Home Remodeling in
Winlock, WA

Winlock's ADU supplemental form, last updated in May 2023, caps an ADU at 800 square feet or half the main house, not counting the garage, whichever is less. A basement ADU can go over 800 square feet as long as it is not larger than the rest of the home.

Lewis County plans fully under the Growth Management Act, so HB 1337 applies here. Since 30 June 2026 it has barred capping an ADU below 1,000 square feet and required two ADUs per lot, and where the local form conflicts, state law wins. Confirm with the city how it is applying that before you design.

Every ADU needs its own locking entrance, separate from the main house door. A detached ADU needs 10 feet of separation from the house, or fire-rated construction on the nearest wall. An attached ADU needs a tested one-hour fire-rated assembly with a sound rating of STC 50 between the units, and smoke and carbon monoxide alarms are required in both.

An ADU made entirely inside the existing house can be exempt from the fire and sound separation if interconnected smoke alarms run through the whole building. Turning unheated space, like a garage or unfinished basement, into living space has to meet the energy code.

The form also flags power. The existing service may need upgrading at the transformer, so ask Lewis County PUD to confirm transformer capacity and overhead line clearances early.

Winlock asks for the property corners to be staked and the lines strung so setbacks can be verified. In the R-1 zone the setbacks are 20 feet at the front, 5 at the sides and 15 at the rear, with homes capped at 35 feet and accessory buildings at 20.

The sewer line has to be inspected, and sidewalks and driveways installed and approved, before a certificate of occupancy is issued. Roof and storm drains cannot be connected to the sewer.