Detached ADUs in Orange County: sizes, rules, cost, and our builds.

What a detached ADU is, and what makes it different.

Standalone unit on your property. Most rental upside, longest schedule, biggest ROI when the property has the room. It suits properties with side or rear setback room for a free-standing unit. It is the only one of the four typologies that starts from nothing, so the scope reads: New foundation, full kitchen + bath, independent utilities. That is the whole trade. A detached unit is not constrained by the shape of the house, so the floor plan, the orientation and the privacy are yours to set — and it is the longest schedule and the largest scope of the four, because a new structure means new footings, new service runs and a full envelope.

The other three stay tied to something you already own. An attached ADU is an addition off the primary residence. Shares a wall, often shares a service line. A Junior ADU is a conversion within the existing footprint. Tightest scope, kitchenette + bedroom only. A garage conversion is an existing garage rebuilt as living space. Fastest move when the garage frame is sound. If you are still choosing between them, what an ADU is compares all four side by side, and the ADU glossary defines the terms a plan checker will use.

How big a detached ADU can be.

California sets the envelope and your city fills in the rest. A local ADU ordinance may cap the total floor area of a detached unit at 1,200 square feet (§66314(d)(5)), so 1,200 is the top of the range where the city allows it, not something you may always build. Underneath that, state law sets floors a city cannot go below: an ordinance may not set a maximum square footage requirement that is less than 850 square feet of interior livable space, or less than 1,000 square feet of interior livable space for an ADU that provides more than one bedroom (§66321(b)(2)). And no local standard may stop an ADU of at least 800 square feet of interior livable space with four-foot side and rear yard setbacks from being built in compliance with all other local development standards (§66321(b)(3)).

There is also a by-right path written into the statute. A local agency must ministerially approve one detached, new-construction ADU that does not exceed four-foot side and rear yard setbacks on a lot with a proposed or existing single-family dwelling. On that unit the agency may impose a total floor area limitation of not more than 800 square feet of livable space, and the height limits described below (§66323(a)(2)).

Our detached builds typically run 500 – 1,200 sqft, Studio – 3 BR. What your lot actually takes is decided by lot coverage, easements, and where the sewer lateral and the electrical service already run — which is the first thing we look at, before anything is drawn. Setbacks, easements, utility lines, neighbor friction reviewed together. We tell you what your vision can carry before drawings.

Setbacks, height, and parking under state law.

Four provisions of the state ADU statute decide most detached projects, and they override a conflicting local ordinance. They are quoted here with their section numbers because that is what a permit counter argument needs.

What your city adds on top.

State law is the floor, not the whole rulebook. Objective local standards still apply — lot coverage, fire access, hillside grading, and the coastal-zone review that a permit in a coastal city carries. The Orange County ADU permit indexnames how each of the 37 cities takes an ADU application and links each city’s own page, so you can read the local overlay next to the state rule before you commit to a footprint.

What a detached ADU costs in Orange County.

Detached ADUs typically run $245K – $440K+. At that band the scope includes new foundation, full kitchen + bath, independent utilities. Where a project lands inside the range is set by size, site conditions and finish level, which is why we publish a band rather than a per-square-foot figure. Here are all four typologies against it.

Impact fees, and what is not an impact fee.

A local agency, special district or water corporation shall not impose any impact fee upon the development of an ADU that has 750 square feet of interior livable space or less. Above that, any impact fee charged shall be charged proportionately in relation to the square footage of the primary dwelling unit (§66311.5(c)(1), the section formerly numbered §66324). An impact fee is not a utility connection fee or a capacity charge: those sit outside the exemption and are still payable, though §66311.5(b) requires them to be proportionate to the unit’s burden on the system. To model a build against rent rather than against fees, the ROI calculator and the ADU cost estimator take your own numbers, and ADU financing and the ADU loan calculator cover how owners pay for one.

How long a detached ADU takes.

A detached ADU typically takes 5–8+ moto build, with design and permitting ahead of that. Plan check typically runs 4–6+ weeks when the package is complete at intake. A permit application for an ADU is considered and approved ministerially, without discretionary review or a hearing, and the permitting agency must approve or deny a completed application within 60 days of receiving it; if it has not done either by then, the application is deemed approved (§66317(a)(1) and (a)(3)). The clock starts once the application is complete, so each correction round delays when that clock begins — and a clean, engineered submittal is the single biggest thing anyone can do about the calendar.

• 01 Free consultation. Setbacks, easements, utility lines, neighbor friction reviewed together. We tell you what your vision can carry before drawings.
• 02 Drawings. We turn your vision into a buildable blueprint — architectural set, structural engineering, soils when needed.
• 03 City permits. We take care of the submission, comments, and resubmittal to the city. You sign once; we carry it all.
• 04 Construction. Foundation, framing, mechanical, electrical, and plumbing, finishes. Weekly progress reports throughout.
• 05 Walk + warranty. Final inspection, owner walk-through, then the tiered warranty (1-year fit & finish, 4-year systems, 10-year structural).

Start from a plan, or go fully custom.

Three signature ADU plans are already drawn, with photoreal renderings of every room: Bellecielo at 830 SQFT, Casielo at 387 SQFT and Elega at 500 SQFT. Start from one and adjust the finishes, the layout and the exterior during design, or start from a blank sheet — see them all on the ADU plans page. Some Orange County cities also run a pre-approved standard-plan program, where a plan the city has already reviewed moves faster through permitting than a custom set; the permit index is where to check whether yours does.

• Custom architectural plans + structural engineering, included
• Title 24 energy calculations handled in-house

Sources

Every statute quoted on this page, in the order it appears. California Government Code, via the Legislature’s own text at leginfo.legislature.ca.gov: §66313 (definition), §66314 (size cap, setbacks, replacement parking, renting and selling), §66321 (minimum sizes and heights), §66323 (the by-right detached unit), §66317 (ministerial review and the 60-day clock), and §66311.5 (impact fees; formerly §66324). Statute text read 2026-08-18. Cost, size and build-window ranges are Avorino’s own published Orange County figures.