ADU zoning rules in San Carlos have been substantially liberalised by California state law over the past five years — but “state law allows it” and “your specific property qualifies” are not the same statement. Here is what the current rules actually permit in San Carlos in 2026, what local regulations still apply, and how to verify eligibility before committing to design fees.
Since California AB 68 (2020) and subsequent legislation including AB 2221 (2023) and SB 4 (2023), local jurisdictions including San Carlos are prohibited from applying most discretionary restrictions to ADU applications. In 2023, San Mateo County permitted over 600 ADUs across its jurisdictions — a figure driven directly by these state preemption laws removing local barriers that previously blocked most ADU applications.
What California State Law Allows in San Carlos
Under current California ADU law, San Carlos homeowners are entitled to build the following on any single-family or multifamily lot, subject to certain conditions: one ADU of any type (attached, detached, or garage conversion), one Junior ADU (JADU) within the existing structure, and in some cases both an ADU and a JADU on the same lot simultaneously.
Key state law provisions that apply in San Carlos: no replacement parking is required when an existing structure is converted to an ADU, no owner-occupancy requirement may be imposed for ADUs permitted after January 1, 2020, and ministerial approval is required — meaning the city cannot hold public hearings or apply discretionary design review to most ADU applications.
What San Carlos Local Rules Still Apply
While state law preempts most local barriers, San Carlos retains authority over specific development standards. These include: minimum lot size (typically 5,000 square feet for a detached ADU in most San Carlos residential zones), maximum ADU size (state law caps detached ADUs at 1,200 square feet), setbacks (4-foot rear and side setbacks apply to most new detached ADUs), and height (single-story detached ADUs in most San Carlos zones are limited to 16 feet).
The setback exemption for existing structures is one of the most valuable provisions in current law: a detached garage built close to the property line — which would not meet setback requirements for new construction — can typically still be converted to an ADU without meeting the standard setback minimums. This opens ADU eligibility to many San Carlos properties that homeowners incorrectly assume do not qualify. For a full breakdown of what a garage conversion ADU involves, see our garage conversion ADU guide.
How to Check ADU Eligibility for Your San Carlos Property
The fastest and most reliable way to check ADU eligibility for your specific property is an in-person assessment by a contractor who has navigated San Carlos ADU permits before. A site visit identifies lot coverage availability, setback compliance, utility connection distance, and any site-specific constraints — the four factors that most affect whether a project is straightforward or complex.
Brian Pezzulich (CA License #957500) has managed ADU permits across every city in San Mateo County. He visits your property, tells you what is possible, what it will cost, and what the realistic timeline is — before any design fees are committed. For current ADU construction costs, see our ADU cost guide for San Mateo County. For the full ADU build and permit process, see our process page and the ADU construction service page. Completed ADU projects are in our home addition portfolio.
Frequently Asked Questions
Can I build an ADU in San Carlos CA in 2026?
Yes — California state law requires San Carlos to approve ministerial ADU permits for most single-family and multifamily properties. Eligibility depends on lot size, existing structure conditions, and setbacks. Most San Carlos residential lots qualify for at least a garage conversion ADU or a Junior ADU within the existing home.
What is the maximum ADU size allowed in San Carlos?
California state law caps detached ADUs at 1,200 square feet. Attached ADUs are generally limited to 50% of the existing home’s floor area, up to 1,200 square feet. A Junior ADU is limited to 500 square feet and must be within the existing structure.
Do I need to live on the property to rent out my ADU in San Carlos?
No. Under California law, owner-occupancy requirements cannot be imposed on ADUs permitted after January 1, 2020. You may rent out your ADU without living in the main house, subject to standard local rental ordinances.
My garage is close to the property line — can I still convert it to an ADU?
Likely yes. California ADU law includes a setback exemption for conversions of existing legally built structures — a garage that does not meet standard setback requirements for new construction can typically still be converted to an ADU without relocating the structure. This is one of the most important provisions for San Carlos homeowners with garages near property lines.
How long does an ADU permit take in San Carlos?
Ministerial ADU permits in San Carlos currently take several months for approval. Projects submitted with complete, professionally prepared packages — including architectural drawings, Title 24 calculations, and plumbing and electrical plans — move through the process more efficiently than incomplete submissions. Brian manages the full permit package on every ADU project to minimise back-and-forth with the building department.
Check What Your Property Can Support
The fastest way to know if your San Carlos property qualifies for an ADU — and what type — is an in-person visit. Brian visits your property, checks zoning, setbacks, lot coverage, and utility connections, and gives you a direct answer before any fees are committed.
Request a Free ADU Eligibility Check or call Brian at 650-966-4190.