How much does it cost to build an ADU in California?

An Accessory Dwelling Unit (ADU) may be used to work from home, accommodate house guests, or provide independent living quarters for an older or younger relative, keeping multigenerational families together while maintaining individual privacy. It may be rented out as a short-term rental or to a long-term tenant. But how much does it cost to build an ADU in California?

Note: All of the gathered information is based on research conducted at the time of publication. All costs are average costs collected from various sources. For an accurate estimate to build your potential ADU, contact Top Home Builders.

Calculating The Costs of Building an ADU

Defined as a self-contained residential unit located on the same property as a main or primary single-family home, it may be attached to the main house, or a separate, standalone structure.

The cost to build an ADU in California may range from $150,000 to $350,000+. Averaging the costs of new detached or attached units brings us to the slightly narrower $200,000 to $300,000. That’s still a wide range. We can narrow down this estimate further by selecting a type and breaking down the process so you can add up the steps that your property and plans will require.

Types of ADU + Cost

As long as it’s not the main residence on the property, and it contains all the necessities for living, it’s an ADU. So, an attic apartment, a converted garage, an addition above a garage, an addition to the side or back of a house, and an independent structure are all ADUs. Adding a kitchen and bathroom to a second story with one or more bedrooms can turn it into an ADU. Each of these types has its own price range.

Here are the main types recognized in California:

Junior ADU (JADU): In California, a JADU may measure no more than 500 square feet. JADUs are typically bedrooms in single-family homes that have an entrance into the unit from the main home and another entrance to the outside from the JADU. The JADU must have a basic kitchen, utilizing small plug in appliances, but is not required to have a private bathroom.
Above-Garage ADU: Built on top of an existing garage, this ADU is a way to add living space without reducing ground-level outdoor space. It may or may not be attached to the rest of the existing home. Caution should be exercised with this type to ensure that carbon monoxide from vehicles and/or a water heater in the garage does not infiltrate the living space.
Attached ADU: This is any ADU structure that has a connection to the primary residence on the property. It may be built on to the existing structure, or it may be a conversion of an existing space. Since it may share one or more walls with the main home, it usually costs less than a detached ADU.
Garage Conversion ADU: Since a garage may be attached or detached, a garage conversion may also be either. Plumbing, electrical, HVAC (Heating, ventilation, and Air-Conditioning) will be necessary in addition to interior finishes and space division.
Detached ADU: This is a separate, standalone structure with no attachment to the primary residence. While it is often built in the backyard, it can be built anywhere on the property. It may be a new construction, or it may use an existing structure such as a garage, old servants’ quarters, or old stable. The ADU cost in California for this type tends to be on the higher end due to the need for independent utilities and construction.
Interior Conversion ADU: This is the type of attached ADU that is created by converting a space already within the primary residence, like a basement, an attic, or a portion of a floor.

Offering the simplest and most affordable housing option, JADUs bridge the gap between a roommate and a tenant. The doors between the two spaces can be secured from both sides, These units share central systems, and require no fire separation.

Each type of ADU has its own set of regulations, benefits, and challenges. Which type will suit you and your property best depends on factors like local zoning laws, property characteristics, and homeowner preferences.

Types of ADU and Cost

Type of ADU	Average Minimum Cost	Average Maximum Cost
Detached	$200,000	$300,000
Attached	$150,000	$250,000
Interior Conversion	$80,000	$150,000
Garage Conversion	$100,000	$175,000
Junior (JADU)	$50,000	$100,000
Above-Garage	$150,000	$250,000
Prefabricated or Modular	$150,000	$250,000
Carve-out	$75,000	$125,000

Factors of Your ADU Cost California

There are numerous factors that can affect the ADU cost in California which homeowners face. These include size, type, site preparation, foundation, framing, professional services, exterior and interior finishes, utilities, waste disposal, and materials. It is wise to budget 10-20% of the total price for contingencies, as well. These are the unforeseen things like site and construction conditions that can bump up costs.

Average Size: 400 to 800 square feet.
Smaller ADUs: Many municipalities have minimum size requirements around 150-300 square feet
Larger ADUs: In some areas, the maximum allowed size for a detached ADU is 1,200 square feet.

The difference between the least expensive option (JADU) and the most expensive (Detached ADU) can be as much as 500% or more. Even within the same type, costs can vary by 100-200% depending on size, location, and quality of finishes.

Site preparation will include assessment, grading, and excavation. Some consider utility connections (water, sewer, electricity, gas) to be included, but that information is listed separately below.

The type of foundation that your ADU requires will depend upon the type of ADU (some require no foundation) and your site conditions, so we have listed a broad price range for several types of foundation, including concrete slab, stem wall, pier and beam, and basic perimeter.

There are various types of framing available for an ADU in California, but the most common is Wood Platform, and the price per square foot doesn’t change if you’re building one or two stories.

Professional services are mostly about design, and include architectural and interior design planning and drawing, any engineering fees that may be required, and project management. Permitting assistance may be basic or advanced, and will really help smooth and speed the permitting process.

Exterior finishes will include whatever covers the framing and insulation, usually some form of siding, and any windows and doors that are added.

These include drywall, interior panels, flooring, cabinets, and appliances. These determine the look and feel of your ADU.

You may need to run each utility from the street or from your existing residence, and how far the connection has to travel will determine its cost, so the further from your home, the higher these costs will be.

It is wise to set aside part of your budget for the unforeseen. This may include site conditions or construction conditions. We recommend 10-20% of your total project cost to be set aside in addition to that cost for this part of the budget.

This will include dumpster rental, debris removal, and recycling fees. Dumpster rental prices often include the cost of removing the dumpster and all of its contents. If so, there will be no separate debris removal cost.

This is a big one. Every step of the project will require materials. Raw materials are listed below in prices per square foot, finished materials such as door knobs and windows are priced by the unit.

Check out this breakdown of costs involved in building an ADU:

ADU Cost Component		Cost
Total Soft Costs		$14,000 – $55,000
Architecture & Design		$5,000 – $20,000
Engineering		$2,000 – $10,000
Project Management		$5,000 – $20,000
Basic Permitting Assistance		$2,000 – $5,000
Total Legal Costs		$12,000 – $50,000
Permits		$5,000 – $15,000
Impact fees		$5,000 – $30,000+ in some areas
Specialized studies (if required)		$2,000 – $5,000+ each
Site Preparation	Site assessment, grading, and excavation	$15,000 – $30,000
Total for Utility Connections		$6,000 – $28,000
	Water	$1,500 – $7,000
	Sewer (Septic System if Necessary)	$2,500 – $11,000
	Electricity (From main house or separate meter)	$750 – $4,500
	Metered gas line from main supply	$1,250 – $5,500
Foundation	Includes the price range for Concrete Slab, or Stem Wall, or Pier and Beam, or Basic Perimeter	$4 to $16 per square foot
Framing	Wood Platform for single or double story ADU	$15 to $32.50 per square foot including labor
Total for Materials		$60,000 – $100,000
	Roofing materials	$120 to $400 per square foot
	Wood framing materials	$3.50 – $7.00 per square foot
	Exterior siding	$12 per square foot
	Windows	$100 to $1,500 each
	Doors	$500 to $5,000 each
	Insulation	$0.30 to $1 per square foot
	Drywall	$4 to $6 per square foot
	Flooring	$3.50 to $21 per square foot
	Plumbing Pipes	$15 to $40 per square foot
	Electrical Wiring	$2 to $4 per square foot
	Electrical Components	$8,000 – $15,000
	Kitchen cabinets	$100 – $300 per linear foot
	Kitchen countertops	$15 – $100 per square foot
	Bathroom fixtures	$1,500 – $2,500
	Kitchen plumbing fixtures	$130 – $1000
HVAC	New system including ductwork and installation (Existing HVAC may be used for an attached ADU)	$5,000 – $15,000
Total Labor		$50,000 – $80,000
	General contractor fees	10% – 20% of the total project costor $30,000 to $70,000
	Specialized trade labor (plumbers, electricians, etc.)	$75 – $180/hour
Total Finishing		$39,000 – $101,000
	Interior Finishes	$30,000 – $80,000
	Trim and molding	$2,000 – $5,000
	Interior painting	$3,000 – $6,000
	Trim and molding	$2,000 – $5,000
	Lighting fixtures	$2,000 – $5,000
Total Waste Disposal		$2,000 – $5,000
	Dumpster rental	$400 – $1000/week
	Construction debris removal	$2,000 – $5,000
	Recycling fees	$0.50 – $2.00 per square foot
ADU Landscaping		$2,000 – $10,000
Contingency (10-15%):		$20,000 – $70,000

Next Steps to Start Building Your ADU

The average cost of an ADU in California comes to $250,000.00. As we have seen, that is by no means exactly what you can expect to pay for an ADU build, as there are a LOT of variables. Many of these, however, are within your control. An ADU will definitely increase the value of your property, and may create a regular income stream, depending upon how you choose to use it.

The main variables are:

Location (urban vs. rural, coastal vs. inland)
Type – A junior ADU is the least expensive, a detached ADU is the most expensive.
Size and complexity of the unit
Quality of materials and finishes
Local labor and material costs
Permit fees and local regulations

Top Home Builders can be your general contractor and supply all of the above according to your choices for a lump sum, saving you work, and worry. We can start the process as soon as you can contact us. Once the plans have been approved, we can give you an accurate estimate of the overall cost and time that building a specific ADU will take.

Frequently Asked Questions

The average minimum cost of a JADU is $50,000. This would be for one bedroom and a kitchenette with a cooktop, and the least expensive interior finishes, built within the walls of your existing home.

For a long-term rental, the median rate of a two-bedroom ADU in California is $2,800 monthly, or $33,600 per year.

For a short-term rental, in desirable vacation locations you might make from $1,000 to $3,000 or more per month. Several urban areas in California have banned short-term rentals of buildings that are not the landlord’s primary residence. Other areas have added requirements for registration and taxation, so be sure to familiarize yourself with the regulations in your area before choosing short-term rental.

Nationwide, an ADU will add 20% to 30% to a home’s value. In California, the addition in value may exceed 30%. A good way to estimate this is to multiply the monthly rental value of the ADU by 100, so a unit renting for $2,800/month would add $280,000 to the overall value of the property.

The California Housing Finance Agency’s (CalHFA) ADU Grant Program provides grants up to $40,000 to reimburse pre-development and non-recurring closing costs (fees associated with borrowing to finance) for the construction of an ADU. Pre-development costs include site prep, architectural designs, permits, soil tests, impact fees, property surveys, and energy reports.

Yes! In a press release dated October 16, 2023, the U.S. Department of Housing and Urban Development, through the Federal Housing Administration (FHA) announced a new policy which allows lenders to count income from ADUs built inside, attached to, or on the same property as a primary residence when underwriting a mortgage.

It will allow the use of 50% of the estimated rental income from a new ADU to qualify for a mortgage under the Federal Housing Authority’s renovation loans, also known as 203(k) Rehabilitation Mortgage Insurance program.

The same policy will also allow 75% of the estimated ADU rental income for some borrowers to qualify for an FHA-insured mortgage on a property with an existing ADU, making your home-with-ADU combination easier to sell.

The ADU cost per square foot in California can vary widely depending on factors such as location, materials, and design. However, on average, you can expect to pay between $300 and $600 per square foot for a new ADU construction.

The ADU unit cost is highest for a detached ADU.

A JADU, or Junior ADU, is the least expensive type.

There are many considerations to keep in mind, many of which depend upon you, your property, and your intended ADU use.

Prefab units are made in a factory, while a custom ADU will be constructed on your property specifically according to your needs and specifications.