How to Convert a Detached Garage Into an ADU in California

Why Convert Your Detached Garage Into an ADU?

Converting a detached garage into an Accessory Dwelling Unit (ADU) is one of the most cost-effective ways to add livable square footage to your California property. Because the basic structure already exists, you can save thousands of dollars compared to building from the ground up. Garage conversions also typically move through the permitting process faster and have shorter construction timelines than new detached builds.

California homeowners have increasingly turned to garage-to-ADU conversions as a strategy for generating rental income, housing aging parents, or creating private spaces for adult children. Under current state law, local governments cannot require additional parking when you convert an existing garage, which removes one of the biggest barriers that used to stand in the way of these projects.

In this guide, we will walk you through every step of converting your detached garage into a fully permitted ADU in California. We will cover structural assessments, permit requirements, construction phases, typical costs, and the design considerations that will help you maximize both livability and return on investment.

California Laws That Make Garage Conversions Easier

Over the past several years, California has enacted a series of laws that significantly streamline the process of converting a garage into an ADU. Understanding these laws will help you navigate the process with confidence and push back against any local requirements that conflict with state mandates.

Key State Legislation

AB 68 and SB 13 (2020): These companion bills eliminated many of the barriers local jurisdictions used to impose on ADU construction. For garage conversions specifically, they removed the ability of cities to require replacement parking when an existing garage is converted. They also reduced impact fees for ADUs under 750 square feet and set a maximum timeline of 60 days for local agencies to approve or deny ADU permit applications.

AB 881 (2020): This bill clarified that ADUs could be built within existing structures such as garages, and it limited the ability of local governments to impose setback requirements on conversions of existing structures. If your garage is already built, the existing footprint is generally grandfathered in, even if it does not meet current setback standards.

SB 9 (2022): While primarily focused on lot splits and duplexes, SB 9 expanded options for homeowners in single-family zones, making it easier to add density through various means including garage conversions.

Law	Year	Impact on Garage Conversions
AB 68	2020	No replacement parking required; reduced impact fees
SB 13	2020	60-day permit approval timeline; fee reductions
AB 881	2020	Existing structures exempt from new setback rules
AB 2221	2023	Streamlined approval; clarified conversion rights

Parking Replacement: You Do Not Need It

One of the most common misconceptions about garage conversions is that you need to replace the parking spaces you are losing. Under California state law, local jurisdictions cannot require replacement parking when you convert a garage into an ADU, regardless of how many parking spaces the garage currently provides. This applies to both attached and detached garages.

If your local planning department tries to impose a replacement parking requirement, you can cite AB 68 and current state ADU law to push back against this demand. The state legislature specifically removed this barrier to encourage more ADU construction.

Step 1: Structural Assessment of Your Existing Garage

Before you commit to a garage conversion, you need a thorough structural assessment to determine what your existing garage can support and what modifications will be required. Not every garage is a good candidate for conversion, and understanding the condition of your structure early will help you budget accurately.

Foundation Evaluation

The foundation is the single most important structural element to evaluate. Most residential garages in California were built with one of two foundation types:

• Slab-on-grade: A concrete slab poured directly on the ground. This is the most common type and usually the easiest to work with for a conversion. The key question is whether the slab is thick enough (minimum 3.5 inches, though 4 inches is preferred) and whether it has adequate reinforcement.
• Stem wall with slab: A raised perimeter foundation with a slab floor. These are generally well-suited for conversion but may need additional insulation beneath the slab to meet energy code requirements.

Your structural engineer will check for cracks, settling, moisture issues, and whether the slab extends properly to support the new wall framing. If the foundation needs reinforcement, this can add $5,000 to $15,000 to your project cost, but it is not usually a project-ending issue.

Roof Structure and Load Capacity

Many garages have lightweight truss systems designed only to support the roof itself, not any additional ceiling loads like insulation, drywall, lighting, and HVAC components. Your engineer will assess whether the existing roof structure can handle these additional loads or whether reinforcement is needed.

Common roof modifications include:

• Adding collar ties or ridge beams for additional support
• Sistering rafters to increase load capacity
• Upgrading sheathing to meet current building codes
• Adding proper ventilation channels between insulation and roof deck

Wall Framing Assessment

Garage walls are often framed with 2x4 studs, which is adequate for most conversion projects. However, your engineer will verify that the framing is in good condition, properly anchored to the foundation, and capable of supporting any new window or door openings you plan to add.

The garage door opening is the most significant framing consideration. Converting this large opening into a wall requires proper header installation, new stud framing, and integration with the existing wall structure. Many homeowners choose to install a large window or sliding glass door in this location to maximize natural light.

Step 2: The Permit Process for Garage-to-ADU Conversions

The permit process for a garage conversion follows the same general path as other ADU projects, but there are some unique considerations. Here is what you can expect when working with your local building department.

Required Documents and Plans

For a typical garage conversion permit application, you will need to submit:

• Architectural plans: Floor plan, elevations, sections, and details showing the proposed conversion. These must be drawn to scale and include dimensions, materials, and specifications.
• Structural calculations: Prepared by a licensed engineer, showing that the existing structure and any modifications meet current building codes.
• Title 24 energy calculations: Demonstrating that the converted space will meet California's energy efficiency standards. This includes insulation values, window specifications, and HVAC efficiency ratings.
• Site plan: Showing the property boundaries, existing structures, setbacks, and the location of the garage being converted.
• MEP plans: Mechanical, electrical, and plumbing plans showing new systems and connections to existing utilities.

Timeline and Fees

Under California law, your local jurisdiction must approve or deny your ADU permit application within 60 days of receiving a complete application. In practice, the timeline often looks like this:

Phase	Duration	Notes
Plan preparation	3-6 weeks	Architect and engineer design
Plan check review	4-8 weeks	City reviews plans for code compliance
Corrections and resubmittal	2-4 weeks	Address any plan check comments
Permit issuance	1-2 weeks	Pay fees, receive approved plans

Permit fees for garage conversions in Los Angeles typically range from $3,000 to $8,000, depending on the scope of work. This includes plan check fees, building permit fees, and school fees. Some cities waive or reduce impact fees for ADUs under 750 square feet, which is a common size for garage conversions.

Step 3: Construction Phases for a Garage Conversion

Once you have your permit in hand, construction can begin. A typical garage conversion takes 3 to 5 months to complete, depending on the scope of work and the availability of subcontractors. Here is a breakdown of the major construction phases.

Phase 1: Demolition and Preparation (1-2 Weeks)

The first phase involves removing the existing garage door, any interior finishes, and any elements that will not be part of the final design. This includes:

• Removing the garage door and track hardware
• Demolishing any interior walls or partitions
• Removing existing electrical wiring that does not meet code
• Cleaning and preparing the concrete slab
• Identifying and marking utility locations

Phase 2: Structural Modifications (2-3 Weeks)

With the space cleared, structural work begins. This typically includes framing the former garage door opening, adding window and door openings, reinforcing the roof structure, and making any foundation repairs or upgrades identified during the structural assessment.

Phase 3: Rough-In (2-3 Weeks)

The rough-in phase is when all of the behind-the-wall systems get installed before the walls are closed up. This is one of the most critical phases because these systems are difficult and expensive to modify after the fact. The rough-in phase includes:

• Plumbing rough-in: Running supply and drain lines for the kitchen, bathroom, and laundry. This often requires cutting into the existing slab for drain lines, which is one of the most disruptive and expensive aspects of a garage conversion.
• Electrical rough-in: Installing a new subpanel (or upgrading the main panel), running circuits for outlets, lighting, appliances, and HVAC, and installing boxes for switches and receptacles.
• HVAC rough-in: Installing ductwork or running refrigerant lines for a mini-split system. Most garage conversions use ductless mini-splits because they are efficient and do not require duct space.

Phase 4: Insulation and Drywall (1-2 Weeks)

After the rough-in passes inspection, insulation is installed in all walls, the ceiling, and sometimes under the slab (if required by energy code). Common insulation options include fiberglass batts, blown-in cellulose, or spray foam. The insulation must meet California Title 24 requirements for the specific climate zone where your property is located.

Once insulation is inspected and approved, drywall is hung, taped, mudded, and sanded. This transforms the space from a construction site into something that starts to look like a real living space.

Phase 5: Finish Work (3-5 Weeks)

The finish phase is where everything comes together. This includes flooring installation, cabinet and countertop installation, plumbing fixture installation, electrical fixture and device installation, painting, trim work, and appliance installation. The finish phase also includes any exterior work such as siding, stucco, or paint to match the existing property aesthetic.

Complete Cost Breakdown for a Garage Conversion ADU

Understanding the full cost picture is essential for planning your garage conversion project. Costs vary significantly based on location, scope, finishes, and local labor rates, but the following breakdown provides a realistic range for California homeowners.

Cost Category	Low Estimate	High Estimate
Design and engineering	$5,000	$12,000
Permits and fees	$3,000	$8,000
Structural modifications	$8,000	$20,000
Plumbing	$8,000	$18,000
Electrical	$5,000	$12,000
HVAC (mini-split)	$3,500	$8,000
Insulation and drywall	$4,000	$8,000
Kitchen and bath finishes	$10,000	$25,000
Flooring	$3,000	$8,000
Exterior finish and paint	$2,000	$6,000
Total	$51,500	$125,000

Most garage conversions in the Los Angeles area fall in the $80,000 to $120,000 range for a standard 400 to 500 square foot unit with one bedroom, one bathroom, and a kitchenette. Higher-end finishes, larger garages, or significant structural work can push costs above $150,000.

Compared to a ground-up detached ADU, which typically costs $150,000 to $350,000 in California, a garage conversion can save you 30% to 50% on overall construction costs because the shell structure already exists.

Design Tips for a Garage Conversion ADU

Designing a livable space within the constraints of an existing garage requires careful planning and creative solutions. Here are the key design considerations that will help you maximize comfort and functionality.

Floor Level and Moisture

Garage slabs are typically several inches lower than the main house floor level and do not always have a moisture barrier beneath them. You may need to add a vapor barrier, raise the floor with a sleeper system, or apply a waterproof membrane to address moisture concerns. This also provides an opportunity to run plumbing drain lines in the raised floor system rather than cutting into the slab.

Natural Light and Ventilation

Garages typically have minimal windows, so adding natural light is a priority. The former garage door opening provides a perfect opportunity for a large window, French doors, or a sliding glass door. California building code requires operable windows in bedrooms (for egress) and bathrooms (for ventilation), so plan window locations carefully.

Ceiling Height

Many garages have ceiling heights between 8 and 10 feet, which is adequate for a comfortable living space. If your garage has exposed trusses, you may be able to vault the ceiling for a more spacious feel, though this adds cost for structural modifications and additional insulation.

Layout Optimization

For a typical two-car garage (approximately 400 to 500 square feet), a well-designed layout might include a combined living and sleeping area, a compact kitchen along one wall, and a bathroom in one corner. Open floor plans work best in smaller garage conversions, while larger garages can accommodate separate bedroom and living spaces.

Common Challenges and How to Solve Them

Every garage conversion comes with its own set of challenges. Here are the most common issues California homeowners encounter and proven solutions for each.

Challenge: Slab Plumbing

Running drain lines through an existing concrete slab is one of the most expensive and disruptive aspects of a garage conversion. The concrete must be saw-cut, excavated, and then patched after plumbing is installed.

Solution: Minimize the distance between kitchen and bathroom plumbing to reduce the amount of slab cutting required. Consider locating the bathroom and kitchen on the same wall to share drain lines. Some homeowners use a raised floor system to run plumbing above the slab, though this reduces ceiling height by 4 to 6 inches.

Challenge: Inadequate Electrical Service

Most garages are served by a single 20-amp circuit from the main house panel. An ADU requires its own electrical panel with multiple circuits for lighting, outlets, appliances, and HVAC.

Solution: You will likely need either a subpanel fed from the main house panel or a new service drop from the utility company. Your electrician will assess the existing main panel capacity and recommend the best approach. Upgrading the main panel to 200 amps is sometimes necessary and costs $2,000 to $4,000.

Challenge: Noise Transfer

Garage walls and ceilings often have minimal insulation, leading to noise issues between the ADU and the main property.

Solution: Use sound-rated insulation (such as mineral wool batts) in shared walls, and consider adding a layer of mass-loaded vinyl beneath the drywall for additional sound isolation. If the garage shares a wall with the main house, double-stud construction or resilient channel can further reduce noise transfer.

Frequently Asked Questions About Garage Conversions

How much does it cost to convert a detached garage into an ADU in California?

A typical detached garage conversion in California costs between $80,000 and $150,000, depending on size, location, structural condition, and finish level. In the Los Angeles area, the average cost is approximately $100,000 to $120,000 for a standard one-bedroom, one-bathroom unit. This is significantly less than a ground-up ADU, which typically starts at $150,000 and can exceed $300,000.

Do I need to replace the parking spaces I lose when converting my garage?

No. Under California state law (AB 68), local jurisdictions cannot require replacement parking when you convert an existing garage into an ADU. This applies statewide, regardless of your local municipality's parking requirements. If your city tries to impose a replacement parking requirement, you can cite state law to override this demand.

How long does a garage conversion take from start to finish?

Including the design, permitting, and construction phases, a typical garage conversion takes 6 to 9 months from initial planning to move-in. The design and permitting phase usually takes 2 to 4 months, while construction takes 3 to 5 months. Some cities have expedited permit processing for ADU projects, which can shorten the overall timeline.

Can I convert a one-car garage into an ADU?

Yes, you can convert a one-car garage into an ADU, though the smaller footprint (typically 200 to 280 square feet) means the unit will function as a studio or Junior ADU (JADU). California has no minimum size requirement for ADUs beyond basic habitability standards, so even a small garage can be converted into a legal dwelling unit with proper kitchen and bathroom facilities.

Will a garage conversion increase my property value?

Yes. Garage conversions typically increase property value by 20% to 30% of the conversion cost. In high-demand rental markets like Los Angeles, the return on investment can be even higher because the rental income potential adds significant value. A garage conversion ADU that rents for $1,500 to $2,500 per month can pay for itself within 4 to 7 years through rental income alone.

Do I need a separate address and utility meters for my garage conversion ADU?

California does not require separate utility meters for ADUs, though some homeowners choose to install them for billing purposes, especially when renting to tenants. Most cities will assign a separate address to the ADU for emergency services purposes, but this process is straightforward and handled as part of the permit process.