ADU & backyard cottage rules in Mukilteo, WA
Mukilteo's residential districts are the RD zones, named for the lot size they were drawn around — RD-12.5, RD-9.6, RD-8.4, RD-7.5 and RD-7.2, with an (S) variant for some. Their standards come from the bulk regulations in Mukilteo Municipal Code 17.20, as updated by Ordinance 1516 in 2025.
Coverage and height differ between the plain and the (S) zones, so look up your exact zone code, not just the lot size. Every RD zone allows two homes per lot under the state's middle-housing rules.
Parking for an ADU is tied to the lot: one off-street space on a lot under 6,000 sq ft, two on a larger lot, and none within a half-mile walk of a major transit stop. Converting an existing building is exempt, and an improved frontage earns a one-for-one on-street credit (MMC 17.56).
The detached ADU setbacks in the table are the ones our engine applies when it places a backyard cottage; a survey and the city decide the final lines.
The rules our engine reads for Mukilteo
| Zone | Coverage | Height | Smallest lot | Setbacks front / side / rear | Detached ADU setbacks | Also |
|---|---|---|---|---|---|---|
| RD-12.5 | 30% | 30 ft | 12,500 sq ft | 25 ft / 5 ft / 25 ft | 25 ft / 5 ft / 5 ft | up to 2 homes per lot |
| RD-12.5(S) | 35% | 35 ft | 12,500 sq ft | 20 ft / 5 ft / 5 ft | 20 ft / 5 ft / 5 ft | up to 2 homes per lot |
| RD-9.6 | 35% | 30 ft | 9,600 sq ft | 25 ft / 5 ft / 20 ft | 25 ft / 5 ft / 5 ft | up to 2 homes per lot |
| RD-9.6(S) | 35% | 35 ft | 9,600 sq ft | 20 ft / 5 ft / 5 ft | 20 ft / 5 ft / 5 ft | up to 2 homes per lot |
| RD-8.4 | 35% | 35 ft | 8,400 sq ft | 20 ft / 5 ft / 5 ft | 20 ft / 5 ft / 5 ft | up to 2 homes per lot |
| RD-7.5 | 35% | 30 ft | 7,500 sq ft | 20 ft / 5 ft / 20 ft | 20 ft / 5 ft / 5 ft | up to 2 homes per lot |
| RD-7.2 | 35% | 35 ft | 7,200 sq ft | 20 ft / 5 ft / 5 ft | 20 ft / 5 ft / 5 ft | up to 2 homes per lot |
Rules checked 2026-10-25. Sources: Mukilteo Municipal Code Table 17.20, bulk regulations (Ord. 1516, 2025).
What this means for your lot
- Find your zone code on your parcel record or Mukilteo's zoning map, then read its row above: the setbacks and the lot coverage limit decide how much of the lot a second home can use.
- Whether a cottage fits is a question of room, not permission: setbacks, what is already built, trees and slope leave a buildable area. The 3D check places a home inside it and tells you when it breaks a rule.
- Washington's HB 1337 sets the floor in urban areas — two ADUs per lot, no owner-occupancy rule, and a size cap no lower than 1,000 sq ft — so the local table above is what is left to check.
Common questions
Can I build a detached ADU (backyard cottage) in Mukilteo?
Washington's HB 1337 requires cities and counties that plan under the Growth Management Act to allow two ADUs per lot in urban growth areas, in any mix of attached and detached. Whether one fits on your lot in Mukilteo depends on its setbacks and coverage, which are in the table on this page.
How big can an ADU be in Mukilteo?
State law does not allow a cap below 1,000 sq ft of floor area. The local cap is not in our tables yet — check the code.
Do I need parking for an ADU in Mukilteo?
1 off-street space per ADU on lots under 6,000 sqft, 2 on lots of 6,000 sqft or more, none within a 1/2-mile walk of a major transit stop; conversions of existing buildings are exempt and an improved frontage earns a 1:1 on-street credit (MMC 17.56). State law allows no more than one space per unit on lots under 6,000 sq ft and two on larger lots, and none within a half-mile walk of a major transit stop.
Do I have to live on the property to rent out an ADU in Mukilteo?
No. HB 1337 bars cities and counties from requiring the owner to live on the property or in the ADU.
Check your address in 3D · Washington's ADU law, HB 1337
General information from public sources, not legal advice. Codes change and cities differ — confirm with your permit office before you design or build.