What Is an ADU? The Complete California Guide (2026)

“What is an ADU?” is one of the fastest-growing housing searches in California. The question barely registered on Google before 2017. By 2023, California homeowners were pulling more than 20,000 ADU permits per year — a tenfold increase from fewer than 2,000 in 2016, according to data from the California Department of Housing and Community Development. The surge is driven by a simple collision: California has a housing shortage of roughly 2.5 million units, and the state passed a series of laws that made it legal to build a second home on most single-family lots. An ADU — accessory dwelling unit — is that second home.

This guide covers what an ADU is, what types exist in California, what they cost, how permits work, and how to find a verified contractor to build one. If you already know the basics and want to compare builders, browse our verified directories for ADU builders in Sacramento, ADU builders in Los Angeles, ADU builders in San Diego, and the ADU builders in the SF Bay Area.

An ADU is a self-contained housing unit built on the same lot as an existing single-family home. ADU stands for accessory dwelling unit. You might hear them called granny flats, in-law suites, backyard cottages, or casitas — the legal term in California is ADU.

To qualify as an ADU under California law, a unit must have:

• Its own entrance separate from the primary home
• A full kitchen (sink, cooking appliance, refrigerator, counter space)
• A bathroom
• A sleeping area

An ADU is a legal, permitted dwelling. It gets its own address. It can be rented to a tenant or used by family members. It is not a shed, a converted room without a kitchen, or a structure built without permits. The distinction matters because only permitted ADUs can be legally rented, insured, and included in property valuations.

The term “additional dwelling unit” is sometimes used interchangeably with ADU, but the correct legal term in California statute is accessory dwelling unit — the unit is accessory to the primary residence on the lot.

California recognizes four main ADU types. Each has different size limits, cost ranges, and construction requirements.

Detached ADU

A standalone structure built separately from the primary home, usually in the backyard. This is the most common ADU type and offers the most design flexibility. Detached ADUs can be up to 1,200 square feet on single-family lots. They require 4-foot side and rear setbacks under state law. Typical cost: $180,000 to $400,000 depending on size, finish level, and local labor rates.

Attached ADU

An extension built onto the existing home — a new wing, addition, or second-story unit connected to the primary structure. Attached ADUs can be up to 50% of the primary dwelling’s floor area, with a maximum of 1,200 square feet. They often share a wall or foundation with the main home. Typical cost: $150,000 to $350,000. Structural work connecting to the existing home can add complexity and cost compared to a detached build.

Garage Conversion ADU

Converting an existing garage into a living unit. California law does not require replacement parking when you convert a garage to an ADU. This is often the most affordable ADU type because the shell — walls, roof, foundation — already exists. You’re adding insulation, plumbing, electrical, a kitchen, and a bathroom. Typical cost: $80,000 to $180,000. For a full breakdown, see our guide on the cost to convert a garage to an ADU in California.

Junior ADU (JADU)

A small unit of 500 square feet or less created within the existing footprint of the primary home or attached garage. JADUs can share a bathroom with the main home (though most have their own). They require an efficiency kitchen — a sink, cooking appliance, and counter, but no full-size range. JADUs have different owner-occupancy rules than standard ADUs. California homeowners can build both an ADU and a JADU on the same lot. Full details in our JADU California guide.

Three forces are driving the ADU wave in California.

State Legislation Removed Local Barriers

Between 2017 and 2024, California passed more than a dozen ADU-related bills. The most significant — AB 68, SB 13, and AB 881 — took effect in January 2020 and did three things: they stripped cities of the power to block most ADU applications, they capped permit processing at 60 days, and they eliminated parking requirements near transit. Before these laws, most California cities made ADU permitting so difficult that almost nobody bothered. After them, permit volumes exploded.

Housing Shortage

California is short roughly 2.5 million housing units. Median home prices in Los Angeles, San Francisco, and San Diego are well above $800,000. ADUs add housing units to existing residential lots without requiring new land, new infrastructure, or zoning changes. They’re the fastest path to adding housing supply in neighborhoods that are already built out.

Rental Income

A detached ADU in Sacramento rents for $1,200 to $2,200 per month. In Los Angeles, $1,800 to $3,500. In the Bay Area, $2,200 to $4,000+. For homeowners paying $3,000 to $5,000 monthly mortgages, ADU rental income offsets a significant portion of their housing cost. Some homeowners build the ADU, rent it out, and use the income to cover the construction loan payments entirely.

ADU costs vary by type, size, city, and finish level. These are 2026 ranges based on market data from California builders.

ADU Type	Size Range	Cost Range
Garage Conversion	200-500 sq ft	$80,000 – $180,000
JADU	150-500 sq ft	$50,000 – $130,000
Attached ADU	400-1,200 sq ft	$150,000 – $350,000
Detached ADU	400-1,200 sq ft	$180,000 – $400,000
Prefab / Modular	400-1,200 sq ft	$120,000 – $280,000

These ranges include permits, site preparation, construction, and standard finishes. They do not include high-end upgrades, complex site conditions (hillside lots, poor soil, difficult access), or major utility work. Permit fees alone range from $3,000 to $15,000 depending on your city.

Los Angeles and the Bay Area are the most expensive markets. Sacramento and parts of San Diego County are more affordable. For a full cost breakdown by city and category, see our guide to ADU construction cost in California.

ADU Financing

Most homeowners finance ADU construction through home equity loans (HELOCs), cash-out refinancing, or construction loans. California also offers the CalHFA ADU grant — up to $40,000 with no repayment required for qualifying homeowners. For a full comparison of options, see our guide to ADU financing California.

ADU permitting in California is ministerial — meaning your city must approve the application if it meets zoning and building code requirements. They cannot use discretionary review to deny it. This is codified in Government Code Section 65852.2.

The process works like this:

1. Design. Hire an architect or designer to create construction drawings. Some cities accept pre-approved ADU plans, which can save $5,000 to $15,000 in design fees. San Jose, Sacramento, and several other cities have pre-approved plan libraries.
2. Submit plans. File your ADU application with your city’s building department. Include architectural drawings, structural engineering, Title 24 energy calculations, and the permit application form.
3. Plan check. Your city reviews the plans for code compliance. State law requires a decision within 60 days of a complete application. In practice, most cities issue corrections — items they want changed before approval. Each correction cycle adds 2-4 weeks.
4. Permit issuance. Once plans are approved, you pay permit fees and receive the building permit.
5. Construction. Your contractor builds the ADU according to the approved plans. The building department conducts inspections at key stages: foundation, framing, plumbing rough-in, electrical rough-in, insulation, and final.
6. Final inspection. The building inspector verifies everything matches the approved plans and meets code. Once passed, you receive a certificate of occupancy.

The full process — from design through final inspection — takes 8 to 14 months for most projects. Permitting accounts for 2 to 3 months of that timeline. Construction takes 4 to 8 months. For detailed timelines and costs, see our guide to getting an ADU permit California.

The contractor you hire is the single biggest risk factor in any ADU project. California has seen ADU contractor scams that left hundreds of homeowners with unfinished projects and six-figure losses. The difference between a successful build and a disaster comes down to verification.

Before signing with any contractor:

• Verify the CSLB license. Every ADU general contractor must hold an active Class B license from the California Contractors State License Board. Check it yourself at cslb.ca.gov. Don’t take their word for it.
• Check the bond. California requires a $25,000 contractor surety bond on file. Confirm it’s current in the CSLB database.
• Confirm workers’ comp. If the contractor has employees, they must carry workers’ compensation insurance. If a worker is injured on your property and they don’t have coverage, you can be held liable.
• Review complaint history. CSLB tracks complaints, legal actions, and disciplinary records. A clean history doesn’t guarantee quality, but active complaints or suspensions are disqualifying.
• Ask for local references. A contractor who has built ADUs in your city should provide addresses of completed projects. Call your building department to verify final inspection was approved.

California law (Business and Professions Code Section 7159.5) limits contractor deposits to $1,000 or 10% of the contract price — whichever is less. Any contractor asking for more is breaking the law.

VerifiedADU maintains curated directories of CSLB-verified ADU builders. Every listing is checked daily — license status, bond, workers’ comp, and complaint history. Contractors who lose their license or accumulate complaints are flagged as removed contractors. For a step-by-step guide on finding and vetting builders, see how to find ADU builders near you.

While California’s statewide ADU laws set the floor, each city adds its own layer of rules — fee schedules, design review processes, and local ordinances. Here’s how the four largest ADU markets compare.

City	Max Size	Permit Fees	Typical Timeline
Sacramento	1,200 sq ft	$3,000 – $8,000	8-12 months
Los Angeles	1,200 sq ft	$5,000 – $15,000	10-16 months
San Diego	1,200 sq ft	$4,000 – $10,000	8-12 months
SF Bay Area	1,200 sq ft	$5,000 – $18,000	10-18 months

All four cities follow the same state maximums for ADU size (1,200 sq ft detached). The differences are in permit fees, plan check timelines, and inspection processes. Los Angeles and San Francisco consistently have the longest permitting timelines due to higher application volumes and more complex inter-departmental reviews. Sacramento and San Diego tend to process ADU permits faster.

For a complete breakdown of California’s ADU laws — including the most recent legislation — see our guide to ADU construction in California.

What does ADU stand for?

ADU stands for accessory dwelling unit. It’s a self-contained housing unit built on the same lot as an existing single-family home. Other terms include granny flat, in-law suite, backyard cottage, and casita. The legal term used in California law is accessory dwelling unit.

What is the difference between an ADU and a guest house?

An ADU has a full kitchen and is designed for independent living — it can be rented to a tenant. A guest house, in many California jurisdictions, is a secondary structure without a kitchen that cannot be used as a separate rental unit. The kitchen is the legal dividing line. If it has a kitchen, it’s an ADU and must be permitted as one.

How much does an ADU cost in California?

ADU costs in California range from $80,000 for a garage conversion to $400,000+ for a large detached unit in a high-cost market. The average detached ADU runs $200,000 to $300,000 for a 600-800 square foot unit with standard finishes. Costs vary significantly by city — Los Angeles and the Bay Area are the most expensive, Sacramento is the most affordable among major metros.

Can I build an ADU on my property in California?

Most single-family homeowners in California can build at least one ADU and one JADU on their property. State law prohibits cities from banning ADUs on residential lots. Multifamily properties can also add ADUs under certain conditions. The main requirements are: the lot must have an existing or proposed primary dwelling, and the ADU must meet setback and size limits.

Do I need a permit to build an ADU in California?

Yes. Every ADU in California requires a building permit. Building without a permit is a code violation that can result in stop-work orders, fines, and mandatory demolition. An unpermitted ADU cannot be legally rented, may not be insurable, and creates disclosure problems when selling the property. Retroactive permitting — where available — costs two to three times the original permit fees.

Can I rent out my ADU in California?

Yes. California law prohibits local governments from banning long-term rental of ADUs. You can rent your ADU to tenants with no owner-occupancy requirement for standard ADUs. Short-term rentals (under 30 days) are restricted — most California cities either ban or heavily regulate short-term ADU rentals. JADUs may have owner-occupancy requirements in some circumstances.

What is a JADU?

A JADU is a junior accessory dwelling unit — a small unit of 500 square feet or less created within the existing footprint of the primary home or attached garage. JADUs have an efficiency kitchen (no full range required) and may share a bathroom with the main home. California homeowners can build both a full ADU and a JADU on the same lot, for a total of three housing units. See our full JADU California guide.

How long does it take to build an ADU?

Most ADU projects in California take 8 to 14 months from permit application to move-in. Design and engineering take 4 to 8 weeks. Permitting takes 4 to 12 weeks depending on your city. Construction takes 4 to 8 months. Projects in Los Angeles and San Francisco tend to take longer due to permit backlogs and correction cycles.

Does an ADU add value to my property?

A permitted ADU typically adds 20-30% to a property’s assessed value. The exact amount depends on ADU size, finish quality, rental income potential, and your local real estate market. A 600 square foot detached ADU on a $500,000 property in Sacramento might add $100,000 to $150,000 in value. Unpermitted ADUs add little to no appraised value and create legal risk during sale.

What is the maximum size of an ADU in California?

Detached ADUs can be up to 1,200 square feet on single-family lots under state law. Attached ADUs can be up to 50% of the primary dwelling’s floor area, with a maximum of 1,200 square feet. JADUs are capped at 500 square feet. Some cities previously had smaller limits, but state law now overrides local size caps that fall below these statewide standards.