Accessory Dwelling Units

Accessory Dwelling Units (ADUs) are a second contained unit on the same grounds (or attached to) a single-family house.  Also referred to as "granny flats" and "in-law suites. Units can be separate (e.g., above a garage), attached or located within the main dwelling unit (e.g., basement apartment). Typcailly includes a full bathroom, kitchen and separate entrance.

Benefits and Problems Addressed

Increase in housing stock & variety:  Accessory Dwelling Units provide housing on individual properties.

Homeowner flexibility: Units can provide income and flexible space for owners. 

Tips & Techniques

Examples:  include (1) apartment over the garage (2) a smaller house (on a foundation) in the backyard (3) apartment within the house or (4) attached aprtment with separate entrance. ADUs can be interior, attached or detached units with a full bathroom and working kitchen.

Uses:  ADUs used primarly for (1) second income, (2) home office, storage and (3) housing for relatives.  Many owners switch among uses.

Typcial requirements: can include (1) unit must be smaller than main dwelling; (2) owner must reide on-site; (3) fire code access, egress and/or sprinklers; (4) setback adn size limitations; (5) 

Barriers to ADUs: include zoning, permit fees, parking requirements and outright bans. Egress under fire codes can  be costly or architectually infeasible. Adding a unit can increase property values in high-cost markets,  often beyond the ability to recoup costs via rent. Some localities (Arlington VA) allow only a bathroom or kitchen but not both.

Hot Buttons: Parking, use for short term rentals, some ordinances require the main house be occupied by owners


Accessory Dwelling Unit Program - San Francisco CA US

Further information:
Accessory Dwelling is a clearinghouse on ADUs.

Accessory Dwelling Unit Survey for Portland, Eugene, and Ashland, Oregon (2012)