Accessory Dwelling Units

What is an ADU?

An Accessory Dwelling Unit (ADU) is one or more rooms with a private bath and kitchen facilities comprising an independent, self-contained dwelling unit within or attached to a single-family dwelling or in a detached building on the same lot as the primary dwelling unit. An ADU is distinguishable from a duplex in that, unlike a duplex, it is clearly subordinate to the primary dwelling unit, both in use and appearance.
An ADU is legal only with an established building permit.

For a complete listing of zoning code requirements for accessory dwelling units, please refer to VMC 20.810 Accessory Dwelling Units.

This project may be eligible for tax relief for up to three years.

Note: You must file your application with the Assessor’s Office before construction is completed.

What applications are needed when I submit for an ADU?

New Detached Structure

Addition or Remodel of Existing Structure

Where can I build an ADU?

ADUs are allowed as limited uses in all residential zones (R-2, R-4, R-6, R-9, R-18, R-22, R-30 and R-35) if in compliance with all of the development standards contained in VMC 20.810.030.

ADU’s are not allowed within non-residential districts or on properties not containing a detached single family dwelling or on properties containing activities requiring a home occupation permit per VMC 20.860.

ADUs are permitted on lots smaller than 4,500 square feet provided they are legally established parcels.

How many ADUs can I build on a single lot?

Only one ADU may be created in conjunction with each single family residence.

Are owners required to live on the property?

No, the property owner is not required to live on the property where an ADU is located.

Is a covenant required?

A covenant is no longer required due to recent changes in state law.

What are the size limitations for an ADU?

Within an existing basement, an ADU can use the entire basement square footage even if it exceeds 800 square feet. Detached and attached ADUs shall not exceed 800 square feet or up to 50% of the size of the main house, not including the garage (whichever is less).

ADUs can be less than 300 square feet in size so long as they comply with state building code minimum dwelling standards. The maximum height of an ADU is limited to 25 feet or less.

Is on-site parking required for an ADU?

No additional on-site parking is required in conjunction with the establishment of an ADU. However, the conversion or construction of an ADU cannot eliminate the required parking for the main dwelling. The main dwelling is required a minimum of one parking space, which can be in a garage or an approved 9’x17’ pad attached to a driveway on the property.

Does an ADU need to be visually subordinate to the main dwelling?

An ADU addition to an existing house or the construction of a new ADU shall comprise no more than 50% of the total visible façade area of the primary structure and other outbuildings as seen from the front of the lot. If a corner lot, the front faces the lot line with the narrowest frontage.

ADUs that are separate or extend from the principal dwelling shall be architecturally compatible with the principal dwelling. Architecturally compatible means that the ADU should match the appearance of the principal dwelling, i.e. similar materials, roof pitch, height.

What are the required setback and lot coverage requirements for an ADU?

An ADU addition to an existing house or the construction of a new ADU shall not exceed the allowable lot coverage or encroach into required setbacks as prescribed in the underlying zone for the primary structure.

Can I convert my existing accessory structure into an ADU?

An existing garage structure or other outbuilding may be converted to an ADU provided that the structure complies with the setback standards for the primary structure as prescribed in the underlying zone, applicable building codes and all other standards in VMC 20.810.

The conversion of a garage cannot eliminate existing required legal parking for the main dwelling unless it is provided at an approved location elsewhere on the property (see above). VMC 20.945.040

Which energy forms are required?

Energy Forms are required for new dwellings, when adding new dwelling area, and when converting unheated space to heated space. When building a new home and an ADU at the same time, separate energy compliance forms are required for each dwelling.

How do I know if I have adequate electrical service?

Contact Clark Public Utilities at 360-992-8558 to request verification of transformer capacity.

Will existing overhead electrical lines need to be moved?

If you are enlarging your home (an addition) or building a new  detached ADU, contact Clark Public Utilities at 360-992-8558 to request required clearances of overhead lines.

Should I hire a professional to design my ADU?

It is not required, however due to the complexity of these projects, we recommend that you hire a professional to assist in the design process.

Additional questions?

For questions on setbacks or development standards, please email

For building code questions, please email