ADU & backyard cottage rules in Auburn, WA
Auburn folded its old residential districts into fewer in 2025. Ordinance 6959 (the Middle Housing Code Update) turned R5 and R7 into R-2 and R10 and R16 into R-3; R20 became R-4, which is not in our table yet. A parcel record may still show a retired code; the table uses the current one.
Auburn is one of the places where the code puts numbers on the ADU itself: an accessory dwelling unit is 300 to 1,000 sq ft (Auburn City Code 18.32.010). Parking is one off-street space per ADU unless the lot is within a half-mile walk of a major transit stop.
Building limits in Auburn are written as impervious surface, not building coverage: 25% in RC, 50% in R-1, 75% in R-2 and 80% in R-3. That is a looser measure for the house and a tighter one for the driveway, patio and cottage together, so a lot with a large paved area has less room than the figure suggests.
Each zone also carries the setbacks a detached ADU must keep, shown in the table.
The rules our engine reads for Auburn
| Zone | Coverage | Height | Smallest lot | Setbacks front / side / rear | Detached ADU setbacks | Also |
|---|---|---|---|---|---|---|
| RC · Residential Conservancy | 25% impervious | 35 ft | 174,000 sq ft | 35 ft / 20 ft / 35 ft | 35 ft / 20 ft / 35 ft | up to 4 homes per lot; ADU 300–1,000 sq ft |
| R-1 · Residential 1 DU/Acre | 50% impervious | 35 ft | 43,500 sq ft | 35 ft / 10 ft / 20 ft | 35 ft / 10 ft / 20 ft | up to 4 homes per lot; ADU 300–1,000 sq ft |
| R-2 · Residential Low | 75% impervious | 35 ft | 4,400 sq ft | 10 ft / 5 ft / 15 ft | 10 ft / 5 ft / 5 ft | up to 4 homes per lot; ADU 300–1,000 sq ft |
| R-3 · Residential Moderate | 80% impervious | 45 ft | 2,700 sq ft | 10 ft / 5 ft / 15 ft | 10 ft / 5 ft / 5 ft | up to 4 homes per lot; ADU 300–1,000 sq ft |
Rules checked 2026-10-25. Sources: Auburn City Code 18.07.030 as amended by Ord. 6959 (City of Auburn Middle Housing Code Update guide, Feb 2025); Auburn City Code 18.32.010 — accessory dwelling unit standards.
What this means for your lot
- Find your zone code on your parcel record or Auburn's zoning map, then read its row above: the setbacks and the impervious 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 Auburn?
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 Auburn depends on its setbacks and coverage, which are in the table on this page.
How big can an ADU be in Auburn?
Auburn sets an ADU at 300–1,000 sq ft. State law does not allow a cap below 1,000 sq ft.
Do I need parking for an ADU in Auburn?
1 off-street space per ADU unless within a 1/2-mile walk of a major transit stop (City of Auburn ADU information sheet, July 2025). 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 Auburn?
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.