California ADU Setback Rules: 4-Foot Side and Rear Yard

What state law guarantees on side and rear setbacks

Government Code Section 66314(d)(7) sets the statewide ceiling on the side and rear setbacks a city may impose on an ADU. The text reads, in full, that no setback shall be required for an existing living area or accessory structure, or a structure constructed in the same location and to the same dimensions as an existing structure, that is converted to an ADU or to a portion of an ADU. A setback of no more than 4 feet from the side and rear lot lines shall be required for an ADU that is not converted from an existing structure, or a new structure constructed in the same location and to the same dimensions as an existing structure.

The 4-foot ceiling applies whether the unit is attached or detached, and it applies on lots that have an existing or proposed single-family dwelling. For an attached ADU that uses an expansion of an existing accessory structure, the 4-foot ceiling applies to the new portion only. The existing footprint keeps its pre-existing position. The same 4-foot ceiling carries into the 800 square foot ADU that the city must allow under Section 66323(a)(2), so an 800 square foot detached ADU on a single-family lot cannot be pushed past 4 feet from a side or rear lot line.

• Maximum 4 feet from each side lot line.
• Maximum 4 feet from each rear lot line.
• No setback required for an existing structure converted to an ADU.
• No setback required for a same-location, same-dimensions rebuild.

How garage conversions and existing structures get a zero setback

Section 66314(d)(7) reaches a zero setback in two specific cases. The first is the conversion of an existing living area or accessory structure, which includes an attached garage, a detached garage, a barn, a workshop, or any other accessory building on the lot that already sits on or inside the property line. The second is a new structure built in the same location and to the same dimensions as an existing structure. A garage sitting on the rear property line stays on the rear property line when it becomes an ADU.

The statute uses the exact phrase same location and to the same dimensions for the second carve-out. Any project that does not match that phrase falls outside the second carve-out and is read against the general side and rear setback rules in Section 66314(d)(7).

The 800 sq ft protection that stops a city from using setbacks as a block

Section 66321(b)(3) is the second setback rule, and it works as a backstop. The statute says a city may not enact, by ordinance, any requirement for a zoning clearance or separate zoning review, any other minimum or maximum size for an ADU, size based upon a percentage of the proposed or existing primary dwelling, or limits on lot coverage, floor area ratio, open space, front setbacks, and minimum lot size, for either attached or detached dwellings, that does not permit an ADU with at least 800 square feet of interior livable space and with 4-foot side and rear yard setbacks to be constructed in compliance with all other local development standards.

In practice the backstop means three things at once. A city cannot deny an 800 square foot ADU because of a deep front setback rule that would push the unit off the lot. A city cannot deny the same 800 square foot ADU because the lot is below the city's standard minimum lot size. And a city cannot deny it because a percentage-of-primary-dwelling rule would shrink it below 800 square feet. The 800 square foot figure is the floor, and the 4-foot setback is the matching ceiling. Cities may adopt more permissive setbacks, and many do.

Front setbacks, height, and lot coverage that travel with the ADU

Section 66314(d)(7) only addresses side and rear setbacks. Front setbacks come from the city's underlying zone, except where Section 66321(b)(3) bars the city from using a front setback rule to block the 800 square foot ADU. Section 66314(d)(7) does not by its terms require a detached ADU to sit behind the primary dwelling; any local rule on that placement comes from the city's underlying zone, the city's ADU ordinance, or the city's design review standards, and is subject to the Section 66321(b)(3) backstop. Cities can also adopt local standards on maximum lot coverage, floor area ratio, and open space, subject to the same 800 square foot backstop.

Section 66314(d)(4) and (d)(5) write in the size ceilings that have to live alongside the setback rules. Total floor area for a detached ADU may not exceed 1,200 square feet, and an attached ADU may not exceed 50 percent of the existing primary dwelling floor area. The 1,200 square foot and 50 percent figures are ceilings. Section 66321(b)(2) sets the corresponding floor. A city may not impose an ADU maximum below 850 square feet for a studio or one-bedroom unit, or below 1,000 square feet for a unit with more than one bedroom. The state's size ceilings and setback ceilings are independent. Both apply.

How the rules apply to a real Orange County or Los Angeles County lot

On a typical single-family lot in Orange County or Los Angeles County, the Section 66314(d)(7) side and rear setback ceiling means an ADU sits at least 4 feet from the side property line and at least 4 feet from the rear property line. A garage conversion along a rear or side property line skips the 4-foot rule under the existing-structure carve-out. A new detached ADU in the rear yard pads the envelope at 4 feet. Section 66321(b)(3) protects the 800 square foot, 16-foot-tall, 4-foot-setback unit when the city's own front setback or lot-coverage rule would otherwise push it off the lot.

L Square Design & Build's published 2026 model lineup runs from the 600 sq ft two-bedroom Model 600 at $180,000, to the 800 sq ft Models 800A and 800B at $240,000 each, to the 1,000 sq ft Models 1000A and 1000B at $295,000 each, up to the 1,200 sq ft Model 1200 at $354,000. State law caps the side and rear setback at 4 feet for any of these models under Section 66314(d)(7). Confirm the actual side, rear, and front setback requirements with the permitting agency for the address before signing a construction contract, and confirm how the city's underlying zone, the city's ADU ordinance, and any design review standards apply to the placement on the lot.