Richland, WA Accessory Structures: ADU Rules (2026)

Richland Municipal Code Section 23.42.020 allows one accessory apartment unit per single-family dwelling in every single-family zone, administered by Development Services as a permitted use rather than a discretionary one. Only one accessory apartment is allowed per legal lot, it must be accessory to a detached single-family residence, and a lot already holding two or more dwellings cannot add one. The unit can be added onto an existing house, built adjacent to it, or constructed together with a new residence, and it must include its own kitchen and bathroom while staying at two bedrooms or fewer.

Size is capped two ways at once: the accessory apartment cannot exceed 40 percent of the primary dwelling's total floor area, and it must fall between 200 and 800 square feet of interior space regardless of the percentage figure. An extra off-street parking space is required beyond the minimum parking otherwise required under RMC Section 23.54.020, and an accessory apartment permit must be obtained and approved before any building permit for the alterations or new construction is issued. The unit has to tie into the same utilities as the main dwelling, apart from telephone and television, and cannot carry separate utility services.

Detached units must sit at least six feet from the primary dwelling, are limited to 15 feet in height and a single story, and must otherwise meet the same lot coverage, setback and building-line standards as the primary residence, though the rear setback can shrink to 15 feet if a solid privacy fence runs along the rear property line. Attached units must share the same roof as the main house, cannot connect only by a breezeway, and need a separate outside entrance not on the same facade as the main entrance.