Marin County ADU Guide

🏡 1. Can I build an ADU on my property?

Depending on the size and configuration of your lot, you may have several or all of the below options — but it's important to weigh the differences beforehand, as you will only be able to build one ADU, and one JADU per lot. Most of the time, lot size or configuration, access issues (such as small, curvy roads), and utility lines, trees, and main house location are the main constraints that may limit the options for your property.

• Detached ADU (stand‑alone cottage or garage apartment):

• 800sf guaranteed minimum floor area, (if site, fire, utility, and safety constraints allow) max 1,200sf. Must have full Kitchen, Bathroom, and a separate entrance. Must have 4ft minimum setbacks from property lines. Max 18 ft tall (or 25 ft if above a garage). If in a Fire Zone, must meet all WUI requirements. Must have solar if it is new construction, and must meet all energy code requirements, regardless of whether it is a new or existing building.

• Attached ADU (may be added to the main house):

• Can be added onto exiting home, but no more than 50% of the existing square footage, and no more than 1,200 sf max. Must follow 4' setbacks, so may be constrained by site or home configuration. Must follow zoning height restrictions, or 28' max, whichever is less. Must have a Full Kitchen (i.e., countertop, refrigerator, range, sink), Full Bathroom (i.e., sink, toilet, shower or bathtub), and a Separate Entrance. May have a doorway connection to the main residence.

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• Conversion ADU (e.g., garage, spare room):

• Garage space may be converted, but you must provide a permanent heating source. May not use "plug-in" type portable heaters. Electric base heaters no longer pass code either, and may not be used. Most homeowners extend their existing HVAC, or opt for a separate "mini-split" system. The garage floor slab may need to be leveled and sealed. You may expand the existing residence up to an additional 150sf. Local jurisdictions may not require replacement parking if any of the following apply:

  
  

  • ADU is within ½ mile walking distance of public transit (bus, BART, train, ferry).

  • ADU is within an architecturally and historically significant district.

  • ADU is part of the existing primary residence or existing accessory structure (garage, carport, etc.). Essentially, all garages.

  • On-street parking permits are required but not offered to the ADU occupant.

  • ADU is within one block of a car-share vehicle.

• Junior ADU, "JADU" (must be built within the main house)

• ≤ 500 sq ft, must be entirely contained within the existing home. Must have separate "kitchenette" (6' minimum countertop space, with sink and cooking surface), but may share a bathroom with main residence. Must have a separate entrance. Local agencies may not require parking as a condition (even when converting an attached garage). Must have owner-occupancy in either the JADU or the main residence.

See also, California State ADU Laws.

See also California Department of Housing and Community Development which published the ADU Handbook that explains all the different types of ADU's in depth. State law guarantees at least an 800 sq ft ADU, but you can build larger — up to 1,200 sq ft depending on design and lot size.

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🛠️ 2. Check the Local Rules

Here’s the checklist:

✔️ Lot use: Must be single- or multi‑family residential

✔️ Number allowed: One ADU or one ADU + one JADU (if not expanding current residence square footage). Applies to all single-family residentially zoned districts.

✔️ Parking:

Usually 1 space per ADU bedroom (attached/detached), unless exempt (e.g., within ½ mile of transit, or conversion ADU)

✔️ Design & materials: May need to match main house (style, color, lighting)

✔️ Must have 7'x10' minimum bedroom size, with minimum egress window sizes per code.

✔️Lighting and electrical: All spaces must meet minimum code requirements, dark-sky compliant exterior lighting

✔️ Landscaping & trees: Check with the local jurisdiction if removing trees will be necessary for the development.

• Local Building Department Websites and Resources

  • ADU Marin, (Contact this Non-Profit first! You won't regret it!)

  • Marin County ADU Guidelines

  • Corte Madera: Corte Madera Municipal Code

  • Mill VAlley ADU Guidelines

  • Tiburon ADU Guidelines

  • San Rafael ADU Guidelines

  • Novato ADU Guidelines

  • Larkspur ADU Guidelines

  • San Anselmo ADU Guidelines

  • Ross ADU Guidelines

  • City of Hayward ADU Checklist (more East Bay Cities to come soon!)

For other counties/jurisdictions, contact us for a free consultation.

📋 3. THE SHORTCUT :

Contact us to determine if your property qualifies for an ADU or JADU, or you can contact the ADU Center at ADU Marin for a FREE feasibility report and a thorough analysis of the options for your property. This service is sponsored by Marin County to help homeowners with planning, budgeting, and financing ADUs!

Once you have determined what type of ADU is feasible for your property, contact us for a free site visit to discuss how to move forward for your ADU! If you would like a free guide on the architectural steps required for a typical project, click here!