What Is the Maximum ADU Size in California? Detached, Attached, JADU

The maximum ADU size in California depends on the type of unit: a detached accessory dwelling unit can be up to 1,200 square feet, an attached ADU is limited to 50 percent of the primary home’s floor area, and a junior ADU tops out at 500 square feet. These caps are set by state statute, and local governments have narrow room to shrink them.¹

Detached ADUs: Up to 1,200 Square Feet

A newly constructed detached ADU, meaning one that stands as its own structure on the lot, can be built up to 1,200 square feet of floor area under California Government Code Section 65852.2 (recodified as Section 66314 and related sections effective 2024).¹ The ceiling is a hard cap set by the state, and it doesn’t depend on how big your main house is. A 900-square-foot primary dwelling still permits a 1,200-square-foot detached ADU.

Local jurisdictions can adopt their own ADU ordinances, but they cannot set a maximum size below 850 square feet for a studio or one-bedroom unit, or below 1,000 square feet for a unit with two or more bedrooms. So if your city caps detached ADUs below 1,200 square feet, that cap still has to allow at least 1,000 square feet for a two-bedroom design.

The state also tells cities their development standards — lot coverage, floor area ratio, setbacks — must at minimum allow an 800-square-foot ADU with four-foot side and rear yard setbacks.¹ That provision is a backstop against indirect regulations that would block a compliant ADU through the side door.

Attached ADUs: Half the Main House

An attached ADU, meaning one that shares a wall with the main house, follows a percentage rule rather than a flat square-footage cap. State law limits an attached ADU to 50 percent of the existing primary dwelling’s floor area.¹ A 2,000-square-foot home allows a maximum attached ADU of 1,000 square feet. A 3,000-square-foot home would theoretically allow 1,500 square feet, though many local ordinances independently cap attached ADUs at 1,200 square feet.

The same local minimums protect attached ADUs. No ordinance can set a maximum below 850 square feet for a studio or one-bedroom, or below 1,000 square feet for a two-bedroom unit.¹ In cities without a compliant ADU ordinance, the default rule allows an attached ADU of up to 50 percent of the primary dwelling with a guaranteed minimum of at least 800 square feet.²

Junior ADUs: 500 Square Feet, Inside the House

A junior accessory dwelling unit is a smaller, simpler unit limited to 500 square feet under Government Code Section 65852.22.³ Unlike a standard ADU, a JADU must be built entirely within the walls of an existing or proposed single-family residence. Enclosed spaces like an attached garage count as part of the residence for this purpose.

A JADU must include an efficiency kitchen with a cooking appliance, food preparation counter, and storage cabinets sized appropriately for the unit. It does not need its own bathroom. Sharing sanitation facilities with the main house is allowed. If the JADU lacks a separate bathroom, it must have its own exterior entrance plus an interior entry connecting to the main living area.³

The Smallest ADU the Law Allows

At the other end of the range, state law prevents local agencies from setting a minimum square footage that would prohibit an efficiency unit. Under the California Health and Safety Code, an efficiency unit can be as small as 150 square feet, provided it includes a kitchen sink, cooking appliance, refrigerator, closet, and a separate bathroom.² That 150-square-foot floor effectively functions as the statewide minimum, though most ADUs in practice are far larger.

Garage conversions get the same floor as new builds. A city cannot tell you that your converted garage ADU must be smaller than 850 square feet, or 1,000 square feet if it has two or more bedrooms.¹

How Height and Setbacks Shape Usable Size

Floor area is only half the story. Your footprint and height together decide what you can actually fit on the lot. State law caps detached ADUs at 16 feet in height as a baseline. That limit rises to 18 feet if the ADU sits within a half-mile walking distance of a major transit stop or high-quality transit corridor, or if the lot already has (or will have) a multistory multifamily dwelling. An additional two feet is allowed on top of the 18-foot limit to match the roof pitch of the primary dwelling.²

For setbacks, the state limits what cities can require to a maximum of four feet from the side and rear lot lines, for both attached and detached ADUs. No setback at all is required when you convert an existing structure like a garage into an ADU, or when you build a new ADU in the same location and to the same dimensions as the structure it replaces.² That zero-setback rule is one of the reasons garage conversions remain the most straightforward ADU path on tight urban lots.

What Counts Toward the Square Footage

As of the 2026 legislative updates, ADU size limits are measured based on interior livable space rather than gross floor area.² That matters when you’re sizing a design against the 1,200-square-foot cap or the 50 percent rule, because wall thickness and non-livable areas no longer eat into your limit.

Size also interacts with impact fees. ADUs with 750 square feet or less of interior livable space are fully exempt from development impact fees. JADUs of 500 square feet or less are also exempt.² For ADUs larger than 750 square feet, impact fees must be charged proportionally based on the ADU’s square footage relative to the primary dwelling. The 750-square-foot threshold often drives design decisions where every foot above the line adds real cost.

How Many ADUs You Can Build Per Lot

Size matters more when you know how many units the law lets you stack onto the property. On a single-family lot, you can build up to three units in combination: one ADU converted from existing space (like a garage), one newly constructed detached ADU, and one JADU.² Only one JADU is allowed per single-family lot.

Multifamily properties follow different math. The property can have at least one ADU converted from existing non-livable space (such as a storage room or laundry area), up to 25 percent of the existing unit count. On top of that, lots with an existing multifamily building can add up to eight detached ADUs, though that number cannot exceed the count of existing units on the lot.² For a proposed (not yet built) multifamily dwelling, the detached ADU limit drops to two.

Under SB 9’s lot-split provisions, a single lot can never be required to hold more than four total units in any combination of primary dwellings, ADUs, and JADUs.²

1
California Legislative Information. California Code Government Code 65852.2 – Accessory Dwelling Units
2
California Department of Housing and Community Development. Accessory Dwelling Unit Handbook
3
California Legislative Information. California Code Government Code 65852.22 – Junior Accessory Dwelling Units