Quick answer
A converted ADU is an accessory dwelling unit made from space that already exists, such as a garage, basement, attic, spare wing or shed, instead of a new building.
It is a way of building, not a separate legal type. In California, a conversion inside existing space gets easier rules on setbacks, parking and utility connections than new construction.
Quick facts
- Starts from
- An existing house, accessory structure or unused part of a building
- Also called
- Interior ADU, when the space is inside the main house
- Setback for the existing walls (CA)
- None may be required 2
- Expansion allowed by right (CA)
- Up to 150 sq ft, for getting in and out only 1
- Lost parking (CA)
- Cannot be required to be replaced 2
- New utility connection (CA)
- Cannot be required for a by-right conversion on a single-family lot 3
- Code that applies
- The building code for a dwelling: ceiling height, escape openings, fire separation 6
Key takeaways
- A converted ADU reuses walls, a roof and a floor you already own.
- Garages, basements, attics, spare wings and sheds can all be converted.
- In California, existing walls do not have to meet new setbacks 2.
- A city must approve a qualifying conversion on a single-family lot without a hearing 1.
- A by-right conversion needs no new, separate utility hookup, so no connection fee for one 3.
- The space must still meet the building code for a home. That is where most of the work is 6.
What it means, in plain English
Most ADUs are new buildings in the backyard. A converted ADU is different. You take a space that is already standing and make it into a home with its own kitchen, bathroom and bed space.
The space can be inside the house, like a basement or a spare wing. People often call that an interior ADU. Or it can be a separate structure, like a detached garage, a pool house or a large shed.
California law does not have a box called “converted ADU.” It has rules for units made “within the existing space” of a home or an accessory structure 1. This page uses “converted” as the plain word for that idea.
Why it matters to you
Conversion can save you the biggest parts of a new build. The foundation, the frame and the roof may already be there. On a small lot, it may also be the only way to fit a unit at all.
The law helps too. In California, an existing structure keeps its place on the lot even if it sits closer to the line than new building is allowed 2. That can turn a “no” into a “yes.”
But old space was not built to be a home. Low ceilings, small windows, no insulation and weak framing are common. You trade some savings for fix-up work. Knowing that trade early keeps the budget honest.
Kinds of conversion
| Starting space | Usual name | First hurdle to check |
|---|---|---|
| Attached or detached garage | Garage conversion | Slab, door opening, insulation |
| Basement | Basement ADU | Ceiling height and an escape window 6 |
| Spare wing or rooms of the house | Interior ADU, or a junior ADU if 500 sq ft or less | Separate entrance and fire separation |
| Attic | Attic ADU | Ceiling height and stairs 6 |
| Pool house, studio, large shed | Accessory structure conversion | Whether it was ever permitted |
| Storage, boiler or garage space in an apartment building | Multifamily conversion | Count limit and state building standards 1 |
How a conversion works, step by step
- Find the space. List every room or building that is not used as living space today.
- Check its history. Was it built with a permit? An unpermitted shed is a harder start.
- Measure it. Floor area, ceiling height and window sizes decide what fits 6.
- Pick the legal path. A full ADU, or a junior ADU if it is inside the house and small enough 4.
- Plan the expansion, if any. By right, it may grow up to 150 sq ft, but only for getting in and out 1.
- Draw the plans. Show the existing walls, what changes, and how it meets the dwelling code.
- Apply for the permit. A qualifying conversion gets ministerial review (no public hearing) 1.
- Build and pass inspection. Then the space is a legal home.
Rules: what California says about conversions
Three numbers in the table do most of the work for a California conversion. They are pulled out below.
| Topic | What the law says | Section |
|---|---|---|
| Approval | A city shall approve, without discretion, one ADU and one junior ADU per single-family lot within existing or proposed space of the home or an accessory structure | § 66323(a)(1) 1 |
| Growth | Up to 150 sq ft beyond the existing accessory structure, limited to ingress and egress (getting in and out) | § 66323(a)(1)(A) 1 |
| Access | The space must have exterior access, and side and rear setbacks enough for fire and safety | § 66323(a)(1)(B)–(C) 1 |
| Setbacks | No setback for an existing living area or accessory structure that is converted, or one rebuilt in the same place to the same size | § 66314(d)(7) 2 |
| Parking | A garage or carport converted for an ADU does not have to be replaced | § 66314(d)(11) 2 |
| Utilities | No new or separate utility connection, and no related connection fee or capacity charge, for a § 66323(a)(1) unit, unless built with a new house | § 66311.5(d) 3 |
| Apartment buildings | At least one ADU in non-livable space of an existing multifamily building, and up to 25% of its existing units | § 66323(a)(3) 1 |
- 150 sq ftMost a by-right conversion of an accessory structure may growOnly for getting in and out
- 0 ftSetback a city may require for the existing wallsAlso for a same-size rebuild in the same place
- 0 spacesLost garage or carport parking that must be replacedWhen the garage becomes the ADU
California only, Gov. Code §§ 66323(a)(1) and 66314(d)(7), (d)(11). Source: [1] [2]
Source and scope
These are California rules, read in Chapter 13 of the Government Code as in force on 8 October 2026. Other states leave conversions to local codes or set their own limits; see ADU laws by state.
Interior ADU: a conversion inside the house
“Interior ADU” is a common name for a full ADU carved out of the main house. It keeps its own bathroom and full kitchen. It is not limited to 500 square feet.
If the space is 500 square feet or less, you can choose a junior ADU instead 4. A junior ADU can share the main bathroom and needs only an efficiency kitchen. But it comes with a deed restriction, and owner occupancy if the bathroom is shared 5.
So the same rooms can become two different legal things. Size, the bathroom plan and how you want to use it decide which.
Worked example: one house, three conversion choices
A hypothetical California single-family lot with a detached garage set 2 feet from the rear line, a basement, and a spare wing of about 450 sq ft. No prices: only the rules that decide each option.
| Option | What the rules say | Result |
|---|---|---|
| Convert the detached garage | Existing walls need no setback 2; growth up to 150 sq ft for entry only 1 | Allowed in place, even 2 ft from the line |
| Tear down and build new in the same spot | A structure rebuilt in the same location and to the same dimensions also needs no setback 2 | Allowed, if the size stays the same |
| Build new, bigger, in the same spot | New construction needs up to 4 ft side and rear setbacks 2 | Must move off the line |
| Turn the spare wing into a junior ADU | Inside the house and under 500 sq ft 4 | Allowed with a deed restriction 5 |
| Garage ADU plus wing JADU | One ADU and one JADU on the same lot 1 | Both allowed together |
The rules give the shape of the answer. Ceiling height, windows and fire separation still have to pass plan check. Use the garage conversion vs new build tool to compare paths.
The building code checks every conversion meets
| Check | Model-code rule | Section |
|---|---|---|
| Ceiling height | Living space and hallways at least 7 ft; bathrooms and laundry at least 6 ft 8 in | IRC R313.1 6 |
| Room size | Habitable rooms at least 70 sq ft (kitchens excepted) | IRC R312.1 6 |
| Escape opening | Every sleeping room and every basement needs one | IRC R319.1 6 |
| Bathroom | A toilet, a sink and a tub or shower in every dwelling unit | IRC R326.1 6 |
| Kitchen | A kitchen area with a sink in every dwelling unit | IRC R326.2 6 |
| Unit separation | Fire-rated walls or floors between two dwelling units: 1 hour, or ½ hour if sprinklered | IRC R302.3 6 |
Where you will see it
- On the permit application: a box for “conversion of existing space” or “new construction.” Pick carefully. It changes setbacks and fees.
- On the plans: existing walls are often drawn one way and new walls another, with a legend.
- On the fee sheet: a conversion on a single-family lot should show no new utility connection charge 3.
- On the bid: line items for demolition, a new slab or footing, insulation and window changes.
- In the city’s comment letter: notes on headroom, egress windows or fire separation.
Rules outside California
Many states and cities allow ADUs inside existing homes. Some were allowing basement and attic units long before backyard cottages. But the special breaks on setbacks, parking and hookups are California’s rules, not national ones.
Elsewhere, a converted garage may have to replace its parking. Existing walls may still have to meet today’s setbacks. Check your state and city. Our ADU laws by state research is a good starting point.
Costs and what drives them
We do not publish a typical conversion price, because the starting space varies too much. These are the drivers that move it. See ADU cost for the full picture.
- Condition of the shell. A sound slab and roof save the most. A cracked slab or rotted framing eats the savings.
- Ceiling height. Too low means lowering a floor or raising a roof, which is major work 6.
- Plumbing route. How far the new drains run to the existing sewer line.
- Windows and doors. Escape openings and a garage door infill 6.
- Fire separation. Rated walls or ceilings where the unit touches the main house 6.
- Fees. In California, no impact fee at 750 sq ft or less, and no separate hookup fee for a by-right conversion 3.
Benefits and limits
What it does well
Converted ADU compared with related terms
| Term | Built from | Key difference |
|---|---|---|
| Converted ADU | Existing space | Easier setback, parking and hookup rules in California |
| Detached ADU | A new stand-alone building, or a converted one | Describes position, not whether it is new |
| Attached ADU | A new addition sharing a wall | New walls need setbacks |
| Junior ADU | Space inside the house, 500 sq ft max | Its own article of law 5 |
| New-construction ADU | A new building | Setbacks up to 4 ft; new hookup may be required 3 |
Common mistakes
- Myth A conversion does not need a permit because the building already exists.
- Reality Turning space into a home changes its use. It needs a building permit and inspections like any ADU.
- Myth Any expansion keeps the conversion rules.
- Reality By-right growth is capped at 150 sq ft and limited to getting in and out 1. A bigger addition is new construction.
- Myth A conversion is always cheaper.
- Reality Often, not always. Fixing height, slab and framing can cost more than building new.
- Myth “Interior ADU” and “junior ADU” mean the same thing.
- Reality An interior ADU is a full ADU. A junior ADU is a separate legal category with a 500 sq ft cap 4.
- Myth Conversion breaks apply in every state.
- Reality The setback, parking and hookup breaks quoted here are California’s.
What to ask before you commit
- Was this structure built with a permit? Does the city have a record?
- Does the city treat my plan as conversion or new construction?
- How much of my planned expansion counts as “ingress and egress”?
- Can the ceiling reach 7 feet without lowering the floor or raising the roof?
- What fire separation is needed between the unit and the main house?
- Will my utility provider treat this as a conversion for fee purposes?
Checklist for your project
- Pull the permit history for the space.
- Measure floor area, ceiling height and every window.
- Decide: full ADU or junior ADU.
- Have a designer check the slab, framing and roof.
- Mark existing walls and new work clearly on the plans.
- Confirm the fee sheet treats it as a conversion.
- Book inspections and keep the final sign-off.
Related guides, tools and research
- ADU typesHow conversions compare with every other form
- ADU type selectorWhich form fits your lot
- Garage conversion researchWhat the data shows on conversions
- ADU cost calculatorRough budget by type
- Legalizing an unpermitted unitWhen the space was built without a permit
- Ministerial approvalHow by-right review works
Questions about converted adu
What is a converted ADU?
It is an ADU made from space that already exists, like a garage, basement, attic or spare part of the house. You add a kitchen, a bathroom and living space inside walls that are already standing. The finished unit must still meet the building code for a home, including ceiling height and escape windows.
What is an interior ADU?
An interior ADU is a full ADU made inside the main house, such as from a basement or a wing. It has its own bathroom and full kitchen. If the space is 500 square feet or less, a California owner can choose to make it a junior ADU instead, which has different rules.
Do I need a permit to convert space into an ADU?
Yes. Turning a garage, basement or spare room into a home changes how the space is used. It needs a building permit and inspections. In California, a city must review a qualifying conversion ministerially, which means by fixed rules and without a public hearing.
Does a converted ADU have to meet setbacks in California?
No setback can be required for an existing living area or accessory structure that is converted to an ADU. The same applies to a structure rebuilt in the same place and to the same size. New walls beyond that can face setbacks of up to 4 feet on the sides and rear.
Can I make my conversion bigger?
In California, a by-right conversion on a single-family lot may grow up to 150 square feet beyond the existing structure. That extra space is limited to getting in and out, like an entry. A larger addition is treated more like new construction, with its own setback and size rules.
Do I need a separate water or sewer line for a converted ADU?
Not for a by-right conversion on a single-family lot in California. The utility cannot require a new, separate connection or charge a related connection fee or capacity charge for it, unless the unit was built along with a new house. Other ADUs may be required to connect separately.
Is a converted ADU cheaper than a new one?
It often is, because the foundation, frame and roof may already exist. But it is not guaranteed. Low ceilings, a cracked slab, rot or poor drainage can be costly to fix. Compare a real bid for the conversion against one for new construction before you decide.
Can I convert a shed or pool house into an ADU?
Often, yes, if it is large enough and can meet the code for a dwelling. California treats an existing accessory structure like other existing space for conversion. If the shed was never permitted, the city may first need to review or legalize it, which can add steps.
Can I convert part of an apartment building into an ADU?
In California, yes. A city must allow ADUs in parts of an existing multifamily building not used as living space, like storage rooms, boiler rooms, attics, basements or garages. It must allow at least one, and up to 25 percent of the existing units, if each meets state building standards.
Do I have to replace parking if I convert my garage?
Not in California. When a garage, carport or covered parking is converted to an ADU, the city cannot require you to replace those spaces. Other states and cities may have different rules, so check locally if you are outside California.
Sources
- California Legislature, Gov. Code § 66323 (as amended by SB 543, Stats. 2025, ch. 520), retrieved .
- California Legislature, Gov. Code § 66314, retrieved .
- California Legislature, Gov. Code § 66311.5 (formerly § 66324; renumbered by SB 543), retrieved .
- California Legislature, Gov. Code § 66313, definitions, retrieved .
- California Legislature, Gov. Code § 66333, junior ADUs (as amended by AB 1154, Stats. 2025, ch. 507), retrieved .
- International Code Council, 2024 International Residential Code, Chapter 3 Building Planning, 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: Rules read from the current Gov. Code Chapter 13 sections on leginfo and the 2024 IRC on the ICC site on 8 October 2026. No prices are given.
Suggest a correction. We fix errors and say what changed.