Accessory Dwelling Units (ADU’s)

An Accessory Dwelling Unit is a smaller, secondary dwelling unit on the same lot or within a house, attached house or manufactured home.  The unit includes its own independent living facilities with provisions for sleeping, cooking, and sanitation, designed for residential occupancy independent of the primary dwelling unit. The unit may have a separate exterior entrance or an entrance to an internal common area accessible to the outside.

ADU’s are great for:

–  Downsizing to a smaller living space
–  Creating a rentable unit on your property (or live in the ADU and rent your main house!)
–  Creating a guest house for short or long term visitors
–  Creating an office or studio space for your business or craft
–  Creating a home on site but separate from the main house for aging family or for retirement

(See ADUs and other projects by Mitchell Snyder Architecture at the Projects page)


The total living area of an ADU may be no more than 75% of the living area of the house up to a maximum of 800 sq. ft.


The maximum height for a detached ADU is 20 feet.  Roof height varies for different roof types.  For example, the most common pitched or hip roof is measured to the average height of the roof where as a shed roof is measured to the highest point:


The combined building coverage for all detached accessory structures may not exceed 15% of the total site area.  Detached ADUs must be set back 40 feet from the front lot line; or located behind the rear wall of the house.


No additional parking is required for the accessory dwelling unit. Existing required parking for the house, attached house, or manufactured home must be maintained or replaced on-site.


ADUs 15 feet or under in height:
No requirement to match the existing house

ADUs over 15 feet in height:
– Exterior finish materials:  The exterior finish materials must be the same or visually match in type, size, and placement, the exterior finish material of the primary structure; or Siding must be made from wood, composite boards, vinyl or aluminum products, and the siding must be composed in a shingle pattern, or in a horizontal clapboard or shiplap pattern. The boards in the pattern must be 6 inches or less in width.
– Roof Pitch:  The roof pitch of the detached covered accessory structure must be the same as the predominant roof pitch of the primary structure; or the roof pitch must be at least 6/12.
– Trim:  The trim on the detached covered accessory structure must be the same in type, size, and location as the trim used on the primary structure; or the trim around all windows and doors must be at least 3 ½ inches wide.
– Windows:  The windows on all street facing facades of the detached covered accessory structure must match those on the street facing façade of the primary structure in orientation (horizontal or vertical); or must be square or vertical at least as tall as it is wide
– Eaves:  The eaves on the detached covered accessory structure must project from the building walls the same distance as the eaves on the primary structure or the eaves must project from the building walls at least 1 foot on all elevations.  If the primary structure has no eaves, no eaves are required.

(See ADUs and other projects by Mitchell Snyder Architecture at the Projects page)

Building an ADU in the setback

ADUs may be built new or by converting an existing accessory structure (such as a garage) in the side and rear setbacks if:

– The structure is no more than 15 feet high, and the walls of the structure are no more than 10 feet high (excluding the portion of the wall within a gable)
– The structure is at least 40 feet from a front lot line, and if on a corner lot, at least 20 feet from a side street lot line.
– The structure has dimensions that do not exceed 24 feet by 24 feet (excluding eaves)
– If more than one structure is within the setback, the combined length of all structures in the setback adjacent to each property line is no more than 24 feet.
– The portion of the structure within the setback must be screened from adjoining lots by a fence or landscaping, unless it is enclosed within the setback by a wall. Screening is not required for enclosed structures.
– Walls located within the setback do not have doors or windows facing the adjacent lot line.
– The structure does not have a rooftop deck.
– Dormers are set back at least 5 feet from the side and rear lot lines.

Adjustment Review

If you would like your design to deviate from any of these requirements an ‘adjustment’ can be applied for with the city of Portland for review.  An ‘adjustment’ is a Type II Land Use Review with a fee of $1922 and a typical timeline of 10-12 weeks.

SDC Fees Waived Through July 2018

Save $7,000 to $15,000 on SDC permit fees through July 31, 2018.  ADUs are typically subject to System Development Charges (SDCs) that are levied by the Portland Parks, Environmental Services, Transportation, and Water Bureaus. SDC fees will be waived for ADU projects where a complete building permit application has been submitted on or before July 31, 2018, provided that the ADU receives a certificate of occupancy no later than June 30, 2019.

(See ADUs and other projects by Mitchell Snyder Architecture at the Projects page)

ADU Resources (links)

City of Portland – Bureau of Development Services Accessory Dwelling Unit Main Page
City of Portland – ADU Guide
City of Portland – Zoning Code for Accessory Dwelling Units
City of Portland – Zoning Code for Detached Accessory Structures (See Section 33.110.250 on page 110-32)
Oregon DEQ’s Excellent Video “Accessory Dwelling Units – Taking the First Step”

Get started

Contact Mitchell Snyder Architecture to discuss your ADU project at the Contact page.  See ADUs and other projects by Mitchell Snyder Architecture at the Projects page.