Can I Build an ADU (Accessory Dwelling Unit) in California?

ADU permit requirements. State law, internal vs. detached, basement apartments, and impact fee waivers.

Often yes — many states now require cities to allow at least internal ADUs.

In California

California constrains what a local ADU ordinance may contain. Government Code 66314 provides that an ordinance 'shall' do a list of things including 'Impose objective standards on accessory dwelling units that include, but are not limited to, parking, height, setback, landscape, architectural review, maximum size of a unit, and standards that prevent adverse impacts on any real property that is listed in the California Register of Historical Resources.' The word doing the work is OBJECTIVE - the statute pushes local discretion into measurable standards rather than case-by-case judgement. California does not leave the code edition to the city. Health and Safety Code 17922 adopts Title 24 of the California Code of Regulations statewide, and requires that those standards 'impose substantially the same requirements as are contained in the most recent editions' of the named model codes, including 'The International Building Code of the International Code Council.' and 'The International Residential Code of the International Code Council.' A city or county may still vary it, but only on narrow grounds: under Health and Safety Code 17958.5 a local change must be 'reasonably necessary because of local climatic, geological, or topographical conditions', and under 17958.7 the governing body 'shall make an express finding' to that effect, file it with the California Building Standards Commission, and 'No modification or change shall become effective or operative for any purpose until the finding and the modification or change have been filed with the California Building Standards Commission.' AND THERE IS A LIVE FREEZE ON NEW RESIDENTIAL AMENDMENTS: 'Commencing October 1, 2025, to June 1, 2031, inclusive, a city or county shall not make a change or modification as described in subdivision (a) or (b), including to green building standards, that is applicable to residential units, unless one of the following conditions is met:' - broadly, that it is substantially equivalent to something already filed and in effect as of September 30, 2025, or the commission deems it necessary as an emergency standard. So for residential work today, expect the statewide code to govern and treat a NEW local residential amendment as the exception that has to justify itself.

State Law May Require It

A growing number of states have passed laws requiring cities to allow at least internal ADUs (within the existing footprint of the home — basement apartments, above-garage units, etc.). Many cities also allow detached ADUs, though rules on size and placement vary. Check whether your state has an ADU-enabling law before assuming your city can simply say no.

Types of ADUs

Internal ADUs include basement apartments, converted attic space, and attached additions. Detached ADUs include backyard cottages, converted garages, and purpose-built accessory structures. Each type has different permit requirements and setback rules.

Key Requirements

Most ADUs need: a separate entrance from the primary dwelling, adequate parking (usually one additional off-street space), fire separation (1-hour rating for internal ADUs), and compliance with all building codes for habitable space. Basement ADUs require egress windows in every bedroom.

Reduced Impact Fees

Some states limit impact fees for ADUs, making them cheaper to permit than a full second dwelling. Many cities waive or significantly reduce impact fees for ADUs. Ask your city about current fee schedules — this can save thousands of dollars.

In many states you can build an ADU thanks to state law requiring cities to allow it. Internal ADUs are the easiest path. Start by checking your state's ADU law and your city's specific ADU ordinance.

Permit fees by city in California

Verified from each city’s published fee schedule. Where a city has no separate fee listed for this project, open the city page for the current requirements and amount.

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Permit fees by city in California
	Permit fee	Source
Fresno	Due at permit issuanceVerifiedvia Residential Building Permit	Residential Building Permit →
Long Beach	Per the City's published fee schedulesVerifiedvia Building Permit	Building Permit →
Oakland	NOT PUBLISHED on the pages readVerifiedvia Residential Building Combination Permit (RBC)	Residential Building Combination Permit (RBC) →
Sacramento	Published separately in the City's building fee pagesVerifiedvia Residential Building Permit	Residential Building Permit →
San Diego	$737.00Verifiedvia Residential Building Permit (Combination Building - Single Dwelling Units, ADUs and Duplexes)	Residential Building Permit (Combination Building - Single Dwelling Units, ADUs and Duplexes) →
San Jose	NOT PUBLISHED on the permit service pagesVerifiedvia Residential Building Permit (Standard Plan Review Service)	Residential Building Permit (Standard Plan Review Service) →

See the national overview for this permit →