The Definitive (Marin County) ADU Guide.

🏡 1. Know Your ADU Type

Many homeowners are interested in ADUs, but are often confused by the rules and regulations for their particular location. California State law guarantees the below minimum floor area and heights, but getting permits for an ADU in California will fall under one of the four separate categories below, and each jurisdiction may impose slightly different regulations for situations beyond the below minimums. So, depending in the size and configuration of your lot, you may have several or all of the options — but it's important to weigh the differences early on, as you will only be able to build one ADU, and one JADU per lot, and there may be existing site constraints that limit the options moving forward for your property.

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

• 800sf guaranteed minimum floor area (if site, fire, utility, and safety constraints allow). Must have full Kitchen, Bathroom, and a separate entrance. Must have 4ft minimum setbacks from the property line. Max 18 ft tall (or 25 ft if above a garage).

• Attached ADU (built onto the main house):

• Must have 4ft minimum setbacks. May be constrained by site or home configuration. Can go up to 25 ft (or zoning height of main residence, whichever is less); Must have a full Kitchen, Bathroom, and Separate Entrance. May have a doorway connection to the main residence.

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

• All previous codes must be followed, but may expand the existing residence up to an additional 150sf. may convert garage space. if not required for parking.

• Junior ADU (JADU):

• ≤ 500 sq ft, must be entirely contained within existing home. Must have separate "kitchenette" (6' minimum countertop space), 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

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

📋 3. If you want to take THE SHORTCUT to determine if your property qualifies for an ADU or JADU, contact the ADU Center at ADU Marin for a free feasibility report and a thorough analysis of the options for your property. ADU Marin is a free service, sponsored by Marin County, to help homeowners with planning, budgeting, and financing ADUs! Click here for a sample report!

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!