California ADU Maximum Size Rules: 2026 Square Footage Caps

What the law guarantees as a floor, not a ceiling

Government Code section 66321(b)(2) states that a local agency shall not establish a maximum square footage requirement for an attached or detached ADU that is less than 850 sq ft of interior livable space, or 1,000 sq ft of interior livable space for an ADU that provides more than one bedroom. The 850 sq ft floor applies to a studio or one-bedroom unit; the 1,000 sq ft floor applies once the unit has two or more bedrooms.

Section 66314(d)(5) separately caps the total floor area for a detached ADU at 1,200 sq ft. That is the statewide maximum a city cannot exceed for a new detached structure built as an ADU, even if a city wants to allow a larger unit.

• Studio or one-bedroom: at least 850 sq ft of interior livable space.
• Two or more bedrooms: at least 1,000 sq ft of interior livable space.
• Detached ADU total floor area: capped at 1,200 sq ft under section 66314(d)(5).

Attached ADUs and the 50 percent rule

If an existing primary dwelling stands on the lot, section 66314(d)(4) limits the total floor area of an attached ADU to 50 percent of the existing primary dwelling. The 50 percent figure is measured against the existing primary, not against lot coverage or floor area ratio.

Section 66321(b)(3) also requires the city to permit an attached or detached ADU of at least 800 sq ft of interior livable space with four-foot side and rear yard setbacks, regardless of any local minimum lot size, lot coverage, floor area ratio, or open space standard that would otherwise block it.

Garage conversions and existing structures

An ADU created within an existing living area or accessory structure does not trigger a new setback, so a garage that already sits two feet from the side property line can become an ADU at two feet, under section 66314(d)(7). The same rule applies when a new structure is built in the same location and to the same dimensions as an existing one.

Section 66314(d)(11) further provides that when a garage, carport, covered parking structure, or uncovered parking space is demolished in conjunction with the construction of an ADU or converted to one, the local agency shall not require that those off-street parking spaces be replaced.

Junior ADU (JADU) size cap

A JADU is created entirely within the walls of an existing or proposed single-family residence, including an attached garage. Under section 66333, a JADU is limited to no more than 500 sq ft and must include an efficiency kitchen and, in most cases, a separate exterior entrance.

When a JADU shares sanitation facilities with the primary dwelling, the property owner must occupy either the JADU or the primary residence, with limited exceptions, per section 66333(b). When the JADU has its own bathroom, no owner occupancy is required.

Height, setback, and lot coverage that follow the size

Section 66321(b)(4) requires cities to allow a detached ADU of at least 16 feet in height, or 18 feet when the lot is within one half mile walking distance of a major transit stop or high quality transit corridor as defined in Public Resources Code section 21155. An attached ADU must be allowed at least 25 feet or the limit that applies to the primary dwelling, whichever is lower.

Section 66314(d)(7) sets a four-foot side and rear setback for a new detached ADU that is not converted from an existing structure. Front setback, lot coverage, floor area ratio, and minimum lot size may not be used to prohibit an 800 sq ft ADU that meets the four-foot side and rear setbacks.

How L Square applies these limits on real lots

L Square Design & Build publishes configured model prices: $180,000 for Model 600, $240,000 for Models 800A and 800B, $295,000 for Models 1000A and 1000B, and $354,000 for Model 1200. These are model prices, not a final total for a specific lot. L Square Design & Build designs and builds within these limits across Orange County and Los Angeles County, applying the rules site by site rather than assuming every parcel fits the same template.

A 700 sq ft detached ADU with a vaulted ceiling built by L Square in Pasadena is documented on the project walkthrough page, with the finished great room, kitchen, bath, and bedroom shown.