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Multnomah County, OR Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified August 2026

ADU rules in Multnomah County, OR, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Governing zone: RR (Rural Residential) base zone
Max ADU size: 800 sq ft or 75% of house
Location limit: Must be inside urban growth boundary
Detached setback: 7 to 20 feet from house
Short-term rental: Prohibited; recorded covenant required

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Oregon under recent ADU streamlining laws. The Multnomah County, OR rules below cover local size, height, and setback requirements.

Summary

In the Rural Residential zone, unincorporated Multnomah County allows one accessory dwelling unit per single-family lot, capped at 800 square feet or 75% of the main house's size, but only on land sitting entirely inside the urban growth boundary.

City-specific rules exist: Gresham, and Portland have their own adu rules rules that differ from Multnomah County's county-level regulations. If you live in one of those cities, check the city-specific page instead.

(L) Accessory Dwelling Unit (ADU), subject to the following standards: (1) The ADU is sited entirely inside the urban growth boundary. ... (4) The floor area of the ADU shall not exceed either 800 square feet, or 75% of the floor area of the single-family dwelling to which the ADU is accessory, whichever is less.

View official code

Full Breakdown

Multnomah County Code Section 39.4360(L) lists an ADU as an allowed use in the Rural Residential (RR) base zone, but only if the lot is entirely inside the urban growth boundary -- rural RR land outside the UGB does not qualify. The ADU's floor area cannot exceed 800 square feet, or 75% of the primary dwelling's floor area, whichever is smaller. It must be built inside, or attached to, the existing house or a qualifying accessory building, or sited detached 7 to 20 feet from the house, with no variances allowed on that spacing. Short-term rental of the ADU is banned and the property owner must record a covenant against it. This governs only unincorporated county land -- Portland, Gresham, Troutdale, Fairview and Wood Village each apply their own city code to ADUs, not this section.

Violations & Fines

Building an ADU without county land-use approval, exceeding the 800-square-foot/75%-of-house cap, violating the 7-to-20-foot detached setback, or renting the unit short-term violates Section 39.4360(L) and is enforced through Multnomah County code compliance action.

Frequently Asked Questions

Can I build an ADU anywhere in unincorporated Multnomah County?

Only on RR-zoned land sitting entirely inside the urban growth boundary. Land outside the UGB, and land within Portland, Gresham, Troutdale, Fairview or Wood Village, follows different rules, not this section.

How big can a Multnomah County ADU be?

No more than 800 square feet, or 75% of the primary dwelling's floor area, whichever is smaller, under Multnomah County Code Section 39.4360(L)(4).

Can I rent my ADU out short-term?

No. Section 39.4360(L)(8)-(9) bans short-term rental of the ADU and requires the owner to record a covenant against it, enforceable until the property is annexed into a city.

Sources & Official References

[1]Multnomah County Code, Chapter 39 Zoning Code

Other rules in Multnomah County

Noise Ordinances: Construction HoursFew Restrictions Shed & Carport RulesSome Restrictions ADU Rental RestrictionsSignificant Restrictions Tiny HomesSignificant Restrictions Animal Ordinances: Mandatory Spay/NeuterSignificant Restrictions Fire Regulations: Outdoor BurningSome Restrictions Landscaping Rules: Tree Removal & Heritage TreesSome Restrictions Short-Term Rentals: Primary-Residence-Only RuleSome Restrictions Water Use Rules: Recycled Water RulesSome Restrictions

All Multnomah County rules

How Multnomah County compares: Easiest Cities to Build an ADU·Compare Multnomah County to another location·View the Oregon accessory structures overview

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ADU Rules in Cities Across Multnomah County

Portland, OR Gresham, OR