Choosing
ADU, JADU, or garage conversion: which fits your lot
Four ways to add a unit, what each one costs you in constraints, and the questions that decide it before anyone draws anything.
The short version
- These aren't four styles of the same thing. They're four different sets of constraints.
- Your lot has probably already ruled one or two of them out.
- What decides it: rear yard space, whether a truck can reach the back, your electrical panel, sewer or septic, and what you want the unit to do.
- Garage conversions permit fastest and surprise you most.
Most people call us having already picked a type, usually because a neighbor built one. That's a reasonable place to start. It's a bad place to finish.
- A
Detached, built new
A standalone building in the back yard. The most flexible option and usually the best for rental income, since it works as a separate home with its own entrance. It also asks the most of your lot: room to clear the setbacks, a way to get materials to the back, and a fresh run of every utility.
- B
Attached
Built off the house, sharing a wall. Often the answer on a narrow lot where a detached unit can't fit. You save a wall and a utility run. You also inherit the main house's height limit and end up touching its roof and foundation, which is where attached projects get complicated.
- C
Garage conversion
Reusing a building that's already there and already inside the setbacks, which is why it usually permits fastest. What you give up is control. The existing slab, framing, and ceiling height decide what the finished unit can be, and nobody knows what's under that slab until it's open.
- D
Junior ADU
Up to 500 sq ft carved out of the existing house, usually a bedroom with its own entrance. The cheapest of the four by a wide margin. The limits are real: 500 sq ft, a small efficiency kitchen, and you have to live on site if it shares a bathroom with the house.
The five questions that actually decide it
- How much back yard is left after a four foot setback on both sides and the rear. Measure it on the ground. The county's parcel map isn't survey accurate and the difference tends to run the wrong way.
- Can a truck and a crew get to the back of the lot. A detached unit behind a house with no side access changes how everything gets built, and how long it takes.
- What's in your electrical panel. A 100 amp panel with no spare slots means a service upgrade, and that puts the utility company on your critical path.
- Sewer or septic. On septic, the existing system's capacity usually decides how many bedrooms you're allowed, and that conversation comes before a floor plan.
- What do you want it to do. A unit for your parents wants a step-free entry, a full bathroom, and proximity. A rental wants separation and its own meters. Same lot, two different buildings.
A word about garage conversions
They're pitched as the cheap option and often are. But a garage was built to be a garage:
- The slab is usually thinner than a house slab, and often has no vapor barrier under it
- There may be no footing where a bearing wall now needs one
- The ceiling height frequently lands under what a habitable room needs once you add insulation and drywall
None of that is a reason not to do it. It's a reason to have someone open it up and look before you agree to a price. The conversion that goes badly is almost always the one that got priced from the driveway.
We'd rather have this conversation before anyone spends money on drawings. Sometimes it ends with a type nobody at the table had considered. Sometimes it ends with us saying the lot won't take what you had in mind, which is a cheaper thing to hear now than in plan check.
Sources
- California Government Code sections 66310 to 66342
- Junior ADU standards, Government Code section 66333
- HCD Accessory Dwelling Unit Handbook
This is our read of the rules, not legal advice. We write these because they're the questions we get asked, and we keep them current. ADU law changes most Januaries, and every city adds its own rules on top of the state's, so before you spend money on anything here, confirm it with your city or county planning counter. Or call us and we'll confirm it for you.
More guides
ADU law
What California ADU law lets you build
State law sets a floor your city can't go below. Here's what that floor is, what changed in January 2026, and which parts your city still controls.
Planning
Nine things to check before you spend a dollar
Site conditions decide more ADU budgets than design choices do. These are the ones that turn into change orders when nobody looks first.
Process
How an ADU permit actually moves
The six stages between an idea and a building permit, which ones have a legal clock on them, and where the time really goes.
Want this answered for your address?
State law is the floor. What your city allows on top of it is the part that decides your project. Tell us where you are and we'll tell you what fits.