Attached ADUs in California: Size, Setbacks, and Permits

An attached accessory dwelling unit in California is a self-contained second home that shares a wall, foundation, or roofline with your primary residence, and state law requires your local agency to approve one ministerially when it meets objective standards for size, height, setbacks, and safety. The size cap is 50 percent of your existing home’s floor area, with state-guaranteed minimums that override that percentage on smaller lots. Height can reach 25 feet and two stories. Setbacks top out at four feet. Cities cannot require you to live on the property, cannot hold discretionary hearings, and lose their ability to deny the application if they miss a 60-day deadline.

What Counts as an Attached ADU

An attached ADU is a complete dwelling that shares at least one wall, a foundation, or a roofline with the primary residence. It must include permanent facilities for living, sleeping, eating, cooking, and sanitation, all functioning independently from the main house.¹ It is a full apartment that happens to be physically connected to your home.

Do not confuse the attached ADU with a Junior ADU. A JADU is capped at 500 square feet, must be built entirely within the existing walls of the single-family residence including an attached garage, and can share bathroom facilities with the main house.² An attached ADU needs its own full kitchen and bathroom and can be substantially larger.

The legal feature that shapes every attached ADU project is ministerial approval. Local agencies must evaluate your application against objective, pre-established standards only. They cannot impose discretionary review, hold a public hearing, or apply subjective design criteria to deny a qualifying project.³ If your plans check every box, the city has no authority to say no.

Size Limits

Attached and detached ADUs are sized differently, and this is where homeowners most often plan wrong. A detached ADU is capped at 1,200 square feet. An attached ADU is capped at 50 percent of the existing primary dwelling’s floor area. The 1,200-square-foot ceiling does not apply to attached units.³

The 50 percent rule has a floor beneath it. Local agencies cannot set maximum sizes lower than 850 square feet for a studio or one-bedroom, or lower than 1,000 square feet for a unit with two or more bedrooms.³ If your primary home is 1,500 square feet, 50 percent gives you 750, but your local agency must still allow an 850-square-foot one-bedroom or a 1,000-square-foot two-bedroom design.

A separate safe harbor catches situations where lot coverage limits, floor area ratios, or open space rules would otherwise squeeze a project below a usable size. No combination of local development standards can prevent you from building at least an 800-square-foot ADU with four-foot side and rear setbacks.³ At the small end, local agencies cannot set minimums that would prohibit building an efficiency unit.

Height Limits

Attached ADUs get the most generous height treatment in the statute. Local agencies must allow an attached ADU to reach 25 feet or the height limit that local zoning applies to the primary dwelling, whichever is lower. The unit cannot exceed two stories.³

If your neighborhood’s zoning allows a 30-foot primary residence, your attached ADU can go up to 25 feet. If zoning caps the primary residence at 22 feet, that lower number controls the ADU as well. The two-story cap holds regardless of the height in feet. Detached ADUs start much lower, at 16 feet on most lots, so the 25-foot allowance is a real advantage of building attached rather than detached.

Setbacks and Parking

For a newly constructed attached ADU, state law caps required side and rear yard setbacks at four feet. No local agency can demand more.³ If you are converting existing living area or an accessory structure, or rebuilding in the same footprint and dimensions as an existing structure, no setback is required at all. Fire and life safety codes still apply on their own terms, and your building department may require fire-rated assemblies when you build close to a property line.

Parking rules have been relaxed so far that most projects need no additional space. No parking can be required if the property sits within half a mile of a major transit stop, if the unit is created by converting existing space, or if the property is in a historic district. When parking is required, the local agency cannot demand more than one space per unit or per bedroom, whichever is less.

Fire Safety Requirements

Because an attached ADU shares structural elements with the primary residence, fire separation matters. A shared wall between dwelling units typically needs a one-hour fire-resistance rating under the California Residential Building Code, meaning fire-rated drywall assemblies, sealed penetrations, and proper firestopping.

Sprinklers follow a simple rule. If the primary dwelling already has a sprinkler system, the ADU must have one too. Adding an ADU does not trigger a requirement to retrofit sprinklers into an existing unsprinklered home. When both the ADU and the primary residence are newly constructed together, sprinklers are required throughout.⁴

Permit Timeline and Ministerial Approval

You submit your application to the local planning or building department with architectural and structural plans, a site plan, and utility connection information. The agency then runs against strict state deadlines.

Under legislation effective January 1, 2026, the completeness window is 15 business days. The agency must decide whether your application is complete and send written notice within that period, and any incomplete notice must specify exactly what is missing.¹

Once the application is deemed complete, the agency has 60 days to approve or deny it. If the agency fails to act within those 60 days, the application is automatically deemed approved.³ No local ordinance can be used to delay or deny a qualifying application on other grounds. This deemed-approved provision gives the law real teeth; cities that stall lose their leverage.

Impact Fees and Utility Connections

Cost surprises in the permitting phase usually come from impact fees and utility charges. State law now blunts both.

ADUs with 750 square feet or less of interior livable space are fully exempt from impact fees. If your attached ADU exceeds 750 square feet, the impact fee must be proportional to the ADU’s square footage relative to the primary dwelling, not calculated as if the ADU were a standalone home.¹

For water, sewer, and electrical connections, local agencies and utilities cannot treat an ADU as a new residential use when calculating connection fees or capacity charges. Any fees imposed must be proportional based on square footage or plumbing fixtures compared to the primary dwelling. ADUs created by converting existing space within the home or an accessory structure cannot be required to install a separate utility connection at all, unless built at the same time as a new primary residence.¹ For newly built attached ADUs that don’t fall under the conversion rules, the utility can require a separate connection, but the proportionality cap on fees still applies.

Rental Rules and Owner Occupancy

California prohibits local agencies from requiring you to live on the property as a condition of having an ADU.³ You can rent out both the main house and the ADU, or live elsewhere entirely.

The one rental limitation the state allows is a minimum lease term. Local agencies can require ADU rentals to run longer than 30 days, which effectively blocks short-term vacation rentals through platforms like Airbnb.¹ Whether your city imposes this varies. Check your local ADU ordinance before planning a short-term rental strategy.

Selling an ADU Separately

In almost all cases, you cannot sell the ADU as a separate unit from the primary residence. The ADU stays with the property. The sole exception involves a narrow program where a qualified nonprofit corporation develops the property and the sale follows specific affordability restrictions, recorded tenancy-in-common agreements, and repurchase options designed to preserve long-term low-income housing.⁵ For the typical homeowner-built project, this does not apply.

Property Tax Effect

Building an ADU will raise your property taxes, but only by the assessed value of the new construction. Under Proposition 13, the county assessor adds the market value of the ADU to your existing assessment without reassessing the primary residence. If your home is currently assessed at $500,000 and the assessor values the ADU at $150,000, your new assessment is $650,000 with the original home’s value unchanged. The ADU portion then increases by no more than 2 percent per year going forward, like the rest of your assessment.

Financing an Attached ADU

Construction costs for an attached ADU in California generally run between $200 and $450 per square foot depending on finish level, site conditions, and local labor costs. High-end custom projects can exceed $500 per square foot. For an 850-square-foot one-bedroom, a realistic budget is roughly $170,000 to $380,000 before permits and fees.

Several financing paths exist. Freddie Mac’s CHOICERenovation mortgage allows borrowers to finance ADU construction through a purchase or refinance, and it offers a no-cash-out refinance option to pay off short-term construction financing after the ADU is complete. Freddie Mac permits one ADU on properties with up to three units, and the ADU must comply with local zoning.⁶

Fannie Mae, as of March 2026, allows projected rental income from an ADU to count toward your qualifying income on purchase and limited cash-out refinance loans for owner-occupied one-unit properties. The ADU rental income cannot exceed 30 percent of your total qualifying income, and only one ADU’s rental income counts even if the property has multiple units.⁷ Being able to use projected rent to qualify can meaningfully expand borrowing power.

Home equity lines of credit and cash-out refinances remain the most common approaches for homeowners with significant equity. Construction loans are another option but typically carry higher interest rates and require draws against a predetermined schedule. Verify early that your lender has experience with ADU projects, since underwriting varies.

Insurance

A standard homeowners policy often will not fully cover an attached ADU, particularly if you rent it out. If the ADU is part of the dwelling structure, it may fall under your existing dwelling coverage, but confirm with your insurer that the added square footage and value are reflected in your policy limits. Underinsuring the structure is a common and expensive oversight.

If you rent the ADU to a tenant, long-term or short, you typically need landlord or rental property insurance to cover tenant-related risks, property damage, and liability for injuries on the premises. Some insurers offer a home-sharing endorsement that can be added to an existing policy for short-term rentals; others require a separate landlord policy. An umbrella policy provides additional liability protection beyond the limits of your homeowners or landlord policy and is worth considering given the exposure that comes with tenants on your property.⁸

1
California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
2
California Legislative Information. California Code Government Code 65852.22 – Junior Accessory Dwelling Units
3
California Legislative Information. California Code Government Code 65852.2 – Accessory Dwelling Units
4
California Department of Housing and Community Development. IB 25-004 Accessory Dwelling Unit
5
California Board of Equalization. Accessory Dwelling Units Sale or Separate Conveyance
6
Freddie Mac. Accessory Dwelling Units Fact Sheet
7
Pennymac. Fannie Mae Updates to Accessory Dwelling Unit ADU Rental Income
8
Liberty Mutual. Accessory Dwelling Units and Short-Term Rentals What Is and Isnt Covered