Accessory Dwelling Units (ADUs) have become an increasingly popular way to add flexible living space, generate rental income, or provide housing for family members. But before you start building an ADU in Marin County, you need to understand the local ADU laws to ensure your project complies with zoning regulations, size limits, and permitting requirements.

If you’re a homeowner in Marin County, here’s what you need to know about ADU laws before you get started.

What is an ADU?

An Accessory Dwelling Unit (ADU) is a secondary residential unit that exists on the same lot as a primary home. ADUs can be detached, attached, or converted from an existing structure (such as a garage or basement).

Marin County also allows Junior ADUs (JADUs), which are smaller, self-contained units within the primary home. JADUs must be under 500 square feet and typically share a bathroom with the main residence.

Marin County ADU Laws: The Basics

If you’re considering adding an ADU in Marin County, here are the key laws and regulations you need to be aware of:

1. Zoning and Eligibility

Most single-family and multi-family residential lots in Marin County are eligible for an ADU, but the exact requirements depend on your location.

  • ADUs are permitted by right on most residential properties.
  • Multi-family properties may be eligible for multiple ADUs (converted from non-livable spaces like basements or storage rooms).
  • Check with your local city or town planning department, as some cities within Marin County have their own ADU rules.

2. Size and Height Limits

The size of your ADU depends on whether it is attached or detached:

  • Detached ADUs: Maximum 1,200 square feet
  • Attached ADUs: Cannot exceed 50% of the main home’s floor area, up to 1,000 square feet
  • JADUs: Limited to 500 square feet
  • Height Limit: Typically 16 feet, but taller structures may be allowed with additional approvals

3. Setbacks

  • Side and rear yard setbacks: 4 feet minimum
  • Front yard setback: Must match the primary home’s required setback
  • No setback required when converting an existing structure (like a garage) into an ADU

4. Parking Requirements

One parking space is required for most ADUs unless your property qualifies for an exemption. Parking is not required if:

  • The ADU is within half a mile of public transit (bus stop, ferry, or train station)
  • The ADU is part of the primary residence (like a garage conversion)
  • On-street parking permits are not required in your area

JADUs do not require any additional parking.

5. Owner-Occupancy Rules

  • ADUs: As of recent California law updates, owner-occupancy is NOT required for new ADUs. This means you can rent out both the main home and the ADU.
  • JADUs: The owner must live on-site in either the main house or the JADU.

6. Utility Connections and Impact Fees

  • ADUs under 750 square feet are exempt from impact fees in Marin County.
  • ADUs over 750 square feet may require separate utility connections and impact fees (varies by location).
  • If converting a garage or basement, you may be able to share existing water and sewer connections.

7. Permitting Process

The good news? Many ADUs qualify for streamlined, ministerial approval, meaning they don’t require public hearings or special permits.

Steps to get started:

  1. Consult your local planning department – ADU regulations may vary by city.
  2. Prepare your ADU design and plans – Work with an architect or designer.
  3. Submit for permits – A building permit is required for all ADUs.
  4. Begin construction – Once permits are approved, you’re good to go!

Most permits are approved within 60 days, as long as the ADU meets Marin County’s requirements.

Why Build an ADU in Marin County?

Adding an ADU is a smart investment with multiple benefits:

✔ Rental Income – Generate passive income with a long-term rental or short-term lease.
✔ Family Housing – Provide housing for aging parents, adult children, or caregivers.
✔ Increased Property Value – Homes with ADUs often have higher resale values.
✔ Sustainable Housing – ADUs help create affordable housing solutions in Marin County.

Final Thoughts

Navigating Marin County ADU laws can seem complicated, but understanding the key requirements will help you plan your project with confidence. Whether you’re looking to build a rental unit, a guest house, or extra living space, an ADU is a great way to maximize your property’s potential.

If you’re serious about building an ADU, make sure to consult with your local planning department to confirm specific regulations in your city or town. Each city in Marin County (such as San Rafael, Mill Valley, and Novato) may have slightly different ADU rules.

Want to make the ADU process even easier? Work with a team that specializes in Marin County ADU development to handle permits, designs, and construction.