Terminology
Mother-in-Law Suite: What It Is and What California Law Allows
Households say mother-in-law suite. The law says accessory dwelling unit. Learning to say the second one is the most useful thing on this page.
Last updated
What is a mother-in-law suite?
A mother-in-law suite is a self-contained living space on the same lot as a single-family home, with its own kitchen, bathroom, and entrance. In California it has a legal name: accessory dwelling unit (ADU). That name matters, because ADUs get ministerial approval, meaning your city must approve a compliant application without a discretionary hearing, a design review board, or a neighbor’s objection.
“Mother-in-law suite” is what people say. “In-law suite,” “granny flat,” “casita,” and “guest house” are the same thing in different regions. None of those phrases appear in California law.
The phrase that does appear is accessory dwelling unit, and the gap between the two is where most homeowners lose months. Walk into a planning counter asking about a mother-in-law suite and you may get a conversation about guest houses, second units, and conditional use permits. Walk in asking about an ADU and you have invoked a body of state law that strips your city of most of its power to say no.
Are you buying one, or building one?
These are different projects and this page is mostly about the second.
If you’re house hunting and want a home that already has one, the thing to check before you make an offer is whether the existing unit is permitted. Unpermitted in-law suites are extremely common. An unpermitted unit can create problems with appraisal, insurance, lending, and eventual resale, and legalizing one after the fact is its own project. Ask the seller for the permit record and check it against the county assessor’s description of the property. If the space doesn’t appear in the permit history, treat it as a garage with a bed in it until proven otherwise.
The rest of this page is for people building.
The three shapes it can take
| Type | What it is | Size limit | Typical trade-off |
|---|---|---|---|
| Detached | Standalone structure in the yard | Local cap, but must allow 850+ sq ft (1,000+ with 2 bedrooms) | Most privacy, highest cost |
| Attached | Addition sharing a wall with the house | Local cap; may be capped at a percentage of the main house | Cheaper utilities, less separation |
| Converted | Existing garage, basement, or rooms | No state size cap applies | Cheapest route, constrained by what exists |
| JADU | Up to 500 sq ft carved out inside the house | 500 sq ft | Cheapest of all; different legal rules |
The converted row is the one people miss. Under Government Code § 66323(a)(1), converting existing space such as an attached garage, a basement, or a portion of the house is not subject to the unit size requirements that apply to new construction. If you’re converting an accessory structure, you’re also allowed up to 150 additional square feet purely to add ingress and egress, like a stairwell or a door landing.
The JADU row is a different animal. A junior ADU is no more than 500 square feet, carved out of the existing single-family residence, and can share a bathroom with the main house. It only needs an “efficiency kitchen,” meaning appliances, a food prep counter, and storage cabinets. For a parent who wants proximity rather than total independence, it’s often the right answer and the cheapest one.
What California law actually permits
The state has spent several legislative sessions removing the tools cities used to block these units. As of 2026, here’s what your city cannot do.
It cannot require you to live on the property. Owner-occupancy requirements on ADUs are prohibited under Government Code § 66315. The provision that would have let this sunset in 2025 was removed, so it’s now permanent. JADUs are the exception. Owner-occupancy can be required, but only when the JADU shares sanitation facilities with the primary home. That change came from AB 1154, effective January 1, 2026.
It cannot impose setbacks greater than four feet on the side and rear for a new detached unit.
It cannot cap your size below 850 square feet, or below 1,000 square feet for a unit with more than one bedroom, if it has a compliant ADU ordinance. If it doesn’t have one, the fallback is 1,200 square feet for a new detached ADU.
It cannot use open space or lot coverage rules to deny an 800-square-foot ADU that keeps four-foot setbacks.
It cannot sit on your application. Under SB 543, effective January 1, 2026, the agency must tell you in writing within 15 business days whether your application is complete, and if it’s incomplete, hand you an itemized list of what’s missing. Miss the deadline and the application is deemed complete. It then has 60 days to approve or deny.
It cannot make you replace parking you lose by converting or demolishing a garage to build the unit.
What it can still do is set height limits, but only within state floors: at minimum 16 feet for a detached unit, 18 feet if you’re within half a mile of a major transit stop or high-quality transit corridor, and 25 feet for an attached unit, or the primary dwelling’s limit if that’s lower.
What it costs
Cost tracks the three shapes above almost perfectly, because the expensive parts are the ones a conversion already has: a foundation, a roof, and walls.
A garage conversion is the cheapest path because the shell exists. New detached construction is the most expensive because nothing does. Attached construction sits in between and can be cheaper on utilities, since you’re extending existing service rather than running new laterals.
For detailed numbers by build type, including the line items most quotes hide, see our full breakdown of mother-in-law suite costs and ADU costs in California.
Two fee thresholds are worth knowing before you settle on a size, because they’re cliffs rather than slopes:
- 750 square feet. At or below, your unit is exempt from local impact fees under Government Code § 66311.5. Above it, fees can be charged and are prorated against the primary dwelling’s square footage.
- 500 square feet. Below this, the unit is treated as not increasing assessable space for school developer fee purposes, which generally takes school fees off the table.
An 800-square-foot unit and a 750-square-foot unit are not 50 square feet apart in cost.
Renting it out
You can, long-term.
State law requires that these units be rented for terms longer than 30 days. That closes off short-term vacation rental use, which is the business model a lot of people quietly have in mind. If your plan depends on nightly rates, the plan doesn’t work.
Long-term rental is explicitly contemplated by the law and is the reason owner-occupancy requirements were removed. You can build one, rent it, and live elsewhere.
One caveat to raise with a tax professional before you commit: adding a unit changes your property tax picture. California generally reassesses the new construction portion rather than the whole property, but the specifics depend on your county and your situation, and it’s cheaper to ask before you build than to discover after.
How long it takes
The permitting side runs on clocks the state sets. The construction side does not, and that is where the variance lives.
15 business days for your city to tell you in writing whether the application is complete. Miss it and the application is deemed complete.
60 days to approve or deny a complete application. 30 days if you used preapproved plans.
Before all of that, budget time for the parts nobody schedules: a soils report if you are building detached, an electrical load calculation, locating the sewer lateral, and the design itself. These are weeks, not days, and they happen before the clock starts.
After approval, construction time depends entirely on type. A junior ADU carved out of existing space is a modest job. A detached new build is a real construction project with a foundation, inspections at each rough stage, and a final. Prefab compresses the on-site portion considerably, which is most of what you are paying for.
The mistakes that cost the most
Designing before the soils report. A report that comes back requiring an engineered foundation after the plans are drawn means redrawing them. Run it first.
Not checking the panel. An electrical service upgrade is the most common five-figure surprise on an otherwise ordinary project, and an electrician can tell you in an hour.
Crossing 750 square feet without noticing. The impact fee exemption ends there. A design that drifts from 740 to 780 square feet during revisions has picked up a fee category, and nobody flags it because it happens gradually.
Assuming your city’s rules are current. Some local ordinances still say things state law no longer permits, such as owner-occupancy requirements or replacement parking. If a planner tells you something that contradicts what is on this page, ask which code section they are citing. The recodification from Government Code 65852 to 66310 through 66342 caught a lot of ordinances out.
Treating an unpermitted unit as a shortcut. It cannot be financed against, it will not appraise, it complicates a sale, and legalising it afterward costs more than permitting it correctly the first time.
Getting started
The order that saves the most money:
- Figure out which of the four types your property supports. A conversion is dramatically cheaper if you have a suitable garage or basement. Check that first, before you fall in love with a detached design.
- Check your city’s preapproved plans. Since January 1, 2025, every California jurisdiction has been required to run a preapproved ADU plan program and post the plans on its website. Using one cuts plan check substantially, and an application using preapproved plans must be approved or denied within 30 days. See our guide to ADU plans and preapproved programs.
- Get a soils report early if you’re building detached. Foundation surprises are the most common source of budget overruns.
- Line up financing before design. What you can borrow shapes what you should draw. See ADU financing options.
If the person moving in is an aging parent, the design priorities shift toward step-free entry, door widths, and bathroom clearances. We wrote that version separately: granny flats and aging-in-place design.
Sources
Common questions
- Is a mother-in-law suite the same as an ADU?
- In California, usually yes. 'Mother-in-law suite' is a household term with no legal meaning. If the space has its own kitchen, bathroom, and entrance, it is an accessory dwelling unit under state law, and ADU rules apply. If it shares a bathroom with the main house and sits inside it, it may instead be a junior ADU, which is a separate legal category with different rules.
- Do I have to live on the property if I build one?
- Not for an ADU. California prohibits local agencies from imposing owner-occupancy requirements on accessory dwelling units. Junior ADUs are different. Owner-occupancy can be required only if the JADU shares sanitation facilities with the primary home.
- Can I rent out a mother-in-law suite?
- Long-term, generally yes. Rentals must be for terms longer than 30 days, which rules out short-term vacation rental use for these units. Whether your city allows anything shorter is a separate local question, but state law sets the 30-day floor.
- How big can it be?
- If your city has a compliant ADU ordinance, its maximum size rule must still allow at least 850 square feet, or 1,000 square feet for a unit with more than one bedroom. Converting existing space such as a garage, a basement, or part of the house is not subject to those size caps at all.