Accessory Structures in Franklin County, WA (2026)
5 rules for unincorporated Franklin County, Washington.
Verified from official government sources
ADU Rules
Franklin County allows one attached accessory dwelling unit per single-family home, capped at 800 square feet and 40 percent of the main house's floor area, or one detached ADU on a 2-acre-plus lot sized between 720 and 1,600 square feet with a conditional use permit, under Sections 17.66.050 and 17.66.060.
Franklin County ADU Size and Permit Rules
Some RestrictionsFranklin County Code §§ 17.66.050, 17.66.060
One accessory dwelling is permitted per single-family dwelling within all single-family districts under the following conditions: ... D.An accessory dwelling shall have a kitchen and bathroom and shall not contain more than two bedrooms.E.Shall not exceed forty (40) percent of the principal dwelling's total floor area, and shall not exceed eight hundred (800) square feet. ... One detached acces...
ADU Owner Occupancy
Franklin County requires one of the two dwellings on a lot with an attached accessory apartment to be owner-occupied for at least eight months of the year. The rule, set in Section 17.66.050(G), keeps ADUs tied to owner-occupied properties rather than allowing both the main house and the apartment to be rented to separate tenants.
Franklin County ADU Owner-Occupancy Rule
Some RestrictionsFranklin County Code § 17.66.050(G)
One accessory dwelling is permitted per single-family dwelling within all single-family districts under the following conditions: ... G.One dwelling must be owner-occupied for eight months of the year.
ADU Rental Restrictions
Franklin County bars renting out rooms for lodging in a single-family home that already has an accessory dwelling unit, and separately bars renting a detached ADU or its rooms at all. Sections 17.66.050(N) and 17.66.060(L) close off using either ADU type as a second income-producing rental alongside room rentals in the main house.
Franklin County ADU Rental Restrictions
Significant RestrictionsFranklin County Code § 17.66.050(N), § 17.66.060(L)
N.The renting of rooms for lodging purposes is prohibited in single-family homes where there is an accessory dwelling unit. ... L.An accessory dwelling unit may not be rented, nor may the rooms therein be rented, nor may the accessory dwelling unit be used for business purposes.
Shed & Carport Rules
Franklin County limits tent or canvas-covered storage sheds to 480 square feet in urban growth area residential zones and the rural community 1 zone, and caps at two the number of no-permit accessory structures on restricted residential lots, each set back five feet from side and rear property lines under Section 17.66.040.
Franklin County Accessory Building and Shed Rules
Some RestrictionsFranklin County Code § 17.66.040(A)
A.General Standards.1.Yard setback requirements are identified in each zoning district. ... 6.Tent and/or canvas covered storage buildings/structures shall be no greater in size than four hundred eighty (480) square feet within urban growth area residential zoning districts and the rural community 1 zoning district. The size may be exceeded upon the approval of a conditional use/special use per...
Tiny Homes
In unincorporated Franklin County, leasing an individual parcel for a tiny house or tiny home with wheels (as defined in RCW 35.21.686) is only allowed through the binding site plan process under Franklin County Code § 16.36.020(B), the same land-division tool used for mobile home parks, and the site plan must comply with every applicable mobile home park and zoning regulation.
Franklin County Tiny Home Leasing Needs Binding Site Plan
Some RestrictionsFranklin County Code §§ 16.36.020(B), 16.36.040(F), 16.36.080(A)
The division of property by the binding site plan process may only be used for the following:A.Division of land for the sale or lease of commercial or industrially zoned property as provided in the Franklin County zoning regulations;B.The division of land for the purposes of leasing space for mobile homes, tiny houses or tiny homes with wheels as defined in RCW 35.21.686, or travel trailers so ...
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