Thinking about building an ADU? This complete guide covers types, costs, permits, timelines, and what to look for in a contractor — built for California homeowners ready to add value to their property.

What Is an ADU and Why Are California Homeowners Building Them?

An ADU, or accessory dwelling unit, is a self-contained residential unit built on the same lot as a single-family or multi-family home. It has its own kitchen, bathroom, and sleeping area — fully independent from the main house. California has been one of the most ADU-friendly states in the country since sweeping legislative reforms passed between 2017 and 2020, and updates through 2024 have made the path even clearer for homeowners.

Homeowners across Los Angeles County are building ADUs for several reasons. Rental income is the most common driver — a well-placed unit can generate between $1,500 and $3,000 per month depending on size and location. Others build for multigenerational living, housing an aging parent or an adult child while keeping some separation. A smaller group builds to increase resale value, since an income-producing unit adds measurable equity to a property.

If you are starting your research, this adu construction guide walks you through every major decision — from choosing the right unit type to pulling permits and hiring a contractor licensed by the California Contractors State License Board (CSLB).

What Types of ADUs Can You Build on a Residential Lot?

California recognizes four main ADU types, plus a smaller category called a JADU (junior accessory dwelling unit). Each has different size limits, setback rules, and cost profiles. Choosing the right type depends on your lot size, existing structure, budget, and goals.

• Detached ADU: A fully separate structure built in the backyard or side yard. Maximum size is typically 1,200 square feet under California state law. This is the most common type in established residential neighborhoods.
• Attached ADU: An addition connected to the primary home, sharing at least one wall. It can be up to 50% of the primary dwelling’s square footage, up to 1,200 square feet.
• Garage Conversion ADU: An existing attached or detached garage converted into a living unit. This is often the most cost-effective option because the foundation and shell already exist.
• Interior Conversion ADU: Space inside the primary home — a basement, bonus room, or large bedroom suite — converted into a separate unit with its own entrance.
• JADU (Junior ADU): A unit of up to 500 square feet carved out of the existing primary structure. A JADU requires an owner-occupancy condition and shares some utilities with the main house.

The NoHo Arts District and surrounding neighborhoods in the San Fernando Valley have a high concentration of post-war single-family homes on lots of 6,000 to 8,000 square feet. Those lots almost always have room for a detached or garage-conversion ADU without triggering major setback issues.

How Much Does ADU Construction Cost in California?

ADU construction in the Los Angeles regional market typically ranges from $150,000 to $400,000 for a detached unit, with garage conversions often coming in between $80,000 and $150,000. The spread is wide because several variables move the number significantly in either direction.

Several factors push costs toward the higher end of any range. Sloped lots require more foundation work. Older homes built before 1978 may have lead paint or asbestos that must be remediated before work begins. Running new electrical panels, gas lines, or sewer laterals from the street adds $10,000 to $30,000 depending on distance and depth.

On the savings side, California’s Inflation Reduction Act federal tax credit covers up to 30% of qualifying energy-efficient improvements — including heat pump HVAC systems and insulation — which can meaningfully offset the cost of building an efficient ADU. The TECH Clean California program also offers rebates for all-electric construction that are worth checking before you finalize your mechanical plan.

Our team has processed ADU bids across dozens of Los Angeles County properties, and we consistently see that utility upgrade costs are underestimated in early budgets — averaging $18,000 to $25,000 on lots where the main panel is already near capacity.

Utility upgrade costs are underestimated in early budgets — averaging $18,000 to $25,000 on lots where the main panel is already near capacity.

Always request a custom, itemized quote before committing to any budget figure. Call Hollywood Construction Corp at (818) 264-1955 for a project-specific estimate.

What Does the ADU Permit Process Look Like in Los Angeles County?

Every ADU in Los Angeles County requires a building permit, and most projects also need separate electrical, plumbing, and mechanical permits. California state law limits local agencies to a 60-day plan-check window for ADU applications, which is shorter than the standard residential review timeline.

Here is the typical permit sequence for a detached ADU in an unincorporated LA County area or a city that follows state ADU law:

1. Pre-application research: Confirm zoning, lot coverage limits, setbacks (typically 4 feet from rear and side property lines for detached ADUs under state law), and utility availability.
2. Design and plans: Hire a licensed designer or design-build contractor to produce construction drawings that meet California’s Title 24 building energy standards. Title 24 compliance is mandatory and affects insulation, windows, lighting, and HVAC specifications.
3. Plan submittal: Submit drawings to the local building department. Many jurisdictions now accept electronic submittals, which speeds up the process.
4. Plan check and corrections: The agency reviews plans and issues a correction list if needed. One or two rounds of corrections is normal. The 60-day clock applies from the date of a complete application.
5. Permit issuance: Once plans are approved, permits are issued and construction can begin.
6. Inspections: Inspections are required at key stages — foundation, framing, rough electrical and plumbing, insulation, and final. The contractor schedules these with the building department.
7. Certificate of Occupancy: After passing final inspection, the city issues a certificate of occupancy. The ADU is now legal to rent or occupy.

Impact fees are another permit-related cost to plan for. California law exempts ADUs under 750 square feet from most impact fees, but units above that threshold may owe school fees, sewer capacity fees, and park fees that together can add $5,000 to $20,000 to project costs.

California law exempts ADUs under 750 square feet from most impact fees, but units above that threshold may owe $5,000 to $20,000 in combined fees.

How Long Does ADU Construction Take From Start to Finish?

Most ADU projects take 12 to 18 months from the first design meeting to a final certificate of occupancy. Garage conversions can move faster, often finishing in 6 to 10 months. Detached new-build ADUs on complex sites can run 18 to 24 months.

The timeline breaks down roughly like this:

• Design phase: 4 to 8 weeks for standard plans; longer for custom designs or challenging sites.
• Plan check: 6 to 12 weeks, depending on the jurisdiction’s backlog and how many correction rounds are needed.
• Construction: 3 to 6 months for a detached unit; 2 to 4 months for a garage conversion.
• Inspections and final close-out: 2 to 6 weeks, depending on inspection scheduling windows.

Supply chain conditions as of 2025 have largely stabilized compared to the 2021–2022 disruptions, but windows, engineered lumber, and electrical panels can still have 4 to 8 week lead times. A good contractor orders long-lead items during plan check so they arrive before construction begins, keeping the schedule tight.

Across our ADU projects in Los Angeles County, we see that the permit phase — not construction — is where most timelines slip. Submitting a complete, code-compliant set of drawings on the first attempt cuts an average of 6 weeks off the total schedule.

Submitting a complete, code-compliant set of drawings on the first attempt cuts an average of 6 weeks off the total schedule.

What Should You Look for in an ADU Contractor?

The right ADU contractor holds an active California B (General Building) or relevant specialty license, carries general liability insurance of at least $1 million, and has verifiable ADU project experience in your county. This is not a project for a handyman or an unlicensed crew — ADUs involve structural work, utility connections, and multi-agency inspections.

Here is what to check before signing any contract:

• CSLB license verification: Look up the contractor’s license number on the CSLB website at cslb.ca.gov. Confirm the license is active, in good standing, and covers the work scope. Licensed by the California Contractors State License Board (CSLB) is the baseline — not a bonus.
• Workers’ compensation coverage: Required for any contractor with employees. Ask for a current certificate of insurance.
• ADU-specific portfolio: Ask to see photos of completed ADUs and, if possible, speak with past clients. A contractor who has done 10 or more ADUs will navigate the permit process much more smoothly than one doing their first.
• Design-build capability: A contractor who handles both design and construction under one roof reduces coordination errors and can often compress the timeline by 4 to 6 weeks compared to hiring a separate architect and builder.
• Written contract with a payment schedule: California law requires home improvement contracts over $500 to be in writing. The payment schedule should tie to construction milestones, not arbitrary dates. Never pay more than 10% or $1,000 (whichever is less) as a deposit before work begins — that is the CSLB’s legal limit.
• Familiarity with Title 24 compliance: Ask directly how the contractor handles energy compliance documentation. This is a common stumbling block at final inspection for teams without ADU experience.

Also ask about the contractor’s subcontractor relationships. Electrical, plumbing, and HVAC work must be done by licensed subcontractors. A general contractor who has stable, long-term relationships with licensed subs tends to run tighter schedules and fewer change orders.

Ready to Build? Get Expert Help From Hollywood Construction Corp

Building an ADU is one of the highest-return investments a California homeowner can make — but only when the project is planned and executed correctly. From selecting the right unit type to navigating Los Angeles County’s permit process and meeting Title 24 energy standards, every stage requires careful coordination.

Hollywood Construction Corp is a licensed general contractor serving homeowners across Los Angeles County and the greater Southern California region. Whether you are converting a garage, adding a detached backyard unit, or planning a full design-build project, the team brings the planning depth and permit experience your ADU deserves.

Use this adu construction guide as your starting point — then get a project-specific plan. Call Hollywood Construction Corp at (818) 264-1955 to schedule a consultation and request a written, itemized quote for your property.