Guide · SeaTac · King County · Updated July 2026
How many units can you build in SeaTac?
If you own or are evaluating a residential lot in SeaTac, the first question is almost always the same: how many homes can legally go on it? Since Washington's HB 1110 middle-housing law took effect, the answer for most SeaTac lots is now more than one. SeaTac allows up to 4 middle-housing units per lot near transit or with affordability, above a 2-unit base — with exact per-zone counts confirmed for your parcel in a report.
SeaTac at a glance: 2 units per lot by right, up to 4 with the city's bonus, plus up to 2 ADUs under HB 1337. Source: SMC 15.400.100/.300 (Ord. 25-1008).
What SeaTac's adopted code allows
SeaTac allows up to 4 middle-housing units per lot near transit or with affordability, above a 2-unit base — with exact per-zone counts confirmed for your parcel in a report.
The details: up to 4 near a major transit stop or with one affordable unit (the Tier 2 floor; exact per-zone counts are confirmed at the parcel level). These are SeaTac's city-wide standards, per SMC 15.400.100/.300 (Ord. 25-1008); the exact figure for a specific parcel — its zone, lot size, transit proximity, and critical areas — is what a Civexa report resolves.
What actually fits: floor area, height, and setbacks
HB 1110 and the city code set the right to a number of units; the zone's building envelope sets what physically fits. Floor-area ratio, lot coverage, height, and setbacks together decide whether those homes pencil as detached cottages, a townhouse row, or a stacked-flat building. Civexa computes this envelope for SeaTac zone by zone, so the unit count you see is one the zoning can actually hold.
What can shrink it: critical areas
Steep slopes, wetlands, streams, and flood zones can override the unit math on a specific parcel. A lot that qualifies for the maximum on paper may be constrained once environmentally critical areas are mapped. Civexa screens FEMA flood, slope, and county critical-area layers for every SeaTac address so you find this out before you make an offer, not after.
Get the exact number for your SeaTac parcel
The figures above are SeaTac's city-wide rules. The number that matters is the one for your address — its zone, lot size, transit proximity, and critical areas. Civexa turns that into a full feasibility report in about a minute: unit count, buildable envelope, a preliminary pro forma, utilities, and permit path.
Run a SeaTac feasibility report →
Frequently asked
How many units can I build on a residential lot in SeaTac?
SeaTac allows up to 4 middle-housing units per lot near transit or with affordability, above a 2-unit base — with exact per-zone counts confirmed for your parcel in a report. That is per SMC 15.400.100/.300 (Ord. 25-1008). The exact number for a specific parcel still depends on its zone, lot size, transit proximity, and critical areas — run a Civexa report for the parcel-level figure.
Can I add ADUs on top of that in SeaTac?
Washington's HB 1337 allows up to two accessory dwelling units on a residential lot. In some cities they count toward the middle-housing total and in others they are separate — your Civexa report applies SeaTac's specific rule.
What unlocks the maximum in SeaTac?
Up to 4 near a major transit stop or with one affordable unit (the Tier 2 floor; exact per-zone counts are confirmed at the parcel level).