Accessory Dwelling Units (ADUs) are an effective option for increasing affordable housing stock, without necessitating sprawl onto undisturbed or agricultural land. For owners, they can provide a revenue stream and represent an efficient long-term investment. They can also offer long-term flexibility for aging-in-place. For tenants, they offer all the necessities of a habitable residence, without the added cost that comes with larger units.

Parcels located near neighborhood services like groceries, banks, and healthcare reduce the need for constant car use, and in some cases require no parking at all. Their smaller scale compared to conventional houses can significantly reduce energy demand, particularly for heating and cooling. ADU construction can also benefit from existing infrastructure – sites that already have sewer, water, and power infrastructure, hooking up to these services is far simpler than more spread out development requiring ties into more distant connections. On the right site, they can be inherently walkable, sustainable, and affordable.

Due to their reduced scope and incentives lowering the cost of construction, they are a naturally affordable option for renters. With certain accommodations, they can easily support Low or Very Low Income residents, and be eligible for commensurate incentives (see the County’s Grant Information and Financial Resources page for more information). Even without such considerations, they can still support Moderate Income residents, who are currently underserved by the local housing market.

In recent years, the State of California and the County of Mendocino have taken a number of steps to facilitate ADU construction and operation in an attempt to address the local housing crisis.

Note: The requirements summarized below are not inclusive. If you have specific questions for your project, please contact the appropriate department. 

An Accessory Dwelling Unit is a secondary dwelling unit for one or more persons on the same parcel as a larger, primary dwelling. An ADU can either be attached or detached to the primary residential structure on the property, but must include complete independent living facilities (including permanent provisions for entry, living, sleeping, eating, cooking and sanitation, and adequate water service and sewage disposal systems).

If the ADU is detached from the primary residence, it has a maximum floor area of 1,200 square feet. If attached to the primary residence, the maximum floor area cannot exceed 50% of the existing living area or 1,200 square feet, whichever is smaller.

One parking space (covered, uncovered, or tandem) is usually required per ADU, but this is not necessary if the ADU is either:

• located within one-half mile of a public transportation stop along a prescribed route according to a fixed schedule
• located within one block of a car share parking spot
• located entirely within the principal residence and results in no net increase in habitable floor area on the property
• located in an area where on-street permit parking is required, but such permits are not available to the tenant
• located within a designated Historic District

To make the planning process as approachable as possible, the County of Mendocino offers two free to use, sample ADU plans:

• Two Bedroom ADU
• Three Bedroom ADU

The smaller ADU plan is for an 816 square foot, 2 bedroom, 1 bath home, and the larger model ADU is an 1,120 square foot, 3 bedroom, 2 bath home.  The plans are provided unstamped, meaning they would need to be redrafted and stamped (and modified, if you wish) by a licensed architect in order to be submitted as part of a Building Permit application.

You would also need to draw up a site plan showing the parcel as it exists now, along with the proposed ADU (and any other proposed structures). All this would need to be submitted to the Planning and Building Services Department along with a completed Building Permit application in order to begin the permitting process.

Additional free ADU plans can be found from other sources online. Any plan can be used, so long as it meets all requirements of the Mendocino County Code.

Junior Accessory Dwelling Units (JADUs)  are a separate type of unit, no more than 500 square feet in area, which are attached to the primary single-family residence on a property. They are typically converted bedrooms, with entrances into the unit both from the main home and from the outside.

A Planning Policy determining the square footage of a JADU was adopted on 3/22/2024. You can find a copy of the Policy here

The JADU is not required to have a private bathroom, but must have an efficiency kitchen (specifically, a sink with a maximum waste line diameter of 1½ inches, a cooking facility with appliances that do not require electrical service greater than one hundred twenty volts or natural or propane gas, and a food preparation counter and storage cabinets that are of reasonable size in relation to the size of the junior accessory dwelling unit). 

No additional parking is required, beyond what is needed for the main residence. The application process for a JADU is the same as for an ADU. 

While an ADU does not require that a property owner live on-site, pursuant to state law a JADU does require owner-occupancy unless the owner is a government agency, land trust, or housing organization [Gov Code 66333(b)]. The owner may reside in either the remaining portion of the structure or the newly created JADU.

Generally speaking, most parcels with a single-family residence can support both an Accessory Dwelling Unit and a Junior Accessory Dwelling Unit (and potentially an additional Family Care Unit or Guest House as well).

ADUs are one of the most affordable types of development around, but the upfront costs can nonetheless be expensive, with overall price-tag potentially breaking six figures.

To help defray these costs and make ADU development accessible to people of all income levels, a number of funding sources have been made available, in the form of grants, loans, and other incentives:

State of California

The State maintains a list of potential funding sources. You can find out more by following the link:

• California Housing Agency ADU Grant Program

Mendocino County 

The County maintains a list of affordable housing development resources as well. You can find out more about these resources by following the link:

• Grants and Financial Resources

Where are ADUs allowed?

A defining quality of an Accessory Dwelling Unit is that they are an accessory use. That is, they are accessory to an existing residential use, and so in general only parcels that already have an existing legal residence will be eligible for an ADU.

There are, however, two instances in which an ADU can be created in advance of a larger residence:

• If both an ADU and primary residence are proposed at the same time, the ADU can be built first, so long as the Building Permit for the primary residence has at least been applied for.
• Alternatively, if the proposed ADU meets all the minimum requirements for a primary residence, it can potentially be built on its own as a primary residence, then retroactively made an ADU once plans for a larger residence have been made.

That said, if a given parcel in Mendocino County does have an existing legal residence, it is almost certainly eligible for an ADU if one does not already exist (and may be eligible for additional units even if an ADU does exist). ADUs are most popular on parcels with single-family dwellings, but they can be permitted on parcels with duplexes or other multifamily residential developments. In some cases, multi-family developments can support multiple ADUs.

In some cases, properties with commercial land uses can support additional residential development as well (like an apartment above a store) – these can be identified on our zoning map by their “Mixed-Use Zoning.” These too can support an ADU in addition to a single-family dwelling.

Please note that sometimes properties can span multiple Assessor’s Parcels. In some cases, these can simply be single legal parcels that have been assigned multiple APNs, but in other cases the multiple APNs can represent separate, legally distinct parcels that happen to currently be under the same ownership.

In determining where on a specific parcel an ADU should be sited, there are a few key items to consider. Generally, an ADU will need to respect the parcel’s required front setback (an area of the parcel so many feet from each property line and adjacent roadway that must remain clear of structures, defined by the parcel’s zoning), but only needs to maintain a 4’ setback from the side and rear property lines (or in some cases no setback at all).

If the parcel is served by a septic system, an ADU cannot be constructed on the leach field (including the secondary or “replacement” leach field that was set aside when the septic system was initially put in place). Recorded easements, utility lines, building envelopes, wells, creeks, ponds, streams and drainage ways must all be respected as well. If a parking space is required, it too must be considered.

Additional requirements for coastal development

in the Coastal Zone, an ADU may require a Coastal Development Permit in addition to a Building Permit. Additional coastal-specific restrictions regarding water availability, blufftop development, and use as Vacation Home Rentals apply as well, as delineated in Mendocino County Code Chapter 20.458. Please contact the Planning and Building Services Department for more information.

in the Coastal Zone, an ADU may require a Coastal Development Permit in addition to a Building Permit. Additional coastal-specific restrictions regarding water availability, blufftop development, and use as Vacation Home Rentals apply as well, as delineated in Mendocino County Code Chapter 20.458.

For more information on ADUs in the Coastal Zone, please contact the Planning and Building Services Department.

The reduced size of ADUs make them inherently more efficient than larger houses when it comes to heating and cooling, but a little additional consideration in the planning phase can dramatically reduce resource usage and utility costs for the life of the structure. Here are a few design elements and best practices in the placement and configuration of the structure you may wish to incorporate:

• ADUs can be either attached or detached to the main residence or to existing buildings, like garages. When structures share walls, they effectively act as insulation for each other and can share heat, symbiotically reducing the energy demand for each other.
  • CalRecycle Green Building
• Passive solar and natural lighting are particularly effective at reducing ADU energy usage without significantly increasing building cost.
  • Passive Solar Homes

Once a location has been found and a plan has been chosen or created, the next step is to complete and submit a complete Building Permit application packet. This includes the application form itself, and any relevant items from the application checklist.

Specific building requirements to be aware of:

• If the primary residence requires sprinklers, the ADU will require them as well.
• Any new ADU will require solar panels, regardless of whether the main residence has them or not.

There is a significant need for housing opportunities for persons with disabilities in the county. While not always required of new residential developments, relatively minor accommodations for accessibility made during the planning stage can enable the final structure to serve a wider range of potential tenants.

Once these materials are completed and submitted, they can take 6-8 weeks to receive final approval.

Once approval has been received, it is time to build! The construction itself can either be performed by the owner or by a licensed contractor.

Unlicensed contractors and non-permitted construction

Failure to acquire a Building Permit carries a number of risks. A new ADU created without a permit is not a legal structure, will not be reviewed for compliance with the Building Code, and would be subject to enhanced fees and/or penalties should the violation be discovered.

Renting out an unpermitted building circumvents health and safety regulations and can drastically increase the owner’s liability should an injury occur. Review of a Building Permit application also includes verification that a contractor involved with the project is licensed and has proof of insurance, an important check on the risk of unlicensed contractors and substandard work.

Note that hiring a contractor is not always necessary. ADUs can be built on a DIY basis, but a Building Permit is always required.

ADUs present an opportunity for everyday homeowners to enter into the rental market, regardless of whether they have past experience as a landlord.

For first-time landlords, it is important to have a solid grounding in the requirements and responsibilities of being a landlord, including what constitutes a habitable, livable space, the limits on rent increases and evictions set by AB 1482, how to compose a fair rental agreement, and what constitutes unlawful discrimination. Before renting out a unit, be sure to familiarize yourself with the California Guide to Residential Tenants' and Landlords' Rights. It may also be possible to hire a professional property management company to manage the property for you.

It is also important to know what the median income levels for the area are, to know if you are charging a rental rate affordable to members of the local community (see the Housing Affordability Survey for current rates).

ADU Handbook

ADU Examples

Discrimination and Fair Housing Law

Mendocino County Code - Accessory Use Regulations (Inland)

ADU Inland Regulations – MCC 20.164.015(K)

JADU Inland Regulations – MCC 20.164.015(X)

Mendocino County Code - ADU Coastal Regulations – MCC 20.458

Policy - Junior Accessory Dwelling Unit (JADU) Square Footage 03/22/2024