Can I build a backyard home in SeaTac?
Since 2023, Washington law lets most city lots hold up to two more homes. SeaTac writes its own rules on top. Here’s what the code says, in plain English — and a free report on what may fit on your lot.
What SeaTac allows.
The headline standards. Tap any one for the full wording and the code section.
ADUs per lot2, attached or detached, in 1 or 2 structuresFull wording
2 — 2 attached, 1 attached + 1 detached, or 2 detached in 1 or 2 structures (SMC 15.465.100(C)(1)(a)). ADUs count toward the maximum units per lot (SMC 15.400.100).
Max size1,000 sq ft maximum, including bathrooms and closetsFull wording
1,000 sq ft maximum, including bathrooms and closets; 220 sq ft minimum, excluding them (SMC 15.465.100(E)). An attached garage does not count toward the ADU's size. Converting the basement of a principal unit at least 5 years old is exempt from the cap.
Max height30 ft to the highest pointFull wording
30 ft, measured to the highest point of the structure containing the ADU (SMC 15.465.100(F)).
Front setback15 ft in RL (same as the house)Full wording
Same as principal: 15 ft in RL; 20 ft to the garage of a new detached dwelling (SMC 15.465.100(D); 15.400.100).
Side setback5 ft in RL (same as the house)Full wording
Same as principal: 5 ft in RL (SMC 15.465.100(D); 15.400.100).
Rear setback15 ft in RL (same as the house); 0 ft at an alleyFull wording
Same as principal: 15 ft in RL. A detached ADU may sit on a rear lot line adjacent to an alley (SMC 15.465.100(D); 15.400.100).
Lot coverage50% maximum building lot coverage in RLFull wording
50% maximum building lot coverage in RL (SMC 15.400.100).
Parking1 space per ADUFull wording
1 space per ADU, plus a 2nd for an ADU over 600 sq ft, within 1 mile of Sea-Tac Airport. 0 spaces within 1/2 mile of a major transit stop and more than 1 mile from the airport. Elsewhere 1 space, plus a 2nd for an ADU over 600 sq ft on lots over 6,000 sq ft. The Director may waive (SMC 15.465.100(H)).
Where a city figure reads stricter than the state floor (RCW 36.70A.681), the floor may control — the report flags it for your parcel.
Living there, selling it, plans, time and fees.
Each answer is the code’s own wording, with the section it comes from. “Not published yet” means the city hasn’t posted a figure — the report asks the city, not the internet.
Are backyard homes allowed?
Yes — on all lots in zoning districts that allow detached dwelling units, within the urban growth area (SMC 15.465.100(C)(1)(a)).
Minimum lot size
None for an ADU; zone minimum applies to new lots (SMC 15.400.100)
Do you have to live there?
No — the current SMC 15.465.100 (Ord. 25-1008) has no owner-occupancy requirement.
Can it be sold on its own?
Yes — attached and detached ADUs may be separated in ownership through a unit lot subdivision or conveyed as a condominium unit (SMC 15.465.100(C)(1)(b)).
Pre-approved plans
None found
How long the city takes
Not published
Impact fees
Not found; RCW 36.70A.681 caps ADU impact fees at 50% of the principal unit's
Plans SeaTac has already approved.
A pre-approved plan skips the design review: SeaTac has already checked the drawings, so the permit review is about your lot.
Loading the plans…
Pictures are the official pages’ own. A pre-approved plan is not a permit: the city still reviews your site.
The code is half the answer. Your parcel is the other half.
Lines and easements
Where the lot lines really are, which setbacks apply on each side, and whether a utility or access easement takes the corner you had in mind.
Critical areas and slope
Steep slopes, wetlands and streams come with buffers that no citywide number shows. The report reads the county’s maps for your parcel.
Sewer, septic, water, trees
Whether sewer reaches the lot or it’s on septic, where the water service is, and which trees a city protects — the things that decide where a unit can actually go.
What SeaTac allows, what could fit on your lot, and a planning cost range. If something won’t fit, the report says so — and shows what might.
Where this page comes from.
Read from the SeaTac municipal code and the city’s own pages on November 5, 2026. The same table feeds the HousUp report, so the report and this page never disagree. Cities amend their codes; before you design anything, confirm the current wording with the city.
2 sources, read November 5, 2026
Find out what your property could become.
Type your address. Within a few hours: what SeaTac allows, what could fit, and a planning cost range. Free, no obligation.