Quick answer
A detached ADU is an accessory dwelling unit built as its own freestanding building on the same lot as the main home, usually in the backyard, with its own kitchen, bathroom and entrance.
Because it shares no wall with the house, it gets the most privacy of any ADU type. In California it also carries its own state size cap, height floors and a four-foot side and rear setback limit.
Quick facts
- Also called
- Backyard ADU, backyard cottage
- California size cap (state standard)
- 1,200 sq ft of total floor area 1
- Size a city must allow
- At least 800 sq ft with 4 ft side and rear setbacks 2
- Height a city must allow
- 16 ft; 18 ft near major transit (+2 ft for a matching roof pitch) or on a multistory multifamily lot 2
- Side and rear setback
- No more than 4 ft may be required for new construction 1
- Approval
- Ministerial (no hearing) in California 4
A home of its own in the yard
Picture a small cottage at the back of the lot with its own path, its own door and its own utility connections. That is a detached ADU. It is the most common picture people have of an ADU, which is why "backyard ADU" gets more searches than the formal name.
Detached is about the building, not the size or the method. A prefab unit set on a foundation, a site-built cottage and a two-story unit over a garage are all detached ADUs if they stand apart from the house.
A detached garage that is converted is also detached, but the rules for converted space are different and live on the garage conversion ADU page.
Everything here describes California’s state rules, named as California’s. Other states and cities set their own limits; start with ADU laws by state.
Key takeaways
- A detached ADU is a small home in its own building on your lot, usually in the backyard.
- It needs its own kitchen, bathroom and entrance, like any ADU.
- In California the state standard caps it at 1,200 sq ft 1.
- No California city may use other rules to block an 800 sq ft unit with 4 ft side and rear setbacks 2.
- A California city must allow at least 16 ft of height, more near major transit 2.
- It gives the most privacy, but it is the most new building of any ADU type.
Why it matters which type you build
Detached is often the first idea, and for good reason. It keeps two households apart. It leaves the main house alone while work goes on. And in California it has the highest state-standard size of any ADU type 1.
But it also asks the most of your lot. The unit needs room after setbacks (the required gap between a building and the lot line). It needs a path for pipes and wires. And it needs a way for workers to bring in materials.
Knowing the rules early stops a common mistake: paying for a design that will not fit.
California’s detached ADU rules
| Rule | What the law says | Section |
|---|---|---|
| Maximum size (state standard) | Total floor area of a detached ADU shall not exceed 1,200 sq ft | § 66314(d)(5) 1 |
| Lowest maximum a city may set | Not below 850 sq ft, or 1,000 sq ft for more than one bedroom | § 66321(b)(2) 2 |
| Protected minimum | No lot coverage, floor area ratio, open space, front setback or minimum lot size rule may block an 800 sq ft unit with 4 ft side and rear setbacks | § 66321(b)(3) 2 |
| Height | At least 16 ft; 18 ft within half a mile walking distance of a major transit stop or high-quality transit corridor, plus 2 ft for a roof pitch matching the main home; 18 ft on a lot with a multistory multifamily building | § 66321(b)(4)(A)–(C) 2 |
| Side and rear setbacks | No more than 4 ft for a unit that is not a conversion | § 66314(d)(7) 1 |
| Parking | No more than one space per unit or per bedroom, whichever is less; tandem on a driveway allowed | § 66314(d)(10) 1 |
| Fire sprinklers | Not required if the main home does not need them | § 66314(d)(12) 1 |
| By-right unit | One detached new-construction ADU with 4 ft side and rear setbacks on a single-family lot; the city may cap it at 800 sq ft and the heights above | § 66323(a)(2) 3 |
Kinds of detached ADU
| Kind | What it is | What to know |
|---|---|---|
| Site-built cottage | Framed and finished on your lot | The most design freedom |
| Prefab or modular unit | Built in a factory, set on a foundation on site | Still needs a permit, foundation and utility hookups; see prefab ADU |
| Manufactured home | A factory-built home under the federal HUD code | California counts it as an ADU when it meets the rest of the law 6 |
| Unit over a garage | A home built above a detached garage | Height and structure decide it; see above-garage ADU |
| Two-story unit | A detached unit on two floors | Needs height above the 16 ft floor; see two-story ADU |
How a detached ADU gets built
- Check feasibility. Zoning, setbacks, easements, trees and slope. Start with the feasibility checker.
- Find the spot. Mark the 4 ft side and rear lines and the space left for the unit 1.
- Pick a plan. Custom, prefab or pre-approved.
- Plan utilities. Map how sewer, water and power will reach the unit.
- Apply. In California the city must approve it ministerially, with no hearing 4.
- Build. Foundation, framing, roof, systems, finishes.
- Final inspection. Nobody moves in until it passes.
Worked example: what fits on a 50 × 120 ft lot
A hypothetical California single-family lot, 50 ft wide and 120 ft deep, with the house at the front. The numbers below are inputs chosen for illustration; your city’s front setback, lot coverage and easements will change them. The formula gives the buildable rectangle; the table runs it on this lot.
| Step | Calculation | Result |
|---|---|---|
| Width available after 4 ft side setbacks | 50 ft − 4 ft − 4 ft | 42 ft |
| Backyard depth behind the house (assumed) | Hypothetical: 40 ft of yard behind the house, less a 4 ft rear setback | 36 ft |
| Footprint a city cannot block | § 66321(b)(3) protects at least 800 sq ft with 4 ft side and rear setbacks 2 | For example 40 ft wide by 20 ft deep, which fits the 42 × 36 ft space, or two stories of 400 sq ft if height allows |
| Single-story height a city must allow | § 66321(b)(4)(A) 2 | 16 ft (up to 18 ft near major transit, plus 2 ft to match the main roof pitch) |
| Largest the state standard allows | § 66314(d)(5) 1 | 1,200 sq ft, if the city’s own limits permit it |
Usable footprint = (lot width − 2 × side setback) × (yard depth − rear setback)
Lot width- Feet from one side property line to the other
Side setback- California: no more than 4 ft for a new detached ADU
Yard depth- Feet from the back of the house, less any required gap between buildings, to the rear line
Rear setback- California: no more than 4 ft for a new detached ADU
Easements, trees, slope and fire-separation rules can shrink the rectangle further.
Source: [1]
The decision rule: the state guarantees roughly an 800 sq ft unit at 16 ft on almost any residential lot that can physically fit it, and caps the state standard at 1,200 sq ft. Anything between depends on the local ordinance. Run your own lot through the size envelope calculator.
Detached vs attached: the trade-offs
Why owners choose detached
- Privacy and sound separation for both households.
- The main house stays untouched during construction.
- Allows the largest state-standard size of any ADU type (1,200 sq ft) 1.
- Prefab and pre-approved plans fit this type best; see the pre-approved plan finder.
What it costs you
- A new foundation, roof and four exterior walls, so more new construction than an attached ADU or a conversion.
- Longer utility runs from the street or the main house; see utility connection requirements.
- Uses yard space, and trees, slopes or easements can rule out the obvious spot.
- Impact fees can apply above 750 sq ft in California (§ 66311.5(c)) 5.
Costs and what drives them
This page gives no price, because a fair price depends on your lot and your plan. Instead, here is what moves the number.
Size comes first. Then the foundation, which slope and soil can make harder. Then the utility runs: the farther the unit is from the street or the main house, the longer the trench. Then the finish level, and the number of kitchens and baths.
Fees also count. In California, a unit of 750 sq ft or less carries no impact fee. Above that, impact fees must be in proportion to the main home 5. Sourced figures are on the ADU cost hub and in California ADU cost research.
Rules outside California
Outside California, the size cap, height limit, setbacks, parking and owner-occupancy rules for a detached unit come from each state and city. Some cities allow detached units only on larger lots. Treat every number on this page as California’s, and check your own state and city code.
Where "detached" shows up in your paperwork
- On the site plan: the distance from the unit to each lot line and to the main house.
- On the permit application: the city’s ADU type box decides which size and height limits apply.
- On a builder’s bid: foundation, utility trenching and a new roof are line items a conversion does not have. Compare bids with the bid comparison tool.
Detached vs attached vs garage conversion
| Detached ADU | Attached ADU | Garage conversion | |
|---|---|---|---|
| Building | New, freestanding | New, joined to the house | Existing garage |
| California state-standard cap | 1,200 sq ft 1 | 50% of the home, 800 sq ft protected 1 | The existing space, plus up to 150 sq ft on the by-right path 3 |
| Side and rear setback | No more than 4 ft 1 | No more than 4 ft 1 | None for the existing structure 1 |
| Privacy | Highest | Shared wall | Depends on where the garage sits |
| Work on the main house | Little | Some | Little, if the garage is detached |
Common mistakes
- Myth A shed or studio with no kitchen is a detached ADU.
- Reality No. Without cooking and sanitation it is an accessory structure, not a home 6.
- Myth 1,200 sq ft is guaranteed.
- Reality No. That is the state-standard cap. A city may set a lower maximum, but not below 850 sq ft, or 1,000 sq ft for more than one bedroom 2.
- Myth A tiny house on wheels counts.
- Reality Not as a California ADU. The definition needs permanent provisions for living. See tiny home ADU.
- Myth You always need a parking space.
- Reality Not always. California caps parking at one space per unit or bedroom, and many exemptions apply 1.
What to ask your city and builder
- What is the city’s maximum size for one bedroom, and for two?
- What height does the city allow here, and am I near major transit?
- Are there easements, sewer lines or trees in the backyard I must avoid?
- How far is the sewer connection, and does it flow downhill?
- Does the unit need its own meter? See separate utility meter.
- Is parking required, or does an exemption apply? See parking research.
Checklist for a detached ADU
- Get a lot survey or an accurate site sketch.
- Mark 4 ft side and rear setbacks and the local front setback.
- Mark easements, trees and slopes.
- Decide one or two bedrooms, and check the city cap for each.
- Plan the utility route before the floor plan.
- Compare custom, prefab and pre-approved plans.
- Apply and answer plan-check comments.
- Compare bids line by line, including foundation and trenching.
Go deeper
- ADU types hub
- ADU size statistics
- Garage conversion vs new buildCompare a conversion with a detached build
- What drives ADU cost
- ADU parking requirements research
Questions about detached adu
What is the difference between an attached and a detached ADU?
A detached ADU is its own building; an attached ADU shares at least one wall with the main home. In California the state standard caps a detached ADU at 1,200 sq ft and an attached one at 50% of the existing home’s floor area, with an 800 sq ft floor for both.
How big can a detached ADU be in California?
The state standard caps it at 1,200 square feet of total floor area. A city may set a lower maximum, but not below 850 square feet (1,000 for more than one bedroom), and cannot use other rules to block an 800 square foot unit with four-foot side and rear setbacks.
How tall can a detached ADU be?
California requires cities to allow at least 16 feet, or 18 feet near major transit (plus two feet for a roof pitch matching the house) or on a lot with a multistory multifamily building. Cities may allow more.
How close to the property line can a detached ADU be?
In California a city may require no more than four feet from the side and rear lot lines for a newly built ADU. Front setbacks and distance from the main house are set locally.
Is a backyard ADU the same as a detached ADU?
Yes. "Backyard ADU" and "backyard cottage" are everyday names for a detached ADU. The legal category is the same.
Do I need a permit for a backyard ADU?
Yes. A detached ADU is a home, so it needs a building permit and inspections. In California, the city must approve an ADU application ministerially, which means against fixed rules with no hearing. A unit built without a permit is not a legal ADU.
Can a detached ADU be two stories?
It can if the local height limit allows it. California sets only a floor: a city must allow at least 16 feet, or 18 feet in some cases. Many cities allow more. Check your city’s height limit before you plan a second floor.
Can I put a prefab unit in my backyard as an ADU?
Yes, if it is set on a permanent foundation, hooked up to utilities and permitted like any other ADU. California’s definition also counts a manufactured home as an ADU when it meets the rest of the law. A unit on wheels does not count.
Does a detached ADU need parking?
Sometimes. California lets a city require no more than one space per unit or per bedroom, whichever is less, and tandem parking on a driveway is allowed. Many exemptions apply. Rules outside California vary.
Sources
- California Legislature, Gov. Code § 66314, retrieved .
- California Legislature, Gov. Code § 66321 (as amended by SB 543, eff. 1 Jan 2026), retrieved .
- California Legislature, Gov. Code § 66323 (as amended by SB 543, eff. 1 Jan 2026), retrieved .
- California Legislature, Gov. Code § 66317 (as amended by SB 543, eff. 1 Jan 2026), retrieved .
- California Legislature, Gov. Code § 66311.5 (formerly § 66324; renumbered by SB 543), retrieved .
- California Legislature, Gov. Code § 66313 (as amended by SB 543, eff. 1 Jan 2026), retrieved .
Expert review
Written by the HyreADU Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Every number was read in the current Chapter 13 section on leginfo.legislature.ca.gov on 7 October 2026. The lot example uses stated hypothetical dimensions. Education, not legal advice: the planning department applies its ordinance.
Suggest a correction. We fix errors and say what changed.