The kids are gone and the back bedroom, the old office and the bathroom between them sit unused nine months a year. You have priced a backyard cottage and flinched at the number. There is a smaller move California created specifically for houses in this position: convert the space you already have into a self-contained unit, without pouring a foundation or touching the yard.
The short answer: a junior accessory dwelling unit is no more than 500 square feet, built entirely within an existing or proposed single-family residence, and one is allowed per lot in single-family zones. It needs its own entrance and an efficiency kitchen, may share the house's bathroom, and a city may not require parking for it (ADU Handbook, JADUs; § 66333).
Key numbers
| Rule | Requirement | Where it comes from |
|---|---|---|
| Maximum size | 500 square feet | ADU Handbook, JADUs |
| How many per lot | One, on a lot zoned for single-family with an existing or proposed house | § 66333(a) |
| Where it may be built | Within the walls of the single-family residence, including attached garages | § 66333(d) |
| Parking | May not be required, even converting an attached garage | § 66334(a) |
| Minimum rental term | Longer than 30 days | § 66333(g) |
| Expansion beyond the house's footprint | Not permitted — the 150 sq ft allowance is for ADUs only | ADU Handbook, JADUs |
What makes a unit "junior"
a unit that is no more than 500 square feet in size and contained entirely within a single-family residence
That second half is the defining constraint. A junior unit is carved out of the house — it adds no building, and it may not push past the walls that already exist (ADU Handbook, JADUs). "Enclosed uses within the residence, such as attached garages," count as part of the single-family residence for this purpose, so an attached garage is fair game (§ 66333(d).
A detached garage, barn or workshop is not. Junior units are not allowed in detached accessory structures — a conversion out there is a regular accessory dwelling unit instead, with its own rules.
And the 150-square-foot egress expansion people hear about does not apply here. That allowance belongs to ADUs created from existing accessory structures, letting them grow slightly to add a staircase or doorway. A junior unit may not be expanded beyond the existing dimensions of the single-family dwelling (ADU Handbook, JADUs).
Two coverage details from the state's guidance:
- Half-plexes qualify. Two primary units on separate lots sharing a dividing wall are half-plexes, and one junior unit is permitted in each in single-family zones.
- Duplexes do not. Two attached units on a single lot make the primary dwelling multifamily, which does not qualify for a junior unit.
The five things the unit itself must have
Where a local agency adopts a junior-unit ordinance, state law requires it to include all of the following (§ 66333):
- One per lot, on a lot zoned for single-family residences with a house built or proposed on it.
- A separate entrance from the main entrance to the residence. There is no requirement for an interior connection to the rest of the house — but if the junior unit has no separate bathroom, it must also have an interior entry to the main living area, so the occupant can reach the shared bathroom without going outside.
- An efficiency kitchen, consisting of a cooking facility with appliances, and a food preparation counter and storage cabinets "that are of reasonable size in relation to the size of the junior accessory dwelling unit."
- A recorded deed restriction, running with the land and filed with the permitting agency, that prohibits selling the junior unit separately from the house — stating that it is enforceable against future purchasers — and restricts the unit's size and attributes to conform with state law.
- Rental terms longer than 30 days. Junior units are not short-term rentals.
That third item has a hard edge worth knowing. A local agency cannot be more restrictive than the state's definition of an efficiency kitchen — so requirements dictating counter or cabinet dimensions, specific electrical or gas connections, or particular appliance types are not authorized (ADU Handbook, JADUs). If a plan check comment demands a full-size range or a specified counter run, ask which authority it rests on.
Owner-occupancy: it depends on the bathroom
Junior units are one of the few housing types in California that can still carry an owner-occupancy condition, and whether yours does turns on plumbing.
Where the junior unit has shared sanitation facilities with the existing structure, the ordinance must require owner-occupancy in the single-family residence — and the owner may live in either the remaining portion of the house or the junior unit itself. Owner-occupancy is not required where the junior unit has separate sanitation facilities, or where the owner is a governmental agency, land trust or housing organization (§ 66333(b)).
In plain terms: sharing a bathroom keeps you living on the property. Building the unit its own bathroom is what buys the freedom to move away and rent the whole house.
Parking, inspections, and cities without an ordinance
No parking. A junior-unit ordinance shall not require additional parking as a condition of granting a permit (§ 66334(a) — and the state confirms this holds even when the unit is converted from an attached garage (ADU Handbook, JADUs). Losing the garage does not oblige you to replace the parking it held.
Inspections are still allowed. Nothing in the article prohibits requiring an inspection — including charging a fee for it — to determine whether the junior unit complies with applicable building standards (§ 66334(b)). Being exempt from parking and setback demands does not exempt the work from the building code.
No local ordinance? You are not stuck. Where a local agency has not adopted a junior-unit ordinance, it must ministerially approve a permit to construct a junior unit that satisfies the requirements of Government Code section 66323(a)(1) and of this article (§ 66339). A city's silence is not a veto.
Junior unit or full ADU?
Both can go on the same lot in many cases, and they are priced and regulated differently. A junior unit is cheaper — no foundation, no new envelope — but it is capped at 500 square feet, must live inside the house, and can bind you to living there. A full ADU can be detached and larger: a city's maximum size must allow at least 850 square feet, or 1,000 square feet for more than one bedroom, and lot coverage or floor area rules may not preclude an ADU of at least 800 square feet with four-foot rear and side setbacks (ADU Handbook, Size Requirements).
State law here is a limit on what cities may require, and local ordinances do not always keep pace. If your city's junior-unit rules look stricter than the above, the California Department of Housing and Community Development reviews local ordinances for compliance — hcd.ca.gov.
Sources
- § 66333 — Junior accessory dwelling unit ordinances (Government Code)
- § 66334 — Parking and inspections (Government Code)
- § 66339 — Approval where no local ordinance exists (Government Code)
- JADUs — 2025 California ADU Handbook (HCD)
- Size Requirements and Restrictions — 2025 California ADU Handbook (HCD)
