation belongs with your bank or credit union. FLYP is not a lender and does not arrange financing.
You can lose money on a project like this. Costs move, rents move, and a build that runs long is exposed to both. Talk to a CPA about the tax treatment of rental income and to an attorney if the plan involves family living arrangements or a future transfer of the property.
What to do next
- Check your specific city or county ADU rules, not just the state law.
- Get a real site assessment: utilities, access, slope, setbacks, trees.
- Get two or three fixed price bids on a defined scope.
- Model it as a rental or as housing for a person, not as a resale gain.
- If you are actually preparing to sell, spend the same money on condition instead.
Washington's law removed the legal obstacle, and that is worth a lot. It did not change the construction cost, and it did not create a resale premium. Build one because you want the unit.
If you are weighing an ADU against getting a house ready to list, we are happy to tell you which one the numbers favor, including when the answer is neither. Start at get started.
