ADU rules in Olympia

Detached ADUs are permitted outright in all 14 residential zoning districts, from R1/5 through Urban Residential, and up to two ADUs are allowed per lot alongside the principal unit in any combination of detached, attached, or internal. Ordinance 7444, effective December 22, 2025, replaced Olympia's old one-ADU-per-lot rule to implement the state ADU mandate in RCW 36.70A.680 and .681. A detached ADU is capped at 24 feet in height (measured at mid-gable) where the house may reach 35 to 45 feet, may not encroach into a required side yard, and must sit at least 5 feet from the rear property line, or right on it where the lot abuts an alley.

The same section allows each ADU to be added to, created within, or detached from the principal unit, so basement and internal conversions, additions, and units attached to a garage, shop building, or even to another ADU all qualify. Where an ADU is attached to an accessory structure, the ADU may be up to 1,000 square feet and the accessory structure up to 800 square feet (1,200 square feet for a detached garage, or larger with a conditional use permit). Conversions of legally established structures built before December 22, 2025, or at least 10 years before the application, are allowed even where the existing structure violates current setback or lot coverage rules, so long as any addition does not increase the nonconformity.

Each ADU is capped at 1,000 square feet of gross floor area, defined for ADUs as interior habitable area including basements and attics but excluding a garage or accessory structure. Covered porches and patios do not count toward the cap but are limited to 120 square feet per ADU and may not be enclosed. Ordinance 7444 raised the cap from 850 square feet and the count from one ADU to two, effective December 22, 2025. Detached ADUs are limited to 24 feet in height in every residential district, and ADUs are subject to the underlying zone's lot coverage, impervious surface, and minimum tree density standards. The Director may allow a deviation to use the entirety of a single floor of a dwelling built two or more years before the application.

OMC 18.04.060(A) sets out the complete list of ADU requirements in nine numbered paragraphs and none of them requires the owner to live in the ADU or in the principal unit, at application or on an ongoing basis. State law forbids one anyway: RCW 36.70A.681(1)(b) bars a city from requiring the owner of a lot with an ADU to reside in or occupy the ADU or another housing unit on the same lot. Owner occupancy still matters for two collateral programs, though: an owner-occupied property with only one rental unit on site is exempt from the rental business license and the five-year rental inspection (but not from annual registration), and a short-term rental homestay requires the owner or a long-term tenant to live there as a primary residence.

Table 38.01 in the parking chapter lists Accessory Dwelling Unit with "None" required for motor vehicle spaces and none required for either long-term or short-term bicycle spaces. The city's ADU handout says the same thing in plain language: a parking space is allowed but not required. This goes further than RCW 36.70A.681(2), which only bars off-street parking mandates within a half mile walking distance of a major transit stop and caps them at one or two spaces elsewhere. One caveat: if the ADU is operated as a short-term rental, one additional space is required when more than two bedrooms are rented in a single dwelling unit, and one more when there are two vacation rentals on a parcel and one is a single-family home. Some neighborhoods also require a residential parking pass for on-street parking.

Table 4.01 shows Accessory Dwelling Units as "P", permitted outright, in every residential district, and OMC 18.04.060(A) states that ADUs are permitted in all residential districts subject to the listed standards. No discretionary land use review is triggered: OMC 18.70.020(B) requires land use approval only for a change of use, a new nonresidential or nonagricultural use, a nonresidential building, or a project containing seven or more dwelling units. An ADU is therefore a building permit reviewed by staff and applied for through the city's online permit portal, with design review (where it applies) folded into the building permit application. Olympia also offers six pre-approved detached ADU stock plans, from a 480 sq ft studio to a 1,000 sq ft three-bedroom rambler, to shorten review.

Nothing in OMC 18.04.060(A) restricts renting an ADU, and an ADU may be sold separately from the house once a condominium or unit lot subdivision is approved and completed. Long-term rentals fall under the Rental Housing Code: the landlord must register each unit with the city annually at $35 per unit, and generally must hold a business license and obtain a third-party inspection every five years, although a single rental unit on an owner-occupied property (the classic ADU case) is exempt from both the license and the inspection while still having to register. Short-term rentals are allowed in all residential districts: a homestay, where the owner or a long-term tenant lives on site as their primary residence, needs only a business license and no land use permit, while a vacation rental needs a Type I short-term rental permit renewed every two years plus a 24/7 local contact, and is capped at two vacation rental units per operator citywide and two per parcel containing a single-family home. No short-term rental is allowed on income-restricted units or on a property holding a multifamily tax exemption certificate under OMC 3.57.