ADU GUIDE (Portland, OR)

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-term 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 Openhouse Architecture at the Projects page)

Types

  • Detached: Freestanding from primary structure

  • Connected: Connected to a primary structure by a roof, deck, or other structural connection

  • Attached: Attached to a primary structure by a common wall or shares a common floor ceiling

  • Internal / Converted: A portion of the existing primary residence or detached structure is converted to an ADU

Size

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 800sf, whichever is less.  When the primary dwelling unit is a duplex, the ADU may be no more than 75% of the living area of the larger of the two primary units up to a maximum of 800sf, whichever is less.  The addition of an ADU living area may not exceed the lot’s maximum allowable Floor Area Ratio (FAR).

Site

The combined building coverage for all detached accessory structures may not exceed 20% of the total site area.  The building coverage of each detached or connected covered accessory structure may not be greater than the building coverage of the smallest primary structure, or 900sf, whichever is greater.  Detached ADUs must be set back 40 feet from the front lot line; or located behind the rear wall of the house.

Zoning Eligibility & Quantity Allowed

  • RF, R20, R10: One ADU is typically allowed on a site with a house, attached house, or manufactured home.

  • R7, R5, R2.5: Two ADUs are typically allowed on a site with a house, attached house, or manufactured home if the lot meets a minimum lot area of R7 (5,000sf), R5 (4,500sf), and R2.5 (3,200sf).  One ADU is allowed on a site with a duplex in the R7, R5, and R2.5 zones when the lot meets the minimum size requirement.  When there are two ADUs or an ADU added to a duplex, one ADU must meet visitable unit standards (see below).

  • Multi-Dwelling, C, EX: Up to two ADUs are typically allowed on a site with a house, attached house, or manufactured home.  If there are two ADUs on the lot, only one may be attached or within the primary structure.  One detached ADU is allowed on a site with a duplex in the C and EX zones.  When there are two ADUs or an ADU added to a duplex, one ADU must meet visitable unit standards (see below).

Parking

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.

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

Height

The maximum height for a detached or connected ADU is 20 feet measured from the lowest grade elevation 5ft from the building.  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 whereas a shed roof is measured to the highest point:

Appearance (Detached & Connected ADUs)

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 structures must be the same in type, size, and location as the trim used on the primary structure or trim around all windows and doors must be at least 3½ 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.

Visitability

If two ADUs are added to a primary structure or an ADU is added to a Duplex one ADU must meet the following:

  • Visitable Entrance: One entrance must be accessible via a route with no stairs and the street lot line or on-site parking space

  • Visitable Bathroom: One bathroom with a sink and toilet must accommodate an unobstructed 60 inch diameter circle or unobstructed area comprised of two rectangles that are at least 36 inches by 60 inches oriented at right angles to each other.  The bathroom must be accessible from the visitable entrance on the same floor or via ramp, elevator, or lift.

  • Visitable Living Area: There must be a min. 200sf living area accessible from the visitable entrance on the same floor or via ramp, elevator, or lift.

  • Visitable Doors: All door openings between and including the visitable entrance, living area, and bathroom must be a min. of 34 inches wide.

  • Exemption: Visitable Unit standards are exempt where the lot has an average slope of 20% or greater, where there is more than 3ft rise between the highest elevation at street lot line and the lowest grade at front setback, or where converting an existing accessory structure or space in a house that is at least 5 years old.

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.

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

Home Occupations

ADUs are not allowed on sites with a Type-B home occupation. A Type-B home occupation is one in which the residents use their home as a place of work, and either one employee or customers come to the site. Examples are counseling, tutoring, and hair cutting and styling.

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.

Property Taxes

Multnomah County assesses and taxes ADUs the same as any other new improvement to the property.  The added value associated with the construction of an ADU will be assessed and taxed at the property’s current rate.  The construction of an ADU will not trigger a reset of the Maximum Assessed Value (MAV) of the property.

SDC Fees Waived for ADUs used as Owner Occupied or Long-Term Rentals

System Development Charges (SDCs) which are typically $14,000 to $18,000 are waived for ADUs that will be used as either Owner Occupied or Long-Term Rental for at least 10 years.  The program requires the property owner to sign a covenant stating that neither the ADU nor the house will be rented as Accessory Short-Term Rentals (ASTRs) for 10 years.   The ADU or house may be used as a rental if tenancy is arranged on a month-to-month basis or longer.  Violating the terms of the covenant will require repayment of waived fees at a rate of 150% of the current SDC fees.  The waiver program option went into effect August 1, 2018 and does not have an expiration date.  There is a $600 application fee for the waiver.

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

ADU Resources (links)

City of Portland:
City of Portland - Bureau of Development Services Accessory Dwelling Unit Main Page
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)

Get started

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