Everything You Need to Know About California ADU Laws

Granny flats, backyard cottages, garage conversions — demystified. Based on the official California HCD Accessory Dwelling Unit Handbook and on the Government Code as it reads today.

General information about California law, not legal advice. ADU rules change from session to session and local implementation varies, so confirm anything here with your local building department, or with a lawyer, before you rely on it.

📄 Download the Free Official HCD ADU Handbook (PDF)

California is in a housing crisis — and Accessory Dwelling Units (ADUs) are one of the most powerful tools homeowners have right now to add property value, generate rental income, and help solve the shortage. The state has dramatically loosened the rules since 2020, making it easier and cheaper than ever to build.

This guide breaks down the official California Department of Housing and Community Development (HCD) ADU Handbook into plain English. Whether you're thinking about a backyard cottage, converting your garage, or creating a suite for a family member — you'll find exactly what's allowed, what's required, and what local governments cannot restrict.

One thing to know before you go looking any of this up: California renumbered its ADU statutes. The rules that sat at Government Code § 65852.2 and § 65852.22 for years now have a chapter of their own, Gov. Code § 66310 through § 66342, and the sections cited below are the current ones. An older guide citing § 65852.2 is not necessarily wrong about the rule — it is citing an address the law has moved out of.

What is an Accessory Dwelling Unit?

An ADU — also called a granny flat, in-law unit, casita, second unit, or backyard cottage — is a residential dwelling with complete, independent living facilities on the same lot as a primary home. That means its own sleeping area, kitchen, bathroom, and separate entrance (Gov. Code § 66313(a)).

The California Legislature has officially declared ADUs "an essential component of California's housing supply" (Gov. Code § 66310(h)). That declaration matters: local governments are legally required to allow them and cannot create unreasonably restrictive barriers to building one (Gov. Code § 66311).

Key fact: ADUs do not count against the allowable density for your lot under zoning — meaning adding one doesn't require any variance or density exception (Gov. Code § 66319).

Types of ADUs in California

Detached ADU — A freestanding structure separate from the main home (new cottage, converted barn, prefab unit). Max 1,200 sq ft (Gov. Code § 66314(d)(5)).
Attached ADU — An addition connected to the primary home, sharing a wall. Max 50% of the existing primary dwelling (Gov. Code § 66314(d)(4)) — but no local rule may stop you building at least 800 sq ft (Gov. Code § 66321(b)(3)).
Converted Space ADU — Existing space (garage, storage area, bonus room) converted to a dwelling. No size limit for existing footprint.
Junior ADU (JADU) — A unit entirely within the existing single-family home, up to 500 sq ft with its own kitchen (Gov. Code § 66313(d)).

Size & Lot Requirements

The "statewide exemption ADU". One of the most important provisions in California ADU law is the unit a local agency has to approve ministerially whatever its own ordinance says: a detached, new-construction ADU with four-foot side and rear setbacks. The agency may cap its floor area at 800 square feet of livable space and may apply the height limits below — and nothing else. No local rule on lot coverage, floor area ratio or open space can prevent you from building one (Gov. Code § 66323(a)(2) and § 66321(b)(3)).

How tall it can be — and this is no longer one number. A local height limit has to allow at least all of the following (Gov. Code § 66321(b)(4)):

16 feet for a detached ADU on a lot with an existing or proposed home on it.
18 feet for a detached ADU where the lot is within half a mile walking distance of a major transit stop or a high-quality transit corridor — plus another two feet where that is what it takes to match the roof pitch of the main house.
18 feet for a detached ADU on a lot with an existing or proposed multistory multifamily building.
25 feet, or the local height limit that applies to the main house, whichever is lower, for an ADU attached to the house. Nothing requires a local agency to allow an attached ADU taller than two storeys.

Minimum rules every locality must follow:

No minimum lot size requirements allowed (Gov. Code § 66314(b)(1))
Maximum size must be at least 850 sq ft for any ADU (Gov. Code § 66321(b)(2)(A))
Maximum size must be at least 1,000 sq ft for ADUs with more than one bedroom (Gov. Code § 66321(b)(2)(B))
Lot coverage, floor area ratio and open space rules cannot block an ADU of at least 800 sq ft with four-foot side and rear setbacks (Gov. Code § 66321(b)(3))
No limit on number of bedrooms is permitted

Conversions of existing structures — a barn, say — are not subject to size caps; only expansions beyond the existing footprint trigger size limits (Gov. Code § 66323(a)(1)(A)).

📄 Download the HCD Handbook for the full size requirements →

Setback Rules

New detached or attached ADU: No more than 4 feet from side and rear lot lines (Gov. Code § 66314(d)(7))
Conversion of existing structure (same footprint): No setback required (Gov. Code § 66314(d)(7))
Expansion of an existing accessory structure: No more than 4 feet for the expanded area
Front yard setbacks: May be required, but cannot prevent a statewide exemption ADU (Gov. Code § 66321(b)(3))

Practical tip: Converting your detached garage into an ADU? Zero setback is required if you stay within the existing footprint. You can even expand up to 150 sq ft for an entry or egress without triggering additional setback rules (Gov. Code § 66323(a)(1)(A)).

Parking Requirements

California law limits parking requirements to a maximum of one space per ADU or per bedroom, whichever is less (Gov. Code § 66314(d)(10)(A)). Guest parking can never be required.

When no parking can be required at all (Gov. Code § 66322(a)):

The ADU is within ½ mile walking distance of public transit
The ADU is in a historic district
The ADU is within the existing primary residence or accessory structure
On-street parking permits are required but not offered to the ADU occupant
A car-share vehicle is located within one block

Garage conversions: When you convert a garage to an ADU, the city cannot require you to replace those lost parking spaces (Gov. Code § 66314(d)(11)).

Impact Fees

ADUs under 750 sq ft: Completely exempt from impact fees (Gov. Code § 66311.5(c)(1))
ADUs 750 sq ft or larger: Fees charged proportionally to the primary home's square footage (Gov. Code § 66311.5(c)(1))
School fees: Only apply to ADUs over 500 sq ft (Gov. Code § 66311.5(c)(3) and Education Code § 17620)
Water & sewer connection fees: ADUs converted from existing space cannot be treated as a "new residential use" (Gov. Code § 66311.5(b)) — a significant cost saving

For example: an ADU half the size of the primary home means the impact fee would be at most half of what would be charged for a new primary dwelling. Local agencies can waive fees entirely.

📄 Download the full handbook for fee calculation details →

Junior Accessory Dwelling Units (JADUs)

A JADU is entirely within the walls of an existing single-family residence (including attached garages), maxing out at 500 square feet (Gov. Code § 66313(d) and § 66333(d)). The key tradeoff is owner occupancy — but only in one case: a JADU that shares sanitation facilities with the house requires the owner to live in the house or in the JADU. Give the JADU its own bathroom and that requirement falls away, as it does where the owner is a government agency, a land trust or a housing organisation (Gov. Code § 66333(b)).

Max size — ADU: 1,200 sq ft (detached). JADU: 500 sq ft.
Owner occupancy required? — ADU: No, and there is no longer an expiry date on that. A local agency may not impose an owner-occupant requirement on an ADU at all; it may only require that any tenancy run 30 days or longer (Gov. Code § 66315). JADU: Yes, where the unit shares sanitation facilities with the house (Gov. Code § 66333(b)).
Detached structure allowed? — ADU: Yes. JADU: No — must be within the home.
Per lot limit — ADU: Varies. JADU: One per single-family lot (Gov. Code § 66333(a)).
Separate bathroom required? — ADU: Yes. JADU: No — it may share with the main home, but sharing is exactly what triggers the owner-occupancy rule above.

You can combine a JADU with a detached ADU on the same single-family lot (Gov. Code § 66323(a)(1)) — giving you two additional units alongside the primary dwelling.

Garage & Existing Space Conversions

Converting an existing structure is the fastest, most affordable ADU path. California's laws make this particularly attractive:

No size restrictions — a barn converted to an ADU is not subject to size caps
No setback requirements — use the existing footprint without setback compliance
No lot coverage limits — conversion doesn't count against lot coverage
No parking replacement — convert your garage and you don't have to rebuild those spaces
150 sq ft expansion allowed — for ingress/egress only, e.g. adding a staircase (Gov. Code § 66323(a)(1)(A))

Fire sprinklers cannot be required in an ADU if they aren't required in the primary residence (Gov. Code § 66314(d)(12) and § 66323(d)).

Solar panels are required for new, non-manufactured, detached ADUs — but panels can be installed on either the ADU or the primary dwelling, and conversions are exempt. That requirement comes from the state energy code rather than from the ADU statutes, so confirm the current version with your building department.

Can My HOA Block My ADU?

No. Assembly Bill 670 (2019) made HOA restrictions on ADUs void and unenforceable, and that rule now lives at Civil Code § 4751. Any CC&R clause or governing-document provision that effectively prohibits or unreasonably restricts the construction of an ADU or JADU on a single-family lot is null by state law — regardless of what your HOA documents say. Restrictions that are genuinely reasonable, in that they neither prohibit an ADU nor unreasonably drive up the cost of one, do still stand (Civil Code § 4751(b)).

ADU Permit Readiness Checklist

Use this before submitting your permit application:

✅ ADU is not intended for separate sale from primary residence
✅ Lot is zoned single-family or multifamily and has a primary dwelling
✅ Attached ADU does not exceed 50% of primary dwelling (minimum 800 sq ft allowed)
✅ Detached ADU does not exceed 1,200 sq ft
✅ Side and rear setbacks are at least 4 feet (or zero for conversions)
✅ Height is within what your local ordinance allows, which cannot be less than 16 feet for a detached unit
✅ Parking does not exceed 1 space per unit or bedroom (whichever is less)
✅ No replacement parking required for converted garage
✅ ADU under 750 sq ft — confirm impact fee exemption applies
✅ Solar panel requirement verified with local building department
✅ Fire sprinkler requirement confirmed relative to primary home
✅ If JADU: owner-occupancy deed restriction ready to record where sanitation is shared

Ready to Start Your ADU Project?

ADUs are one of the most cost-effective ways to increase property value and generate passive income in California. Understanding your rights under California law is the first and most important step, and the sections above are where those rights are written down.

County-level contacts and programmes for California are listed in our California county resource directory.

📄 Download the Free California ADU Handbook (PDF) — Official HCD Publication

Sources: California Department of Housing and Community Development, Accessory Dwelling Unit Handbook. California Government Code, Title 7, Division 1, Chapter 13 — Accessory Dwelling Units, at Gov. Code § 66310 to § 66342, which is where the rules formerly at Gov. Code § 65852.2, § 65852.22 and § 65852.26 now live. Civil Code § 4751.