Home Remodeling in
Grand Mound, WA

Thurston County Ordinance 16575, adopted on 16 December 2025, allows up to two ADUs per lot in the county's urban growth areas, Grand Mound included. That brings the county in line with HB 1337, the state ADU law, which also bars capping an ADU below 1,000 square feet.

It is the biggest difference between Grand Mound and its rural neighbours. Just outside the growth area, in places like Rochester, the limit is still one ADU per lot.

Because the rules only changed recently, confirm with the county which standards apply to your parcel before you commit to a design.

Thurston County reads Chehalis River flood stages from the gauge near Grand Mound. Minor flooding, over low-lying roads and pasture, begins around 139.53 feet. Moderate flooding, where individual homes are threatened, is about 142.53 feet, and major flooding, a widespread threat to communities and main roads, is about 144.03 feet.

If your lot is in a mapped flood hazard area, new floor area has to sit at least 2 feet above the base flood elevation, with an elevation certificate before construction and an as-built certificate from a surveyor afterwards.

Thurston County's Building Development Center is at 3000 Pacific Ave SE, Suite 100, in Olympia, on (360) 786-5490. It is open Monday to Friday from 8 a.m. to 4 p.m., and planning staff are available there from 10 a.m. to 2 p.m.