adu

Everything You Need to Know About California ADU Laws

June 01, 2026 7 min read Larry Trimboli

Granny flats, backyard cottages, garage conversions — demystified. Based on the official California HCD Accessory Dwelling Unit Handbook.

📄 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.

What is an ADU?
Types of ADUs
Size & Lot Requirements
Setback Rules
Parking Requirements
Impact Fees
Junior ADUs (JADUs)
Garage & Space Conversions
HOA Restrictions
Permit Checklist

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.

The California Legislature has officially declared ADUs "an essential component of California's housing supply" (Gov. Code § 65852.150). That declaration matters: local governments are legally required to allow them and cannot create unreasonably restrictive barriers to building one.

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.

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.
Attached ADU — An addition connected to the primary home, sharing a wall. Max 50% of primary dwelling (minimum 800 sq ft allowed).
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. Max 500 sq ft.

Size & Lot Requirements

The "Statewide Exemption ADU"

One of the most important provisions in California ADU law is the statewide exemption ADU: a detached ADU of up to 800 square feet, 16 feet in height, with 4-foot side and rear setbacks. No local rule — including lot coverage, floor area ratio, or open space restrictions — can prevent you from building one.

Minimum rules every locality must follow

No minimum lot size requirements allowed
Maximum size must be at least 850 sq ft for any ADU
Maximum size must be at least 1,000 sq ft for ADUs with more than one bedroom
Lot coverage rules cannot block the statewide exemption ADU (800 sq ft)
No limit on number of bedrooms is permitted

Conversions of existing structures (like a 3,000 sq ft barn) are not subject to size caps — only expansions beyond the existing footprint trigger size limits.

📄 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
Conversion of existing structure (same footprint): No setback required
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

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.

Parking Requirements

California law limits parking requirements to a maximum of one space per ADU or per bedroom, whichever is less. Guest parking can never be required.

When NO parking can be required at all

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.

Impact Fees

ADUs under 750 sq ft: Completely exempt from impact fees
ADUs 750 sq ft or larger: Fees charged proportionally to the primary home's square footage
School fees: Only apply to ADUs over 500 sq ft
Water & sewer connection fees: ADUs converted from existing space cannot be treated as a "new residential use" — a significant cost saving

For example: a 2,000 sq ft primary home with a proposed 1,000 sq ft ADU means the impact fee would be at most 50% 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. The key tradeoff: JADUs require owner occupancy of either the main home or the JADU itself.

Rule	ADU	JADU
Max size	1,200 sq ft (detached)	500 sq ft
Owner occupancy required?	No (until 2025)	Yes
Detached structure allowed?	Yes	No — must be within home
Per lot limit	Varies	One per single-family lot
Separate bathroom required?	Yes	No — may share with main home

You can combine a JADU with a detached ADU on the same single-family lot — 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 3,000 sq ft 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)

Fire Sprinklers & Solar Panels

Fire sprinklers cannot be required in an ADU if they aren't required in the primary residence.

Solar panels are required for new, non-manufactured, detached ADUs — but panels can be installed on either the ADU or the primary dwelling. Conversions are exempt.

Can My HOA Block My ADU?

No. Assembly Bill 670 (2019) made HOA restrictions on ADUs void and unenforceable. Any CC&R clause 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.

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)
✅ 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

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. A 2019 Terner Center study found one affordable housing unit in the Bay Area cost roughly $450,000 through traditional construction — an ADU often comes in at a fraction of that price, with rental income frequently paying off the project within a few years.

Understanding your rights under California law is the first and most important step.

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

Sources: California Department of Housing and Community Development, Accessory Dwelling Unit Handbook, September 2020. Government Code §§ 65852.2, 65852.22, 65852.26, 65583. Civil Code § 4751.