Accessory Dwelling Units — also called ADUs, granny flats, in-law suites, garage conversions, and backyard cottages — have become one of the fastest-growing categories of residential construction in the United States. New state laws, particularly in California, have dramatically lowered the barriers to building ADUs. But permits are still required, and the process varies significantly by location.
What Counts as an ADU?
An ADU is a secondary housing unit on a single-family or multi-family lot. It can be attached to the main house (like a basement apartment or addition), detached (like a backyard cottage or converted garage), or internal (a converted attic or basement). Each type has slightly different permit requirements, but all require permits in every U.S. jurisdiction.
A Junior ADU (JADU) — a smaller unit created entirely within the existing footprint of the main house, often from a converted bedroom — is a separate category in California law with streamlined requirements.
California: The ADU Revolution
California has been the epicenter of ADU law reform. A series of state laws passed since 2017 — including AB 2221 and AB 68 — now require cities to approve compliant ADU applications within 60 days using "ministerial" review. This means cities cannot deny ADUs that meet objective standards, and cannot require discretionary hearings or neighbor approval.
Key California ADU rules as of 2026: minimum ADU size of at least 850 square feet (or 1,000 sq ft for 2-bedroom), setbacks of only 4 feet from rear and side property lines, no owner-occupancy requirement, and no parking requirement if within half a mile of public transit. Fire sprinklers are required if the main house has sprinklers. ADUs can be sold separately from the main house under certain conditions.
ADU Rules in Other Cities
Denver expanded ADU-friendly zoning citywide in 2021, allowing detached ADUs on most single-family lots. Portland has an active ADU amnesty program for previously unpermitted units. Seattle's DADU (Detached Accessory Dwelling Unit) program is well-established, though fees are high reflecting overall Seattle construction costs. Chicago's 2021 ADU ordinance allows coach houses and basement conversions in 5 pilot areas, with plans for citywide expansion.
Houston, true to its no-zoning tradition, has relatively permissive ADU rules with fewer restrictions than most major cities. Austin has ADU-friendly policies but enforces compatibility standards around height and setbacks relative to neighboring properties.
What the ADU Permit Process Involves
ADU permits are among the most complex residential permits because they create a complete new dwelling unit. You'll typically need architectural drawings showing the floor plan, elevations, and site plan. Structural plans, energy compliance calculations, and separate plumbing and electrical permits are all required. If the ADU has a separate utility connection, utility coordination is needed.
For garage conversions, additional requirements include demonstrating adequate natural light and ventilation in habitable rooms, meeting minimum ceiling height requirements (typically 7 feet), and providing proper egress from sleeping areas.
ADU Permit Costs
ADU permits are the most expensive residential permits in most cities, reflecting their complexity. In Los Angeles, total permit fees for a compliant ADU typically run $1,500–$5,000, not including architect fees. In Seattle, permit fees alone can reach $1,500–$10,000 for a DADU. In NYC, ADU (basement apartment) permits start around $2,000 and can go significantly higher depending on scope.
Professional fees — architect, engineer, and permit expediter — can add $5,000–$20,000 to the cost depending on complexity and city. In California, impact fees (fees charged by local governments to fund infrastructure) were limited by state law for ADUs under 750 square feet, significantly reducing costs for smaller units.
The ROI Case for ADU Permits
Unlike most permit discussions that focus on compliance risk, ADU permits have a strong positive ROI argument. A permitted ADU adds significant appraised value to your property — often $150,000–$400,000 in high-cost markets like Los Angeles and Seattle. Unpermitted ADUs don't contribute to appraised value and can actually hurt your sale by requiring disclosure and remediation. For the rental income and property value upside, the permit investment is almost always worth it.