Attached ADU in California: Size, Height, and Setbacks

An attached accessory dwelling unit in California is a self-contained second home that shares a wall, roof, or foundation with your main house, and state law requires your city to approve one so long as the plans meet a specific set of objective standards. Those standards cover size, height, setbacks, parking, fire safety, and fees, and they override most of what your local zoning code would otherwise impose. If your project checks the boxes, the approval is ministerial: no hearing, no discretionary design review, no neighbor veto.

What Qualifies as an Attached ADU

An attached ADU is a complete, independent dwelling physically connected to the primary residence through at least one shared wall, foundation, or roofline. It must have its own permanent facilities for living, sleeping, eating, cooking, and sanitation, all functioning without reliance on the main house.¹

This is a different animal from a Junior ADU. A JADU is capped at 500 square feet, must sit entirely within the existing walls of the single-family home (including an attached garage), needs only an efficiency kitchen, and can share a bathroom with the main house.² An attached ADU needs a full kitchen and bathroom of its own and can be considerably larger.

The structural legal protection that runs through everything below is ministerial approval. Your local agency has to evaluate the application against pre-set, objective standards and nothing else. It cannot deny a qualifying project on aesthetic grounds, cannot subject it to a public hearing, and cannot stack on discretionary review.³

How Big an Attached ADU Can Be

Attached and detached ADUs follow different size rules, and this is where many homeowners get the law wrong. A detached ADU is capped at 1,200 square feet. An attached ADU is capped at 50 percent of the primary dwelling’s existing floor area, and the 1,200-square-foot ceiling does not apply.³

The 50 percent rule has a floor beneath it. State law bars local agencies from setting maximum size requirements lower than 850 square feet for a studio or one-bedroom unit, or lower than 1,000 square feet for a unit with two or more bedrooms.³ So if your home is 1,500 square feet, half of that is 750 square feet, but your city must still allow at least an 850-square-foot one-bedroom or a 1,000-square-foot two-bedroom.

There is a further backstop. No combination of local development standards (lot coverage, floor area ratio, open space) can block you from building at least an 800-square-foot ADU with four-foot side and rear setbacks.³ On the other end, cities cannot set minimums that prohibit an efficiency unit.

Height Limits

Attached ADUs get the most generous height allowance in the statute. A local agency must permit an attached ADU to reach 25 feet or whatever height limit 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 to 25 feet. If zoning caps the primary residence at 22 feet, that lower number controls. Two stories is an absolute ceiling regardless of the number in feet.

Detached ADUs start lower: 16 feet on most lots, rising to 18 feet near major transit stops or on parcels with multistory multifamily buildings. The 25-foot allowance belongs to attached units specifically, and it is one 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. If you are converting an existing living area or accessory structure into an ADU, or rebuilding in the same footprint and dimensions as an existing structure, no setback is required at all.³ Fire and life safety codes still apply on their own terms, which can mean fire-rated materials or assemblies when you build near the property line.

Parking rules have been loosened to the point that most ADU 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 within a historic district. When parking is required, your city cannot demand more than one space per unit or per bedroom, whichever is less.

Fire Separation and Sprinklers

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

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

The Permitting Timeline

You submit the permit application to your local planning or building department with full architectural and structural plans, site plans, and utility connection information. From there, the state controls the clock.

Under legislation effective January 1, 2026, the agency has 15 business days from receipt to determine whether your application is complete and send you written notice. An incompleteness 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 that 60 days, the application is deemed approved by operation of law.³ No other local ordinance, policy, or regulation can be invoked to stall or deny a qualifying application. The deemed-approved rule is what gives the statute real force.

Impact Fees and Utility Connections

Cost surprises in the permit phase usually come from impact fees and utility connection charges, and state law now limits both.

Impact fees are fully waived for any ADU with 750 square feet or less of interior livable space. For an attached ADU above 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 house.¹

For water, sewer, and electrical hookups, local agencies and utilities cannot treat an ADU as a new residential use when setting connection fees or capacity charges. Any fees charged must be proportional, based on the ADU’s 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 the ADU is being built at the same time as a new primary residence.¹ For a newly built attached ADU outside the conversion rules, a utility may require a separate connection, but the proportional fee cap still applies.

Renting the Unit and Owner Occupancy

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

The one rental limit the state allows is a minimum lease term. Local agencies may require that ADU rentals run longer than 30 days, which has the effect of blocking short-term vacation rentals.¹ Whether your city has adopted that restriction varies, so confirm with your local ordinance before building around a short-term rental plan.

Selling the ADU Separately

For almost every homeowner, the ADU cannot be sold apart from the primary residence. It stays with the property. One narrow exception exists where a qualified nonprofit corporation develops the property under specific affordability restrictions, recorded tenancy-in-common agreements, and repurchase options designed to preserve long-term low-income housing.⁵ That exception does not reach the typical homeowner-built project.

Property Tax

Building an ADU raises 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 current assessment without reassessing the primary residence. If your home is currently assessed at $500,000 and the assessor values the new ADU at $150,000, your new assessment is $650,000. The ADU portion then rises by no more than 2 percent per year, on the same track as the rest of your assessment.

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