ADU Feasibility Checklist

1. 1

   Question 1: Does your zoning allow an ADU, and which office decides?

   The answer depends on the jurisdiction that governs the parcel and the zone it sits in. A lot inside a city, a lot in an unincorporated urban growth area, and a rural lot can follow very different rules.

   How to check

   • Find the parcel on your county’s GIS map or assessor site. Note the parcel number (APN), the zone, and whether it is inside city limits or an urban growth area.
   • Confirm with the city or county planning office, using the parcel number. A property can have a city mailing address and still be governed by the county.
   • The state approval on a factory-built home (an L&I insignia in Washington, a prefabricated-structure insignia in Oregon) covers the home itself. Your city or county still approves zoning, setbacks, the foundation, utility connections, and occupancy (in Washington, RCW 43.22.460).

   Washington

   State ADU rules apply inside urban growth areas of cities and counties that plan under the Growth Management Act (RCW 36.70A.680). They take effect with each jurisdiction’s periodic plan update, and where local rules were not updated, the state rules override conflicting ones. Rural lots follow county rules.

   Oregon

   Cities over 2,500 people and counties over 15,000 must allow at least one ADU per detached house in areas zoned for detached homes inside the urban growth boundary (ORS 197A.425). Outside it, rural ADUs are a county option with extra conditions (ORS 215.495).

   Washington ADU laws →Oregon ADU rules →County-by-county guides →

   Your notes for question 1
2. 2

   Question 2: How big and how tall can the ADU be, and how many are allowed?

   Floor-area caps, height limits, lot coverage, and the number of ADUs per lot decide which homes are candidates before anything else is designed.

   How to check

   • Find the maximum ADU floor area. Some codes use a fixed number, others a share of the main house, and they differ on whether porches or garages count.
   • Note the height limit, the maximum lot coverage, and how many ADUs the lot may have.
   • Compare the limits with the Wolf model footprints at the end of this checklist.

   Washington

   In urban growth areas, cities and counties must allow at least two ADUs per lot, cannot cap ADU size below 1,000 square feet, and cannot set a height limit under 24 feet unless the house’s limit is lower (RCW 36.70A.681).

   Oregon

   Size caps and other siting and design rules are set locally and vary by city (ORS 197A.425). Where a county allows rural ADUs, they are limited to 900 square feet on lots of at least two acres (ORS 215.495).

   Your notes for question 2
3. 3

   Question 3: What conditions come with an ADU: owner occupancy, parking, and rentals?

   Occupancy, parking, and rental rules decide whether the plan works for a parent, a caregiver, or a tenant.

   How to check

   • Ask whether the owner must live on the property and how many off-street parking spaces are required.
   • If you may rent the ADU, check long-term and short-term rental rules separately. Many cities license or limit short-term rentals.
   • Ask whether the ADU needs its own address or utility meters.

   Washington

   In urban growth areas, owner occupancy cannot be required. Parking cannot be required within a half-mile walk of a major transit stop; elsewhere it is limited to one space per unit on lots up to 6,000 square feet and two on larger lots, with narrow exceptions (RCW 36.70A.681). Short-term rentals may be restricted (RCW 36.70A.680).

   Oregon

   Inside the urban growth boundary, cities and counties may not require owner occupancy or additional off-street parking for an ADU, but they may regulate vacation rentals (ORS 197A.425).

   Your notes for question 3
4. 4

   Question 4: Is the property in an HOA or subject to recorded covenants (CC&Rs)?

   Private covenants can restrict secondary homes, rentals, or exterior design. A building permit does not settle whether a covenant can be enforced.

   How to check

   • Get the recorded CC&Rs and the association’s current rules. Your title report or the county recorder lists recorded covenants.
   • Note when the association was created and when the covenants were recorded, and anything they say about accessory dwellings, rentals, or architectural review.
   • Have a disputed restriction reviewed by a real-estate attorney before you commit to a design.

   Washington

   Associations created after July 23, 2023, in urban growth areas cannot restrict ADUs more than the city or county could (RCW 64.38.160, RCW 64.90.350). Older associations are not covered by that limit.

   Oregon

   A covenant signed on or after January 1, 2021, that allows a house cannot be used to ban an ADU inside an urban growth boundary; from January 1, 2027, such bans are void there no matter when they were recorded. Covenants signed on or after January 1, 2026, also cannot ban prefabricated homes. Rural ADUs are not covered (ORS 93.277).

   HOAs and ADUs in Washington →

   Your notes for question 4

1. 10

   Question 10: Where will the water come from?

   Sharing the existing service, adding a meter, and drawing from a well each carry different approvals, fees, and trench work.

   How to check

   • Ask the water provider whether the ADU may share the existing meter, whether the service has capacity, and what a new meter or connection costs.
   • On a well, ask the county health department whether a second home may use it and how that is reviewed.

   Washington

   A well that serves more than one home can fall under the state’s small water system rules, depending on what the county health board has adopted (WAC 246-291).

   Your notes for question 10
2. 11

   Question 11: Where will the wastewater go: sewer or septic?

   An existing pipe or tank does not mean another home can connect to it. Sewer capacity and septic sizing are approved separately from the building permit.

   How to check

   • On sewer, confirm capacity, the permitted connection point, the route and slope for the new line, and the connection fees.
   • On septic, gather the approved system records and have the health authority or a septic designer review the added flow and bedrooms, the replacement or reserve area, and any upgrade.

   Washington

   A permit is required to connect to or expand a septic system, and added bedrooms count as an expansion that must meet current standards, including the reserve area (WAC 246-272A). Some cities and counties do not allow ADUs on lots without public sewer (RCW 36.70A.680).

   Oregon

   An ADU can share the house’s septic system only if it works properly, there is room for replacement areas for both homes, and the added flow stays within 300 gallons a day or half the system’s design capacity, whichever is smaller. Otherwise it needs its own approved system, which starts with a site evaluation (Oregon DEQ).

   Your notes for question 11
3. 12

   Question 12: Does the electrical service have room for another home?

   Wolf homes are all-electric, with a heat pump, an electric water heater, and an electric range. A panel or utility service upgrade can add real cost and lead time.

   How to check

   • Have an electrician compare the ADU’s load with the existing panel and service, and ask whether the ADU needs its own meter.
   • Ask the electric utility about service capacity, a new meter, and the route for the new line.
   • Appliances are separate from the model price, so include the ones you choose in the load review.

   Your notes for question 12

Footprints are the nominal box dimensions. Roof overhangs, porches, decks, ramps, and landings add to them, and codes differ on whether setbacks and lot coverage are measured to the wall or the eave. Use the dimensioned drawing for the exact outline.