What is an ADU?

You may know them as granny flats, mother-in-law units, backyard cottages, or even tiny houses. When located on the property of a single family home, these are all part of a category of living spaces called Accessory Dwelling Units, or ADUs for short. Often ADUs are standalone homes in a backyard, or are apartments converted from attics, basements or garages. Regardless of where they’re located, they are growing in popularity among homeowners looking to expand living space, either for family use or for rental.

Here in Portland, ADUs are becoming especially popular, due to the rising costs of housing. Looking for ways to maximize their existing home’s space and layout, increasing numbers of Portland-area homeowners are building ADUs.

Most Portland lots are zoned to allow ADUs, and the City of Portland is encouraging more ADU development. The Portland City Council recently voted to permanently waive development fees required to build ADUs in the city, saving an estimated $16,000 per unit.


If you’d like to learn more about ADUs, here’s a list of some of the best online resources.

Accessory Dwellings A one-stop source for information about ADUs of all types, including multigenerational homes, laneway houses, granny flats and in-law units.

City of Portland This official City site has information on permits, requirements, plan reviews and inspections.

Portland State University Learn about PSU’s Small Backyard Homes Initiative

Backdoor Revolution Learn about and purchase the definitive ADU development book, “Backdoor Revolution” by ADU expert and Portland resident Kol Peterson.

Fees and charges update Learn about June 2018 changes to City charges and fees for ADUs