As the housing affordability crisis continues, a greater mix of housing types, not just more housing, is needed to meet differing income and generational needs. But the housing types that contribute to a greater mix often involve densities that are higher than what local zoning rules allow. Different ordinances and codes can help increase supply with “missing middle” housing while meeting various market demands.
One example is accessory dwelling unit (ADU) ordinances. These are aimed at generating a dwelling unit that is secondary to the main house on the lot. ADU ordinances are used when there is a need for more diverse affordable housing opportunities within a lower intensity, primarily single-family detached, context. The standards can be in addition to the base zoning or as an independent set of standards.
Typically, an ADU ordinance allows either the conversion of existing structures, such as garages, into secondary dwelling units or new accessory structures to be built within a parcel by utilizing existing available space, usually in the backyard. There are three types of ADUs:
- Detached ADU: A structure completely separate from the primary building, including detached garage conversions or additions.
- Attached ADU: An addition to the primary building outside of the original building’s footprint.
- Internal ADU: Conversion of a part of the primary building to an ADU (e.g., over an attached garage, in an attic, or in a small portion of the primary building’s ground floor).
ADUs are an affordable type of home to construct because they do not require paying for additional land and major new infrastructure. They serve different populations, ranging from students and young professionals to young families, people with disabilities and senior citizens. ADUs can also retain the scale and character of a neighborhood, and can have an efficient approval process.
Here are two examples from Portland, Ore., that highlight different approaches to creating ADUs:Garage Conversion ADU
The owner built this ADU on a 5,000-square foot-lot (50 feet x 100 feet) located near his primary residence by converting the attached garage on the split-level 1973 house into an ADU. It is an 800-square foot, two-bedroom unit with one bedroom that is fully accessible and a comfortable fit for two people. The primary dwelling unit is the upper level of the same structure and contains 900 square feet, with three bedrooms and one bathroom.
Both are long-term residential rental units. One off-street parking spot was required for the primary unit. The parking spot is located in the front yard setback, which required a variance. The project started in 2018 and took approximately one month to design, one day to permit and five months to build. The owner worked closely with a designer who has extensive construction experience. The owner did the permitting himself, served as the general contractor for this project, and did about 30% of the actual construction as well.
Each unit rents for $1,850 per month. Collectively, they produce $1,000 more per month than the 30-year principal, interest, taxes and insurance (PITI) payments.Attached ADU
This new construction, two-bedroom ADU is located in the southeastern Portland Woodstock Neighborhood. The owners built a new unit to house their mother, who has a disability, for a lot less than buying a new home. The 800-square-foot ADU is attached to an existing traditional farmhouse, totaling approximately 1,500 square feet, via a covered breezeway. The design represents the traditional, regional farm vernacular, including cedar siding, shingle roofing, and gable roof forms.
Passive-solar design principles are incorporated to obtain maximum winter heat gain and summer cooling. The ADU also has a high-efficiency building envelope, FSC-certified wood products, an energy-efficient, mini-split mechanical system, high-efficiency LED lighting, occupancy-controlled ventilation, and low- or zero-volatile organic compound (VOC)/formaldehyde products selected throughout. The slab-on-grade construction offers barrier-free Americans with Disabilities Act (ADA) accessibility throughout the unit.
The project was developed under the current code, which waives the system development fees, saving about $13,000. Parking is not required for either the primary residence or the ADU due to the proximity to public transportation.
Both case studies are featured in NAHB’s “Diversifying Housing Options with Smaller Lots and Smaller Homes” report, available through the Land Use 101 toolkit.