You need a permit to build an ADU anywhere in the Bay Area. There is no legal way around it. Whether you live in San Jose, Oakland, San Francisco, or a small town in Marin, every accessory dwelling unit needs a building permit before construction starts. This applies to detached backyard cottages, garage conversions, basement units, and attached additions. Even a small unit under 500 square feet needs one. The good news is that California law now forces cities to approve most ADU applications quickly and without public hearings, so getting a permit is far easier than it was ten years ago.
Here is the part most homeowners don’t know: the permit process in California is now “ministerial.” That means the city cannot hold a public hearing, cannot let neighbors object, and cannot use personal judgment to deny your project. If your plans meet the written rules, the city must approve them, and state law gives them 60 days to do it. Cities that drag their feet or invent extra rules can be reported to the state, and the state has pushed back hard on Bay Area cities that tried it. This is a big shift from the old days, when a single unhappy neighbor could stall a backyard cottage for years. Some cities have even published pre-approved ADU plans you can use, which cuts both cost and review time.
What the Permit Actually Covers
An ADU permit is not one single document. It usually bundles several reviews into one application. Planning staff check zoning items like setbacks, height, and unit size. Building staff check the construction drawings for safety: structure, fire separation, exits, electrical, plumbing, and energy code. If your lot is on a septic system, near a creek, or in a fire hazard zone, extra reviews may apply. You may also need separate sign-offs for sewer or water connections, though state law limits when utilities can charge you connection fees. Smaller units, generally those under 750 square feet, are exempt from local impact fees, which can save you tens of thousands of dollars.
What You Can Build by Right
State law sets a floor that every Bay Area city must allow. On most single-family lots, you can build one detached ADU up to 800 square feet and 16 feet tall with four-foot side and rear setbacks, and the city cannot say no. Many cities allow more than that, sometimes up to 1,000 or 1,200 square feet. You can also add a junior ADU, which is a small unit up to 500 square feet carved out of your existing house. Garage conversions are allowed almost everywhere, and cities cannot require you to replace the parking you lose. Multifamily properties can add ADUs too, often more than one. Local rules vary on the details, so check your city’s ADU page before drawing plans.
What Happens If You Skip the Permit
Building without a permit is tempting because it seems faster and cheaper. It is neither in the long run. Unpermitted units show up when you sell the house, refinance, or file an insurance claim. A buyer’s inspector will flag the work, lenders may refuse the loan, and insurers can deny claims tied to unpermitted construction. Cities can also fine you and order the unit removed or opened up for inspection, which sometimes means tearing out finished walls. If a tenant lives in an unpermitted unit and something goes wrong, your legal exposure grows fast. California does offer amnesty paths in many cities to legalize older unpermitted units, but that process usually costs more than doing it right the first time.
How Long It Takes and What It Costs
Plan on two to four months from application to permit in most Bay Area cities, sometimes faster if your plans are clean or you use a pre-approved design. The 60-day clock starts once your application is complete, and cities often ask for corrections, which pauses the clock. Permit fees vary widely by city and project size, but a rough range for a detached ADU is $5,000 to $15,000 in fees, on top of design and engineering costs. Larger units above 750 square feet may trigger impact fees, so many owners size their unit just under that line on purpose.
Steps to Get Started
Start with your city’s planning department website, since almost every Bay Area city now has an ADU handout that lists local size limits, setbacks, and fees. Many offer free pre-application meetings where staff will look at your idea and flag problems early. From there, hire a designer or pick a pre-approved plan, submit your application, respond to corrections quickly, and schedule inspections as you build. Keep every approval document, because you will want them when you sell or rent the unit.
The Bottom Line
You need a permit for any ADU in the Bay Area, full stop. But the process today is faster, cheaper, and more predictable than most homeowners expect, because state law has stripped cities of most of their power to say no. Treat the permit as part of the project budget, not an obstacle. A permitted ADU adds real, bankable value to your home, can be legally rented, and won’t come back to haunt you at sale time. An unpermitted one does the opposite. Spend the money, follow the process, and you end up with an asset instead of a liability.