ADU (accessory dwelling unit) Rules and Regulations in Columbia County, OR (2026)

In Columbia County, the ADU rules for your property depend on whether it’s inside a city or on unincorporated rural land. Find out who reviews your parcel and which standards apply before you pick a model.

If the property is in an incorporated city, the city’s land-use code controls the local side. If it’s rural residential land outside city limits, start with Columbia County’s rural ADU framework.

Where ADUs are allowed

Columbia County’s ADU page describes a path for rural residential zoned properties in unincorporated parts of the county, adopted under Ordinance No. 2023-1.

Cities in Columbia County apply their own standards. Check the current city code for Columbia City, Clatskanie, Rainier, Scappoose, St. Helens, or Vernonia instead of relying on an older county packet’s city comparison chart. County-administered land inside an urban growth boundary also has a separate path under CCZO 224.1; the rural rules below do not automatically apply there.

For local service information, see St. Helens and Scappoose.

Lot size and floor-area limits

For the rural residential path, the county page lists these thresholds:

• The lot or parcel must be at least 2 acres (a minimum lot size of two acres).
• The ADU is limited to a maximum of 900 square feet of useable floor area.
• One ADU may be approved per lot or parcel under the rural residential standards.
• The ADU must be located no farther than 100 feet from the primary dwelling.

A parcel can meet all four and still need more work on access, utilities, and site conditions.

Setbacks and height

Published county guidance lists the 100-foot rural siting requirement but does not give one setback or height limit for every property. Confirm the dimensional standards for your zone and parcel directly with the county.

Water, septic, and sanitation

The county framework ties approval to infrastructure being addressed. The page lists, among the development factors:

• An approved water supply.
• Wastewater and sanitation requirements.
• Road access to the property.

Applicants are expected to provide supporting documentation such as water source, utility and sewage disposal, and road access information. On rural land, septic capacity and an approved water source often decide whether an ADU is feasible, so check them early.

Fire protection and wildfire standards

The property must be served by a fire protection district, and rural ADUs may also be subject to wildfire-related standards. Confirm the specific requirements for your address.

Owner-occupancy, rentals, and vacation use

The rural ADU path prohibits vacation occupancy. Confirm the proposed residential use with Planning before relying on rental income. For urban properties, use current city or county standards alongside Oregon’s urban ADU protections; an older owner-occupancy comparison chart is not a current rule.

Parking

The county page doesn’t list a numeric parking standard for rural ADUs, but parking is one of the items that must be shown on the required detailed site plan (alongside distances to property lines, septic locations, entrances, and driveway length). Confirm any specific parking requirement with the jurisdiction that reviews your parcel.

Partition restriction

The county page notes that a rural ADU approval cannot be used to justify a partition that would separate the ADU from the primary dwelling. The ADU stays on the same parcel as the existing home.

Permits and next steps

Columbia County advises applicants to contact the Planner On Duty before submitting an application, and to submit a detailed site plan showing items such as distances to property lines, septic locations, parking, entrances, and driveway length. Confirm jurisdiction and check the parcel against the thresholds above before committing to a design.

Wolf builds modular homes that are delivered into Oregon. Placement, foundation, utility, and inspection approvals for your property still go through Columbia County or the city, such as Scappoose or St. Helens.

For the statewide background, read our Oregon ADU rules guide, and to compare unit sizes against the 900-square-foot limit, see the Wolf model lineup.

Get a free Wolf property evaluation

Send us the Columbia County address and intended use. We will start with jurisdiction, zoning, water, septic, fire service, and model fit. Request your free property evaluation.

This is general information, not legal advice. Confirm current rules, fees, and application requirements with Columbia County or the city that has jurisdiction over your property.