ADU Rules: Can You Build a Granny Flat in Your Backyard?

Accessory dwelling units go by many names: granny flats, in-law suites, backyard cottages, casitas. Whatever you call them, the rules for building one depend entirely on where you live.

California changed the game

California's ADU laws (AB 68, SB 13, and subsequent updates) are the most permissive in the country. The state effectively overrode local restrictions, allowing at least one ADU on any residential lot by right. Local cities cannot require owner-occupancy, cannot require additional parking in most cases, and must approve permits within 60 days. Los Angeles, San Jose, San Francisco, and San Diego have all seen ADU construction surge as a result.

Other states are following

Oregon (SB 1051), Washington (HB 1337), and Connecticut have passed statewide ADU legislation that limits local government's ability to restrict them. In these states, you can generally build an ADU on any single-family lot without a conditional use permit or public hearing. The trend is toward more permissive rules, but the specifics vary.

Size limits

Most cities cap detached ADUs at 800 to 1,200 square feet. California allows up to 1,200 square feet for detached ADUs. Denver allows 1,000 square feet. Seattle allows 1,000 square feet for detached accessory dwelling units (DADUs). Junior ADUs (JADUs), which are converted spaces within the existing house, are typically capped at 500 square feet.

The parking question

Historically, the biggest barrier to ADUs was parking requirements. Cities required one or two additional parking spaces for an ADU, which was impossible on many lots. California eliminated parking requirements for ADUs near transit and in many other situations. Other states are following suit. If your city still requires parking for ADUs, check whether recent state legislation has preempted that requirement.

Renting your ADU

Most cities allow you to rent your ADU as a long-term rental. Short-term rental (Airbnb) use is a different question. Many cities that allow ADUs prohibit or restrict using them as short-term rentals. California, for example, allows ADUs as long-term rentals but individual cities can restrict STR use. Check both your ADU rules and your STR rules before listing.

What it costs

ADU construction costs range from $100,000 to $300,000 for a detached unit, depending on size, location, and finishes. Permit fees range from $1,000 to $15,000. Some cities, including Los Angeles, have waived or reduced ADU permit fees to encourage construction. Prefabricated ADUs can reduce costs but still require the same permits and inspections as site-built units.