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Walla Walla County, WA Accessory Structures: ADU Rules (2026)

Some Restrictions

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

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

Key Facts

Minimum primary home size: 800 square feet before an ADU is allowed
ADU size cap: 1,100 sq ft or 50% of primary home
ADUs per lot: One accessory dwelling unit per lot
Mobile homes/RVs: Not permitted as accessory dwelling units
Shared features required: At least 4 of 6: road, septic, water, meters, yard, parking
Enforcing agency: Walla Walla County Community Development Department

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 Washington under recent ADU streamlining laws. The Walla Walla County, WA rules below cover local size, height, and setback requirements.

Summary

Unincorporated Walla Walla County allows one accessory dwelling unit per lot under WWCC 17.08.015: the primary home must be at least 800 square feet, the ADU is capped at 1,100 square feet or half the primary home's size, mobile homes and RVs cannot serve as ADUs, and the ADU must share at least four utilities or features with the main house.

These county ordinances apply to unincorporated areas of Walla Walla County. Incorporated cities within the county may have their own rules that take precedence over county-level regulations.

C.The size of the primary residence must be at least eight hundred square feet to allow an accessory dwelling unit;D.The accessory dwelling unit, excluding any garage area and other nonliving areas, shall not exceed one thousand one hundred square feet, or fifty percent of the total square footage of the primary residence (excluding any garage area and other nonliving space), whichever is less;E.There shall be only one accessory dwelling unit per lot;F.The accessory dwelling unit and the primary residence must meet all lot coverage and setback requirements of the applicable zone as defined herein the Walla Walla County zoning code;G.Accessory dwelling units shall meet all Walla Walla County health department requirements for water and septic/sewer requirements;H.Mobile homes and recreational vehicles shall not be permitted as accessory dwelling units;I.Accessory dwelling units must be dependent upon the primary residence and must share at least four of the following criteria with the primary residence:i.Road access;ii.Septic system;iii.Water system;iv.Utility meters;v.Yard; andvi.Parking area.

View official code

Full Breakdown

Chapter 17.08 of the Walla Walla County Code defines and conditions accessory dwelling units for the unincorporated county, covering the wine-country farmland, Burbank, Touchet, Dixie, and every other unincorporated area outside Walla Walla, College Place, Waitsburg, and Prescott. Before an ADU can go in, the primary residence on the lot must already measure at least 800 square feet under condition C. The ADU itself is capped by condition D at 1,100 square feet, or 50 percent of the primary residence's square footage excluding garage and nonliving space, whichever number is smaller, and condition E limits a lot to one ADU.

Condition F ties the ADU and the primary residence to the lot coverage and setback rules of whatever zone the property sits in, meaning the size caps in this section work alongside the density and dimensions table in Chapter 17.18. Condition G requires the ADU to meet Walla Walla County health department water and septic or sewer requirements before it can be occupied, and condition H bars mobile homes and recreational vehicles outright as ADUs regardless of size. Condition I requires the ADU to remain dependent on the primary residence, sharing at least four of six listed features: road access, septic system, water system, utility meters, yard, or parking area. Community Development reviews all of these conditions together during building permit review for a new accessory dwelling unit in the unincorporated county.

Violations & Fines

An ADU built over the 1,100-square-foot/50-percent cap, added as a second unit on a lot that already has one, or installed as a mobile home or RV is a WWCC Title 17 violation. Community Development issues a notice of violation and order under Chapter 14.13, and failing to correct it by the stated deadline brings a $250-per-day civil penalty plus possible revocation of the building permit; willful refusal is a gross misdemeanor punishable by up to 90 days in jail or a $1,000 fine.

Frequently Asked Questions

How big can an ADU be in unincorporated Walla Walla County?

WWCC 17.08.015(D) caps it at 1,100 square feet or 50 percent of the primary residence's square footage, excluding garage and other nonliving space, whichever is less. The primary residence must also already be at least 800 square feet under condition C before an ADU is allowed at all.

Can I put a second ADU or a mobile home on my lot?

No. Condition E allows only one accessory dwelling unit per lot, and condition H specifically bars mobile homes and recreational vehicles from counting as an ADU no matter how they're set up on the property. The ADU must instead be part of the primary residence or a separate accessory building meeting county building codes.

Does my ADU need its own septic system?

Not necessarily its own, but it must meet Walla Walla County health department requirements for water and septic or sewer service under condition G, and it must share at least four of six listed features, such as the septic or water system, with the primary residence under condition I.

Sources & Official References

[1]Walla Walla County Code (Municode)

Other rules in Walla Walla County

Animal Ordinances: Chickens & LivestockSome Restrictions Fence Regulations: Height LimitsSome Restrictions Shed & Carport RulesSome Restrictions ADU Owner OccupancySome Restrictions ADU Rental RestrictionsLight Restrictions ADU Impact FeesSome Restrictions Noise Ordinances: Decibel LimitsSome Restrictions Parking Rules: Abandoned VehiclesSome Restrictions Parking Rules: Loading ZonesSome Restrictions Fence Regulations: Material RestrictionsLight Restrictions

All Walla Walla County rules

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

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