Cost and money

What does an ADU cost?

Checked against state laws and official agency sources · Last reviewed October 9, 2026

What ADUs have cost: the research

A research team at UC Berkeley's Center for Community Innovation ran California's first statewide survey of ADU owners. The Terner Center summarized the results on April 22, 2021. Among its findings:

• The median statewide construction cost of an ADU was $150,000, or $250 per square foot.
• 37 percent of new ADUs cost less than $100,000 to build.
• 71 percent cost less than $200,000.

An earlier report from the Terner Center and the Center for Community Innovation, from August 2020, put the average California ADU cost at $167,000. It found an average of $148,000 in Los Angeles and $237,000 in the Bay Area, a gap it said was "driven largely by differences in labor costs." It also said Bay Area costs "can exceed $800 per square foot, equaling $400,000 for a 500 square foot ADU."

These are California figures from 2020 and 2021. Treat them as a starting point, not a quote. Get bids for your own lot. [1][2]

The parts of the bill

Construction is the biggest part, but not the only one. California's housing finance agency lists the "pre-development costs" of an ADU as "site prep, architectural designs, permits, soil tests, impact fees, property surveys, and energy reports."

A simple way to budget is to price each of these separately:

• Design: plans and drawings. In California, each city or county must have a program for pre-approved ADU plans, which can cut design work (see below).
• Permits and plan review: set by your city. See our city pages, such as Los Angeles and San Diego, and the permit process guide.
• Impact fees: fees that help pay for public facilities. California limits them for small ADUs.
• Utility connections: connection fees or capacity charges for water and sewer.
• Site work: site prep, soil tests and surveys.
• Construction: the building itself.

[3][4]

Impact fees: California's 750-square-foot rule

California law limits impact fees on ADUs. California's housing department (HCD) puts it this way in its ADU Handbook (updated March 2026):

• "An ADU is exempt from incurring impact fees from local agencies, special districts, and water corporations if it contains 750 square feet of interior livable space or less."
• For a bigger ADU, "impact fees shall be charged proportionately in relation to the square footage of the ADU to the square footage of the primary dwelling unit." HCD's example: a 1,000-square-foot ADU next to a 2,000-square-foot house "could result in 50 percent of the impact fee" charged for a new house.
• A junior ADU (JADU) of 500 square feet of interior livable space or less is also exempt from impact fees.

School fees are separate. HCD says school districts may levy them for ADUs "larger than 500 square feet of interior livable space." ADUs and JADUs under 500 square feet "are not subject to school impact fees."

Change starting January 1, 2027. The Governor approved SB 1117 on September 29, 2026 (Chapter 809, Statutes of 2026). For up to two ADUs on a lot, impact fees on an ADU over 750 square feet will be charged proportionately "and only on the area in excess of 750 square feet of interior livable space." Lots with more than two ADUs keep the current proportional rule. HCD's 50 percent example above reflects the rule before this change. SB 1117 is not an urgency bill, so it takes effect under the normal rule for new California laws: on "January 1 next following a 90-day period from the date of enactment," which is January 1, 2027.

These limits are California law. For other states, see our state pages, such as Washington, Oregon and Colorado. [4][5][6][7]

Utility connection fees

In California, an ADU or junior ADU is not treated as a new home for utility fees. State law says an ADU or JADU "shall not be considered by a local agency, special district, or water corporation to be a new residential use for purposes of calculating connection fees or capacity charges for utilities, including water and sewer service, unless the unit was constructed with a new single-family dwelling."

For an ADU or JADU built within the space of an existing or proposed house or accessory structure, the provider can't require a new or separate utility connection or charge a related connection fee or capacity charge. The exceptions are a unit built with a new single-family home, or an ADU sold separately.

For other ADUs, such as a new detached unit, the provider may require a new or separate connection. HCD says the connection fee or capacity charge "shall be proportionate to the burden of the proposed ADU, based on its square footage or drainage fixture unit as compared to the primary dwelling."

Ask your water and sewer provider for their exact charges early. [4][8]

Garage conversions and junior ADUs

Converting space you already have changes the fee picture in California. ADUs and JADUs built within existing space can't be required to get a new or separate utility connection or pay a related connection fee, unless built with a new single-family home. JADUs of 500 square feet or less also avoid impact fees.

We have not found a recent government or university study that compares construction costs by ADU type, so we don't publish per-type figures. For the details, see our guides to garage conversions and junior ADUs. For factory-built units, see prefab ADUs. HCD notes that "by definition, an ADU includes manufactured homes." [4][8]

Ways to lower design and fee costs

Pre-approved plans (California). State law required each local agency to "develop a program for the preapproval of ADU plans by January 1, 2025" and to post the plans on its website. For a detached ADU using a pre-approved plan, the agency must approve or deny the application within 30 days.

Fee waivers and local help. HCD says "some local agencies are creating funding opportunities for ADU/JADU predevelopment and construction costs, or they are waiving fees." In Colorado, the state's ADU Grant Program funds certified local governments. They can use it for pre-approved ADU plans, technical help, and "waiving, reducing, or providing financial assistance for ADU-associated fees." Ask your city whether it takes part.

For loans and grants, see ADU financing. [4][9]

Estimate your own cost and rent payback

Use our ADU cost and rent payback calculator to plug in your own bids, fees and expected rent.

For a sense of rents, the same 2021 survey found "the median rental price of a new ADU in California is $2,000." It ranged "from $1,925 in the Central Coast region to $2,200 in the San Francisco Bay Area." Rents where you live may be very different. [1]

Questions people ask