An is a second, complete home on a lot that already has one. Attached to or inside the house, such as a basement or over-garage apartment, it is an . Standing on its own in the back yard, it is a , which is what people mean by a backyard cottage. Either way it has its own kitchen and bathroom. It also has its own entrance, and for the agencies involved, the entrance is what distinguishes it from a spare bedroom.
Much of what is written online about Seattle ADUs is out of date. Guidance published between 2019 and 2024 is more than dated: on several points it says the opposite of the current code. This guide is the picture as of August 2026. Where something is uncertain we say so, and every code section is named so you can check us.
How to read this guide
Words with a dotted underline have a definition attached. Hover over one, or tap it on a phone, and the meaning appears without leaving the page. Every one of them is also written out at greater length in the glossary, with the code section it comes from.
Every fee schedule quoted here resets on 1 January. Seattle's zoning code changed twice in eighteen months. Treat every number as the starting point for a phone call, not as a quote.
What changed, and when
Three changes landed close together, and they interact.
State law now sets minimums that cities must allow. House Bill 1337, passed in 2023 and now 36.70A.681, sets floors that cities inside an cannot go below: at least two ADUs per lot, a maximum size that cannot be set below 1,000 square feet, a height limit that cannot be set below 24 feet, no owner-occupancy requirement, no off-street parking within half a mile of a , and capped at half what the main house would pay. Where a city has not adopted conforming rules by its deadline, the statute says the state requirements supersede, preempt and invalidate the local ones on their own, without anyone having to sue.
Seattle rewrote its ADU code twice. 127211 took effect on 30 June 2025 and repealed 23.44.041 outright, moving the ADU rules into a new SMC 23.42.022. Then Ordinance 127376, the One Seattle Plan zoning compliance package, rewrote that section again with effect from 21 January 2026. If you are reading a page, a plan set or a builder's proposal that cites 23.44.041, it is describing a code section that no longer exists.
King County rewrote its own rules in 2024 through Ordinances 19824 and 19881, amending 21A.08.030.B.7. County rules differ sharply between the urban growth area and the rural area, and several PDFs still circulating describe the repealed pre-2024 county rules: one ADU per lot, owner occupancy, a required extra parking space. Those are gone.
First question: which set of rules applies to you
Many disagreements about ADU rules in this region come from people describing different jurisdictions. Before reading further, work out which one applies to your address.
| If your address is | Zoning and building | Electrical permit | Septic and plumbing |
|---|---|---|---|
| Inside Seattle city limits | SDCI, under SMC 23.42.022 | SDCI, under the | Public Health, Seattle & King County |
| Unincorporated King County, inside the urban growth area | King County Permitting, under KCC 21A.08.030.B.7.a.(7) | Washington L&I | Public Health, Seattle & King County |
| Unincorporated King County, rural area | King County Permitting, under KCC 21A.08.030.B.7.a.(8) | Washington L&I | Public Health, plus a well and septic review that often decides the project |
Seattle and the other cities are all inside King County. For zoning and building permits, a city's own code applies within its limits, and the county code applies only to unincorporated areas. Some county services cross city lines: the county assessor values every property, Public Health reviews septic systems countywide, and the King County capacity charge applies wherever sewage goes to a county treatment plant. If you are in one of the other King County cities (Maple Valley, Kent, Covington, Black Diamond, Renton, Enumclaw and the rest), your zoning comes from that city. None of the Seattle or unincorporated county detail here applies directly. The state law floors do apply if you are inside the urban growth area. The utility, charge and electrical material in the costs part, the electrical part and the metering part is written to cover the whole county.
Seven things to check early
These are the items most likely to change a project's scope or budget. Each links to the part of the guide that deals with it properly.
- Your cottage very likely counts toward your . This is new in 2026, and much of the older guidance still says otherwise. Seattle rules.
- An lot needs 3.75 acres, not five, before it can have a detached cottage. Thousands of legal rural lots fail that test. King County rules.
- One mature tree can take most of a back yard. A protected tree generates a circle of unbuildable ground with a radius of one foot for every inch of trunk. Trees and the site.
- A septic property needs a . On septic, the ADU application is what prompts the county to check whether the house has a reserve drainfield, and there is often nowhere left to put one. Septic, wells and fire.
- A billed after completion. About $46 a month for fifteen years, billed to the property owner on a separate King County bill. Fees and charges.
- Homeowners generally can't do their own wiring on a rental. The homeowner exemption does not cover a new building intended for rent, and the affidavit route that allows it is narrower than it first appears. The electrical work.
- Separate metering protects you on electricity and very little on water and sewer. The reason is structural, and the metering part walks through it. Metering and bills.
- Your homeowner policy probably does not cover a rented cottage. Coverage can be excluded on two separate grounds for the same building. Being a landlord.
Low-cost checks before design
Each item on this list is inexpensive, and any one of them can show that a project won't work before you have paid an architect.
- Pull a title report. Read your covenants and your easements. A recorded covenant forbidding more than one dwelling is enforceable by any neighbor in the association, and a permit is no defense.
- Ask SDCI or King County Permitting whether your parcel carries a designation. The designation affects many things, including your state law protections.
- Have an arborist map the before you buy plans, not after.
- If you are on septic, get the and an inspection first. It is the least expensive way to find out whether the project is viable.
- Call your insurance carrier.
- File the Home Improvement Exemption application before construction is complete. There is no retroactive relief.
- Have the electrical service capacity calculated properly, before the design is fixed and long before the trench is dug.
That last item is our work. A costs little compared with redesigning a project around a service that turns out to be too small. The difference between routing a feeder while the ground is open and routing one afterwards can run to thousands of dollars. If you are at the sketch stage, that is the right time to call.
The guide
Eight parts, in the order a project usually happens. Each stands on its own if you only need one.