Guide · Auburn · King County · Updated July 2026
How many units can you build in Auburn?
If you own or are evaluating a residential lot in Auburn, 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 Auburn lots is now more than one. Auburn allows 4 units per lot by right in R-1/R-2/R-3/R-F, and up to 6 with a transit or affordability bonus — with R-3 reaching as high as 20 via a lot-area ladder.
Auburn at a glance: 4 units per lot by right, up to 6 with the city's bonus, plus up to 2 ADUs under HB 1337. Source: ACC 18.07.030 & 18.02.067 (Ord. 6959).
What Auburn's adopted code allows
Auburn allows 4 units per lot by right in R-1/R-2/R-3/R-F, and up to 6 with a transit or affordability bonus — with R-3 reaching as high as 20 via a lot-area ladder. The RC zone is different — density-based at about 1 unit per 4 acres (174,000 sf minimum lot).
The details: a transit or affordability bonus lifts the base to 6; a per-lot-area ladder can add more (R-3 reaches as high as 20). These are Auburn's city-wide standards, per ACC 18.07.030 & 18.02.067 (Ord. 6959); 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 Auburn 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 Auburn address so you find this out before you make an offer, not after.
Get the exact number for your Auburn parcel
The figures above are Auburn'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 Auburn feasibility report →
Frequently asked
How many units can I build on a residential lot in Auburn?
Auburn allows 4 units per lot by right in R-1/R-2/R-3/R-F, and up to 6 with a transit or affordability bonus — with R-3 reaching as high as 20 via a lot-area ladder. That is per ACC 18.07.030 & 18.02.067 (Ord. 6959). 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 Auburn?
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 Auburn's specific rule.
What unlocks the maximum in Auburn?
A transit or affordability bonus lifts the base to 6; a per-lot-area ladder can add more (R-3 reaches as high as 20).