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Zoning Requirements for an Accessory Dwelling Unit

Learn more about the requirements and allowances in Portland's Zoning Code that govern the placement of accessory dwelling units in residential areas.
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The Portland Zoning Code allows Accessory Dwelling Units (ADUs) to be added on a site accessory to a house, attached house, manufactured home, or duplex in Residential zones, Commercial zones, and the Central Employment (EX) zone as described in Chapter 33.205 of the City Zoning Code.  

ADUs provide additional housing units that are compatible with the look and scale of single-dwelling development, make more efficient use of existing housing stock and infrastructure, and provide a mix of housing options. They can be created by converting part of an existing primary structure, adding area to an existing primary structure, converting an existing accessory structure, or constructing a new building.   

The Zoning Code defines an Accessory Dwelling Unit as a smaller, auxiliary dwelling unit on the same lot as a primary structure. Tender houses for floating homes are not included in this definition; a floating home cannot have an ADU. An ADU 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).

Thinking about building a workshop, garage, or accessory dwelling unit in your backyard?

Use the Detached Covered Accessory Structure Worksheet to understand the zoning requirements for your project:

Summary of zoning standards

The base zones (Chapters 33.110, Single-Dwelling Zones, Chapter 33.120, Multi-dwelling Zones, Chapter 33.130, Commercial Zones, and Chapter 33.140, Employment, and Industrial Zones) provide standards for all detached accessory structures, including detached ADUs. 

Chapter 33.205 of the Portland Zoning Code provides the additional requirements for all Accessory Dwelling Units (ADUs).

Visit the Portland Zoning Code.

Below is a summary of those standards.

General allowances

The zones that allow ADUs are Residential, Commercial, and Central Employment (EX).

One ADU is allowed on sites with a house attached house, or manufactured home. In the RF through R2.5 zones, the site must also be eligible for a primary structure per 33.110.202, and sites with an attached house must meet a minimum size threshold (see below).

In the R20 through R2.5 zones, one ADU is allowed on sites with a duplex (two units within a single detached structure) when the sites have frontage on a City-maintained street or frontage on pedestrian connections created solely for pedestrians and bicycles that connects to a City-maintained street. In the multi-dwelling, commercial and EX zones, one ADU is allowed on sites with a duplex regardless of whether the street is maintained by the City. In all cases, on a site with a duplex, the ADU must be detached from the duplex.  

Up to two ADUs can be allowed in some circumstances:

  • In the R20 through R2.5 Zones, two ADUs are allowed on sites with a house, attached house, or manufactured home when the site meets a minimum size threshold, is not located in the Constrained Sites ("z") overlay zone, and has frontage on a maintained street or dedicated pedestrian connection to a maintained street. Only one of the ADUs can be internal. Only one ADU is allowed on sites with a duplex.
  • In the multi-dwelling, commercial, and EX zones, two ADUS are allowed on sites with a house, attached house, or manufactured home regardless of site size or street frontage. Only one of the ADUs can be internal to the primary structure on the site.

ADUs are not allowed with other development types (e.g. triplexes, fourplexes, floating homes, or multi-dwelling structures).

Short-term rentals and home occupations

ADUs are allowed on sites with a Type A accessory short-term rental as described in Chapter 33.207 of the Portland Zoning Code. A Type A accessory short-term rental is where no more than two bedrooms are rented to overnight guests and the guest rents the bedroom(s) for fewer than 30 consecutive days. ADUs can be used as Type A accessory-short-term rentals with the appropriate permit. However, bedroom(s) used as accessory short-term rentals cannot be located in the ADU if an SDC waiver was given based on a recorded covenant as described in Section III.B.2.b.

ADUs are allowed on sites with a Type B accessory short-term rental as described in Chapter 33.207 of the Portland Zoning Code, if the accessory dwelling unit meets in sub-section 33.815.040.B.1 of the Portland Zoning Code. However, those bedroom(s) cannot be located in the ADU if an SDC waiver was given based on a recorded covenant as described in Section III.B.2.b. A Type B accessory short-term rental is where 3 to 5 bedrooms are rented to overnight guests.

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.

Size allowances

General.  The maximum size of an ADU may be no more than 75% of the living area of the primary structure (or larger of two units in a duplex) or 800 square feet, whichever is less. The size limit does not apply to an ADU proposed within a basement of a primary structure that is at least five years old. Living area is calculated by excluding the following areas from the overall gross building area: 

  • The thickness of the exterior walls;
  • Garage areas;
  • Basement areas where the ceiling height measured from the floor is less than 6 feet 8 inches; and
  • Any other building areas where the floor-to-ceiling height is either less than 5 feet, or areas not accessible by a stairway. 

For example, if an existing house has 1,000 square feet of living area after subtracting all spaces described above, the ADU size is limited to 750 square feet.  For a house that has 2,500 square feet of living area, 75% of the floor area would be 1,875 square feet. In this case, however, the ADU size is limited to no more than 800 square feet.   

Additional requirements for detached or connected ADUs.  In addition to the size limitations listed above, detached ADUs or ADUs connected to the primary structure are limited by the following conditions: 

Height.  The maximum height for a detached ADU is 20 feet when located outside of the required setbacks for the zone. If a detached ADU is allowed to be located within the required setbacks for the zone, the maximum height for the ADU is 15 feet.  

Building coverage.  The building coverage for a detached ADU may not be larger than the building coverage of the house, attached house or manufactured home. The combined building coverage for all detached accessory structures may not exceed 20% of the total site area.   

Location.  Detached ADUs must be set back 40 feet from a front lot line or behind the rear building wall of the primary structure. 

Exterior details. For detached ADUs that are more than 15 feet high, additional standards for the exterior of the ADU are required.  Exterior finish materials, roof pitch, trim, eaves, window orientation and dimension must be the same or visually match those of the house, attached house or manufactured home in type, size and placement, OR must be made from wood, composite boards, vinyl or aluminum products composed in a shingle pattern, or in a horizontal clapboard or shiplap pattern with boards 6 inches or less in width. The detached ADU must also meet certain standards relating to roof pitch, trim, windows, and eaves. See Section 33.110.250 for Single-dwelling zones or 33.120.280 for Multi-dwelling, Commercial, or EX zones. 

ADUs in some historic designations (33.445) may be subject to additional design requirements. These ADUs must meet the Community Design Standards (33.218), where allowed to use Community Design Standards, or they must be approved through Historic Resource review.  

Floor Area.  Floor area limits apply in all zones except RF, and in the R10 and R20 zones, floor area limits only apply on lots that are less than 10,000 sq. ft. in area. In the R7 through R2.5 zones, additional floor area is earned for each ADU proposed. 

Parking. Additional on-site parking is not required for an ADU. However, if parking is required for the existing dwelling unit(s), that parking must either be retained or replaced on-site. Any replacement parking must meet the parking standards in Chapter 33.266 of the Portland Zoning Code. 

Minimum Lot Size for ADU proposals in the RF zone. When one ADU is proposed the minimum lot size is described in Section 33.110.202, When Primary Structures are allowed.

Minimum Lot Size for ADU proposals in the R20 through R2.5 zones. When one ADU is proposed on a lot with a house or manufactured home, the minimum lot sizes are found in Section 33.110.202 When Primary Structures are allowed.

When one ADU is proposed on a lot with an attached house, the minimum lot sizes are as follows (Table 205-1):

Minimum Lot Area (Table 205-1) 
ZoneMinimum Lot Area
R2010,000 sq. ft.
R105,000 sq. ft.
R73,500 sq. ft.
R52,500 sq. ft.
R2.51,500 sq. ft.

When two ADUs are proposed on a lot with a house or one ADU is proposed on a lot with a duplex, the minimum lot sizes are as follows (Table 205-2):

Minimum Lot Area (Table 205-2) 
ZoneMinimum Lot Area
R2012,000 sq. ft.
R106,000 sq. ft.
R74,200 sq. ft.
R53,000 sq. ft.
R2.51,500 sq. ft.

Visitability. When two ADUs are proposed on a lot with a house or one ADU is proposed on a lot with a duplex, at least one dwelling unit must meet standards for visitable entrances, bathrooms, living area and doors. See Subsection 33.205.040.C.5 for additional information. 

Adjustments.  If any of the ADU development standards are not met, an Adjustment (variance) review is required, as outlined in Zoning Code Chapter 33.805. An Adjustment review is a discretionary land use review that requires approximately 8-10 weeks to process. Adjustment requests will be approved or denied based on the approval criteria in Chapter 33.805; there is no guarantee of approval.

Still need help?

If you have questions after reviewing the information on this page, please schedule a free 15-minute appointment with a city planner or set up an Early Assistance meeting for a deeper discussion.