An accessory dwelling unit usually gets sized up as a construction project and a tax question. In Seattle it is mostly a regulatory decision, and the regulations keep applying long after the final inspection passes. Nothing below is a reason not to build. You should know it before you start, though, because several of these items have to be handled before the first tenant moves in.
Registration and daily penalties
registration is required, and the fees are modest
Seattle's Rental Registration and Inspection Ordinance covers rented accessory dwelling units explicitly, alongside single family rentals and multi-unit properties. On the 2026 schedule registration is about $126 for the first unit plus $31.50 for each additional unit, valid two years. A city inspection is about $241.50 for the first unit, with follow-ups free. Fees include a 5 percent technology surcharge.
The first inspection comes within five years of registration, then a roughly one in ten chance of a second in the following five year period.
The penalty is $150 per day for the first ten days and $500 per day after that. Over a year of renting without registration, that adds up to a large sum. SDCI may also bar you from raising the rent if it determines the unit fails RRIO standards.
Seattle rules that change how you operate
180 days' notice for any rent increase
Seattle requires 180 days of written notice before raising rent or any periodic housing cost, including parking, storage and utilities. It is among the longest notice periods in the country. In practice you decide in June what you will charge in January.
Seattle also has . An increase at or above a threshold reported at 10 percent can entitle a qualifying tenant who chooses to move to three months' rent in relocation assistance. The money is advanced to the tenant and billed back to you. A big catch-up increase after several flat years is a very different decision from a series of small ones.
Ending a tenancy requires a listed reason
Seattle's ordinance requires one of a defined list of reasons to end a tenancy. Wanting the space back is not on the list by itself.
Some provisions apply only to ADUs. If an owner ends an accessory unit tenancy after receiving a development standard violation, relocation assistance of $2,000 or two months' rent is owed, depending on tenant income. These provisions are also written around a permitted accessory dwelling unit. That is one more way an unpermitted unit leaves the owner worse off.
The first qualified applicant gets the unit
Seattle's rule requires you to offer the unit to the first applicant who meets the screening criteria you published, rather than choosing among qualified applicants. It was challenged and upheld by the Washington Supreme Court.
In a large building this is mostly paperwork. With a cottage twenty feet from your kitchen window that shares your driveway and yard, it feels very different, and owners say it is one of the few items here they wish they had understood before building. What you do control is the screening criteria. You set them in advance, and then you have to apply them consistently.
Unincorporated King County has its own tenant protections, adopted separately and different from Seattle's. Don't assume one set carries over to the other. If you are outside the city, a Seattle landlord guide may not describe your obligations at all.
Insurance: the coverage gap on a rented outbuilding
An uninsured loss on a rented cottage can cost more than the cottage did to build.
[Other structures coverage] usually will not provide coverage for other buildings on your property that renters occupy, or buildings you use for business. Washington State Office of the Insurance Commissioner, homeowner insurance guide
Your cottage falls under both halves of that sentence. It is another building on your property, and renters occupy it.
The coverage limit is usually too low as well
The same guide notes that other structures coverage is typically about 10 percent of the dwelling coverage.
A home insured for $800,000 of dwelling coverage therefore carries roughly $80,000 for other structures. A cottage that cost $400,000 to build is under-insured by a factor of five before you reach the rental-occupancy exclusion that may void it entirely. Those are two separate problems with the same building.
What you want instead is a written for a rented dwelling. While it is being built you need , because a homeowner policy does not cover a structure under construction or materials on site. Check your liability limits too: a tenant and the tenant's guests (and dog) are new exposures on your land. Picking the forms is a job for a licensed Washington agent, not a web page.
Non-disclosure puts the whole policy at issue
If you rent the cottage without telling your carrier, you are holding a policy issued on materially inaccurate information about how the property is used. The realistic downside goes past a denied claim on the cottage, to a dispute over the whole policy, house included.
We are not equally sure of everything on this page. The coverage gap is verified; it is quoted above from the state regulator's own guide. What a carrier can do about non-disclosure under Washington law (deny, rescind, or something narrower) depends on the policy wording and on statute we have not verified. A five minute call to your carrier avoids the question. Make that call before the tenant moves in.
If there is already an unpermitted unit
Plenty of King County houses were bought with a basement or garage unit already in them. Check its permit status before renting it. An unpermitted unit creates problems in several areas.
- Enforcement. SDCI describes illegal units as living spaces, usually with separate kitchens and bathrooms, installed without a permit, and publishes fines of $150 to $500 per day plus inspection costs, with additional fines in Neighborhood Residential zones. If a permit cannot be obtained you may be ordered to stop renting, and tenants forced to relocate may qualify for relocation assistance paid by you.
- Property tax still applies. State guidance is that new construction value goes on the rolls where a building permit was issued or should have been issued. The unit is taxed whether or not it is legal.
- Registration is still required. A rented unit still owes RRIO registration, and RRIO penalties compound daily. Registering leads to an inspection, and not registering lets the penalties keep accruing.
Legalising is often possible, and both SDCI and King County publish routes for establishing an existing unit. In King County it triggers the whole rural analysis at once (septic, well, fire access, critical areas and impervious surface), and it can't be done in stages. It is still usually better than leaving the unit unpermitted. The electrical side is work we do every day.
Covenants and short-term letting
Covenants. State law's protection against covenants that prohibit accessory dwelling units reaches only associations created after 23 July 2023, and only inside an urban growth area. That excludes nearly every existing Seattle-area association. Headlines saying Washington has banned HOAs from blocking ADUs don't apply to established neighborhoods. Any neighbor in the association can enforce a recorded covenant forbidding more than one dwelling. A building permit is not a defense to that. The city does not enforce private covenants, and it will issue a permit for a building your covenants forbid.
Short-term letting. If the plan was to run the cottage on a nightly basis rather than a tenancy, that is a separate licensing regime with its own limits, and state law expressly preserves a jurisdiction's power to restrict short-term rental use of accessory dwelling units. Check it before you build around that model. The two business cases are not interchangeable.
Where to start
The advice across all eight parts is the same: do the inexpensive checks first. A title report, an arborist, a critical areas question, a septic inspection, a phone call to your insurer, and a properly run . Together those cost a small fraction of a set of drawings, and any one of them can tell you to stop before the expensive part starts.
The load calculation is the part we handle. If you are at the sketch stage, that is the right time to call: the difference between routing a feeder while the ground is open and routing one afterwards is measured in thousands.
Sources
- SDCI, Rental Registration and Inspection Ordinance, SMC 22.214, and the RRIO program fee schedule effective January 2026.
- SDCI, housing cost increases, and Seattle Municipal Code chapter 7.24.
- Seattle Office for Civil Rights, tenant screening rules, on first-in-time; Seattle Ordinance 127211 carrying forward SMC 22.205.010 on accessory unit tenancies.
- Washington State Office of the Insurance Commissioner, homeowner insurance guide, March 2025.
- SDCI, illegal living units; Washington Department of Revenue guidance on building permits and new construction value.
- RCW 64.38.160 and RCW 64.90.350 on covenants; RCW 36.70A.680(5)(a) preserving short-term rental restrictions.
- Chapter 59.18 RCW, the Residential Landlord-Tenant Act.
This is a survey of rules that apply to a rented accessory dwelling unit, not legal advice, and we are electricians rather than attorneys. Seattle and King County tenant rules differ from each other and both change. Fee and penalty figures are the 2026 published schedules and reset annually. Insurance form selection and the consequences of non-disclosure are matters for a licensed Washington agent and, if it comes to it, a lawyer. Read in August 2026.