2,580 of the 3,028 lots in Steilacoom (85%) are zoned for houses inside an urban growth area. On those lots, Washington's HB 1337 requires the city or county to allow at least two accessory dwelling units (ADUs). Another 277 lots are outside the urban growth area, where Pierce County's own ADU rules decide.
40 of those lots look to be on a septic system: an ADU needs the health department to approve the system's capacity.
148 have mapped landslide, erosion, flood or wetland areas that can limit where an ADU goes.
168 lots are zoned for uses other than houses, where the ADU rule does not apply.
The law does not lift setbacks or lot-coverage limits, and planned developments and HOAs can add their own rules: on a small lot a separate ADU may not fit, though converting space inside the house may. Each lot's report checks its own room.
Questions
Does Washington's two-ADU law apply in Steilacoom?
On 2,580 of Steilacoom's 3,028 lots, yes: they are zoned for houses inside an urban growth area, where HB 1337 requires at least two ADUs to be allowed. On 277 lots outside the urban growth area, Pierce County's own rules apply.
What can stop an ADU on a lot in Steilacoom?
Room on the lot: setbacks and lot coverage still apply, so a small or built-over lot may only fit an ADU inside the house. Then a septic system without spare capacity (40 lots here look to be on septic), mapped hazard or wetland areas (148 lots), and the rules of a planned development or HOA. Each lot's report checks its own room.
How do I check a specific lot?
Search its address on Colvos Parcel. The report shows the lot's zoning, whether it is inside the urban growth area, sewer or septic, and mapped hazards, each with its source.
From the Pierce County Assessor-Treasurer, the county's zoning, sewer, hazard and growth-area maps, and the Tacoma-Pierce County Health Department. Records as of October 2, 2026.