California has spent the last several years making accessory dwelling units, or ADUs, easier to build. Most single-family lots in Los Angeles County can now hold at least one. That doesn't mean every lot is simple. The shape of your yard, where the sewer line runs and whether you're on a hillside can matter as much as the law itself.
The four kinds of ADU
An ADU is a self-contained home with its own kitchen, bathroom and entrance, built on the same lot as a house. A junior ADU, or JADU, is a smaller unit carved out of the house you already have. Here's how the common options line up:
| Type | Where it goes | Size, generally | Good fit when |
|---|---|---|---|
| Detached ADU | A new building in the backyard | Up to 1,200 sq ft in many cities | You have yard space and want the most privacy |
| Attached ADU | An addition joined to the house, with its own entrance | Often limited relative to the size of the house | The yard is small or the house sits well on the lot |
| Garage conversion | An existing garage turned into a living unit | The size of the garage, sometimes with a small addition | You have a garage you rarely park in |
| JADU | Space inside the existing house | Up to 500 sq ft | You have a spare bedroom or wing near an outside wall |
A detached ADU usually costs the most because it needs a new foundation, structure, roof and utility lines. A garage conversion reuses walls, a slab and a roof that are already there, which is why it's often the least expensive way to add a rentable unit. A JADU needs no new foundation at all, but it's small, and it has to have an efficiency kitchen and its own entrance from outside.
Many single-family lots can have one ADU and one JADU at the same time, so a garage conversion plus a JADU is a real option if you want two rentals or a rental plus family space.
What state law generally allows
State ADU law sets a floor that cities and the county can build on but generally can't go below. As of this writing, state law generally:
- Allows an ADU on lots zoned for single-family homes that already have, or will have, a house
- Allows a JADU of up to 500 sq ft within the walls of the existing home
- Requires cities to accept side and rear setbacks of 4 feet for a new detached or attached ADU
- Doesn't require replacement parking when a garage is converted to an ADU or torn down to build one
- Doesn't require the owner to live on the property for an ADU
- Requires the owner to live in the house or the JADU only when the JADU shares a bathroom with the main house
- Requires cities to approve or deny a complete ADU application within 60 days
- Bars impact fees on ADUs under 750 sq ft
Cities layer their own standards on top: height limits, design rules, maximum sizes and where the unit can go on the lot. These local rules can't block a basic ADU, but they do shape what you can build.
ADU law has changed almost every year since 2017, and each city writes its own ordinance on top of it. Before you commit to a design, check the current rules with your city's planning department, or with LA County Regional Planning if you live in an unincorporated area.
What to check about your lot
Two houses on the same street can have very different ADU projects. Before you pay for design, look at these:
- Utilities. Where the electric service, water line and gas line come in, and how far they'd have to run to reach the new unit. Long runs add cost.
- Sewer or septic. On city sewer, the question is whether the existing line can take another kitchen and bathroom and how the new unit will tie in. On a septic system, the system may need to be evaluated or upgraded, and the county health department may be involved.
- Slope. A sloped yard can mean retaining walls, grading and extra engineering.
- Easements. Utility or access easements can rule out part of the yard. Your title report or a survey will show them.
- Trees. Some cities protect certain native trees, such as oaks, and large trees near the building area can limit where a foundation can go.
- Fire zones and hillside rules. Lots in very high fire hazard severity zones need wildfire-resistant construction, and some cities, including Los Angeles, add hillside requirements such as road access, fire sprinklers or limits on certain unit types.
- Historic districts and HOAs. Design review can apply to what's visible from the street, and HOA rules may matter for some features even where they can't ban an ADU outright.
A short feasibility visit, before any drawings, usually answers most of these questions.
The stages of an ADU project
Every ADU moves through the same five stages. The design and permit stages often take as long as construction.
1. Feasibility. A site visit and a look at zoning, setbacks, utilities and any special rules for your lot. You come out of it knowing what size and type of unit fits, and roughly what it might cost.
2. Design. A designer or architect draws the unit, and a structural engineer adds the structural design. You'll also need Title 24 energy calculations, and on some lots a survey or a soils report. This is when you choose the layout and finishes. In the City of Los Angeles, pre-approved standard plans can shorten this stage and plan check.
3. Plan check. The city or county reviews the drawings and usually sends back a list of corrections. Your designer answers them and resubmits. Some garage conversions and smaller ADUs can move through faster counter review; larger or more complex units go through regular plan check, which can take a few months with corrections.
4. Utilities. Utility work often runs alongside plan check: applying for any new service or meter, sizing the electrical panel and planning the sewer connection. Utility companies run on their own schedules, so it pays to start early.
5. Construction. Foundation, framing, roof, rough-in of plumbing, electrical and HVAC, insulation, drywall and finishes, with inspections along the way. A detached ADU typically takes several months to build; a garage conversion or JADU is usually shorter. From the first visit to move-in, plan on six months to a year for a detached ADU, and less for a conversion.
Utility connections and separate meters
An ADU needs electricity, water and a sewer connection, and often gas. You have two basic choices for each: share the main house's service, or set up a separate one.
Sharing is simpler and usually costs less up front. The trade-off is that you'll need another way to split the bills if you rent the unit. Separate meters let a tenant pay their own utilities, but they can add connection charges, new service lines and wait time from the utility.
For a JADU, or an ADU built inside an existing house or garage, state law generally doesn't let a city require a new or separate utility connection. For a new detached ADU, the city or utility may require one and charge a connection fee. Electric service in the City of Los Angeles comes from LADWP; most of the rest of the county is served by Southern California Edison or a city-owned utility.
Don't forget the electrical panel. Adding a kitchen, a bathroom and heating and cooling often pushes an older panel past its capacity, and an upgrade is common.
Renting it out and property tax
An ADU can be rented long-term, and for many owners rent is the reason to build. A few things to know:
- State law lets cities bar ADU rentals shorter than 30 days, and many do. Don't plan on a short-term vacation rental without checking.
- In most places, an ADU can't be sold separately from the main house.
- Local rent and tenant protections may apply once someone moves in. Check the rules for your city before you sign a lease.
- Talk to your insurance agent before construction starts and again before a tenant moves in.
On property tax, California generally assesses only the new construction. The ADU's value is added to your assessment, and the existing house keeps its current assessed value rather than being reassessed. Your county assessor can confirm how that works for your property, and a tax professional can tell you how rental income affects your taxes.
If you already have a converted garage or back unit that was built without permits, don't assume you can just start renting it. Many cities have paths to legalize existing units, but the work has to be inspected and may need upgrades first. Our permit guide explains why unpermitted work matters.
Prefab or site-built
A site-built ADU is framed on your lot, piece by piece. A prefab ADU is built partly or fully in a factory, then delivered and set on a foundation your contractor prepares. Prefab can shorten time on site and makes some costs more predictable, while site-built gives more freedom with odd lot shapes, slopes and custom layouts. Either way, you still need permits, a foundation, utility connections and inspections, and good access to the backyard matters more for prefab. HRG's prefab option is HRG Prefab, if you want to compare the two for your lot.
Pick the type of unit, enter the size and a few details, and get a price range for your area with a timeline.
The planner opens on a detached ADU. You can also go straight to a garage conversion or a JADU.
Common questions
Can I build an ADU and a JADU on the same lot?
On many single-family lots, yes. State law generally allows one of each. Your city may have rules on how they're arranged, so confirm with the planning department.
Do I lose my parking if I convert the garage?
State law generally doesn't require you to replace parking when a garage becomes an ADU. You lose the covered spaces, of course, so think about where cars will go, including a tenant's.
Do I have to live on the property?
Not for an ADU. For a JADU, owner occupancy generally applies only when the JADU shares a bathroom with the main house. Rules have changed over the years, so check what your city applies today.
How long does an ADU take?
Timelines vary. A garage conversion or JADU often takes several months from start to finish. A detached ADU commonly takes six months to a year, with design and plan check making up a large share of that.