Home Remodeling in
Rochester, WA

In rural Thurston County, one ADU is allowed per lot. An attached ADU can be no more than half the size of the main house. A detached ADU can be up to 1,000 square feet, or 1,200 if a transferable development right is used, and must sit within 100 feet of the main house and share its driveway.

Ordinance 16575, adopted on 16 December 2025, is what allowed detached ADUs in the rural area without needing extra land. The same update ended the county's old Family Member Units, so a second home for a relative is now built as an ADU.

HB 1337's two-ADU rule applies only inside urban growth areas, so it does not apply in rural Rochester.

A septic system is sized by the number of bedrooms it serves. If a remodel, addition or ADU changes that count, the building permit gets an Environmental Health review, and the existing system may need to be enlarged or repaired.

The county may ask for the system to have been inspected, and pumped if needed, within the three years before you apply. Thurston County does not let homeowners design or install their own septic systems, so a certified designer or installer does that part.

In a mapped special flood hazard area, Thurston County requires the lowest floor, basement included, and all flood-susceptible materials to sit at least 2 feet above the base flood elevation. The foundation needs flow-through vents so flood water can pass and drain.

You need an elevation certificate confirming that height before construction, and an as-built certificate from a professional surveyor after. In high groundwater areas, construction must also be set back 50 feet from the area that floods with open water.