california-adu

name	california-adu
description	California state-level ADU and JADU rules from the HCD ADU Handbook (Jan 2025) and 2026 Addendum. Covers Government Code §§ 66310-66342 including setbacks, height, size, parking, permitting, fees, and ownership. Use this skill for any California ADU question about state requirements.
version	1.0
source	HCD ADU Handbook, January 2025 (54 pages) + January 2026 Addendum (pages 55-58)
authority	California Department of Housing and Community Development
law_as_of	January 1, 2026

California ADU Regulatory Decision Engine

This skill contains the California state-level ADU (Accessory Dwelling Unit) and JADU (Junior Accessory Dwelling Unit) rules extracted from the HCD ADU Handbook (January 2025) and the January 2026 Addendum. It covers Government Code §§ 66310-66342 (including new §§ 66311.5, 66333.5) as amended through January 1, 2026.

What this covers: State law — the floor that cities cannot go below. Cities may be more permissive but cannot impose standards more restrictive than what is stated here.

What this does NOT cover: City/county-specific ordinances, federal requirements (FHA, ADA), or project-specific engineering.

How to Use This Skill

When answering an ADU question, follow the 4-step decision tree below. Each step tells you which reference files to load. Load only what you need — most questions require 3-5 reference files, not all 28.

After loading references, check the Quick-Reference Thresholds table below for the key numbers that come up in almost every query.

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Decision Tree Router

STEP 1: Classify the Lot Type

What kind of lot is the ADU proposed on?

Lot Type	Load These References
Single-family lot	unit-types-66323.md, unit-types-adu-general.md
Multifamily lot (2+ existing units)	unit-types-multifamily.md, unit-types-66323.md
JADU only (within existing/proposed single-family dwelling)	unit-types-jadu.md
Unsure / mixed question	unit-types-66323.md (start here — it has the combinations table)

STEP 2: Classify the ADU Construction Type

How is the ADU being built?

Construction Type	Load These References
New detached construction	standards-height.md, standards-size.md, standards-setbacks.md
Conversion from existing space (garage, basement, etc.)	standards-size.md, zoning-nonconforming.md
Attached addition to existing dwelling	standards-height.md, standards-size.md, standards-setbacks.md
Manufactured/factory-built home	special-manufactured.md

STEP 3: Check Situational Modifiers

Does any of the following apply? Load the additional references.

Situation	Load These References
Near public transit (½ mile of rail/ferry, ¼ mile of bus)	standards-height.md (18 ft bonus), standards-parking.md (exemptions)
In coastal zone	zoning-hazards.md
In fire hazard severity zone	zoning-hazards.md, standards-fire.md
In historic district	zoning-general.md, standards-design.md
Property has an HOA	ownership-hoa.md
Rental / short-term rental question	ownership-use.md
Selling or converting ADU to condo	ownership-sales.md
SB 9 lot split involved	special-sb9.md
Nonconforming zoning or building code violations	zoning-nonconforming.md
Solar / energy code question	standards-solar.md
Fire sprinkler question	standards-fire.md
Parking question	standards-parking.md
Design review / subjective standards question	standards-design.md

STEP 4: Check Process, Fees, and Compliance

What stage is the project at?

Topic	Load These References
Permit application / timeline / pre-approved plans	permit-process.md
Impact fees / connection fees	permit-fees.md
Financing / grants / loans	permit-funding.md
Unpermitted existing ADU (legalization)	compliance-unpermitted.md
Local ordinance question / HCD enforcement	compliance-ordinances.md
Housing element / RHNA credit	compliance-housing.md
Utility connections	compliance-utilities.md
Legislative history / bill tracking	legislative-changes.md
Definition of a term	glossary.md

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Quick-Reference Thresholds

These numbers come up in almost every ADU query. Check here first before loading a reference file — you may already have the answer.

Threshold	Value	Reference File
JADU maximum size	500 sq ft of interior livable space	unit-types-jadu.md
66323(a) converted/detached max	800 sq ft	unit-types-66323.md
Minimum ADU size (efficiency unit)	150 sq ft	standards-size.md
Minimum ADU size (0-1 bedroom)	850 sq ft of interior living space	standards-size.md
Minimum ADU size (2+ bedrooms)	1,000 sq ft of interior living space	standards-size.md
Maximum detached ADU (no local ordinance)	1,200 sq ft of interior living space	standards-size.md
Fee exemption threshold	≤ 750 sq ft of interior livable space (§ 66311.5)	permit-fees.md
Maximum side/rear setback	4 ft	standards-setbacks.md
Detached height — base	16 ft	standards-height.md
Detached height — near transit or multistory primary	18 ft	standards-height.md
Detached height — transit + additional 2 ft for roof pitch	18 ft (+2 ft)	standards-height.md
Attached ADU height	25 ft (or primary dwelling height)	standards-height.md
Completeness check deadline	15 business days	permit-process.md
Approval/denial deadline	60 days	permit-process.md
Minimum rental term	30 days	ownership-use.md
Unpermitted ADU amnesty cutoff	Before January 1, 2020	compliance-unpermitted.md
Multifamily detached ADU cap

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Key Principles

1. State preemption: State law sets minimum ADU rights. Cities cannot impose standards more restrictive than those in these reference files. Cities can be more permissive.

2. Ministerial approval: ADU permits must be approved ministerially (no discretionary review, no public hearings, no CEQA). See permit-process.md.

3. No density count: ADUs and JADUs do not count toward allowable density for the lot. See zoning-general.md.

4. Objective standards only: Cities may only apply objective (measurable, verifiable) development standards to ADUs — not subjective design review. See standards-design.md.

5. Owner-occupancy: Required for JADUs only if the JADU shares sanitation facilities with the primary structure. Not required for ADUs (except for separate conveyance under AB 1033). See ownership-use.md.

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Reference File Catalog

See AGENTS.md for the complete catalog of all 28 reference files with descriptions and key code sections.