California ADU
Size-Limit Screen
Compare an adopted local maximum with the minimum size allowance in State ADU Law, or screen a limited state fallback path. The result is not a parcel entitlement.
Who it is for: California homeowners, architects, and contractors who have identified the applicable ordinance path at pre-design and need a transparent first-pass size check.
Still verify: local ordinance status, parcel eligibility, bedroom definition, objective standards, setbacks, height, fire/building code, utilities, coastal or other overlays, and rules effective when the application is submitted.
How to use this calculator
The formula
No compliant local ordinance: HCD describes 1,200 sq ft for a new detached ADU and, for an attached ADU, up to 50% of the existing primary dwelling while allowing at least 800 sq ft.
JADU: maximum 500 sq ft of interior livable space.
Qualifying conversion: conversion of existing space is not subject to unit-size requirements; the existing eligible space and project facts control.
Worked example
When this estimate will be off
- The local-ordinance result compares only a maximum-size number; it does not validate that the ordinance, project classification, or parcel facts are correct.
- The 800-sq-ft protection against lot coverage, floor-area ratio, open-space, and similar constraints is a separate rule from the 850/1,000-sq-ft local maximum-size standard.
- Multifamily §66323 combinations and detached-unit counts are outside this first release.
- A conversion result does not verify that the existing space qualifies or determine its measured livable area.
- Local agencies may adopt less restrictive rules, including maximums above 1,200 sq ft.
Frequently asked questions
A common mistake: The local-ordinance result compares only a maximum-size number; it does not validate that the ordinance, project classification, or parcel facts are correct.
No. HCD identifies 1,200 square feet as the new-detached fallback where there is no compliant local ordinance, and it states that local agencies may adopt less restrictive maximums above 1,200 square feet.
They serve different functions. HCD describes 850/1,000 square feet as minimum maximum-size standards in a local ordinance, while 800 square feet is protected from certain site constraints and appears in other state-created paths.
No. It means only that the entered maximum is not below the numerical 850/1,000 threshold for the entered bedroom count. All other eligibility and development standards remain.
The calculator returns existing-space review rather than inventing a maximum. HCD states qualifying conversions are not subject to unit-size requirements, but the space and project still must satisfy the applicable criteria.
Sources
- California HCD Accessory Dwelling Unit Handbook, updated March 2026 — Size Requirements and Restrictions at pages 37–38; accessed August 29, 2026.
- California Government Code §66321 — Local ordinance size standards and the 800-sq-ft protection.
- California Government Code §66314 — No-compliant-ordinance attached and detached standards.
- California Government Code §66323 — Conversion and specified ministerial ADU paths.
Sources and review record
Sources and method basis
- California HCD Accessory Dwelling Unit Handbook, updated March 2026, pages 37–38
- California Government Code §§66314, 66321, and 66323 size paths
- https://www.hcd.ca.gov/policy-and-research/accessory-dwelling-units
Limitations
- Single-family size-path screen only; does not validate ordinance status, parcel eligibility, multifamily combinations, objective standards, building/fire code, utilities, overlays, or entitlement.
This free California ADU Size Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.