Anderson Valley Housing Association
Affordable Housing & Housing Support in Anderson Valley

Accessory Dwelling Units

Accessory Dwelling Units are one way we can address the need for housing in our community.

Accessory Dwelling Units

One of the best ways to create new housing while enhancing your property value is to add an accessory dwelling unit, or ADU. You can earn rental income while you own the property, and your market value will also be higher. There are many existing ADU plans online that you can learn from or purchase. Mendocino County offers two pre-approved ADU construction plans on its website, at no cost. These plans are not fancy, but they can save you significant time and money. Another way to add an ADU affordably is to buy one that’s already designed and built – it’s transported to your property and set down on a permitted foundation.

What is an ADU?

The typical ADU is a free-standing single-family house of limited size. Mendocino County regulations allow for three types of ADUs:

  • Detached (separate) single-family residence of 1200 square feet or less with its own kitchen, bathroom(s) and bedroom(s)

  • Attached single-family residence of 1200 square feet or less with its own kitchen, bathroom(s) and bedroom(s); the floor area may not exceed 50% of the floor area of the existing residence

  • A “living space” of 500 square feet or less, attached to or contained within an existing single-family residence; this living space has its own kitchen but is not required to have its own bathroom. The county calls this a “Junior ADU” in its regulations; some people call them “granny flats” or “in-law units.”

The basic definitions are in the county code: Section 20.008.020 - Definitions (A) and Section 20.008.020 - Definitions (J). You can find them on the same webpage.

What Are the Rules for ADUs?

Most of the rules for ADUs are common sense, such as providing water, waste treatment and parking. Here’s a quick summary:

  • You must have an existing single-family dwelling unit on the property, or a permit for one. (Remember, permits have a time limit!)

  • Water and septic systems must be sufficient for the combined total use of the inhabitants in the residence and ADU.

  • Permitting requirements are like those for a primary residence (height limits, setbacks, lot coverage, construction drawings, plan checks, fees, etc.), with one major exception: Class K, which simplifies permitting a bit for owner-occupied habitations, does not apply to property that is rented out.

  • ADUs are not permitted if you already have other forms of accessory housing on the property (such as farmworker or employee housing), or if the property is already permitted for “dwelling group clustering” (aka “cluster housing”).

You must (generally) provide a parking space for the ADU, although there are a few exceptions to this rule. Review the regulations for ADUs carefully.

Where Can I Find Existing ADU Plans?

Mendocino County has links to its pre-approved ADU plans right on the Planning and Building webpage. There is a two-bedroom plan and a three-bedroom plan. Both are detached (freestanding) structures that maximize the 1200-square-foot limit. Even if these plans don’t appeal to you creatively, they can help you understand the space limit and how to work within it.

Some home construction companies offer well-engineered ADU plans for a nominal cost, in hopes that you will hire them to build the ADU. But you don’t have to do that to get the plans. Small House Catalog, for example, is based in Washington state, but the company offers creative, low-cost ADU designs that you can bid out to local builders.

Yet another option is to buy a manufactured ADU. These structures are also known as “pre-fab” (for “pre-fabricated,” meaning they are built in a factory rather than onsite). Here in Northern California we have a company called PreFabADU, which offers nine plans ranging from truly tiny (150 square feet) to the legal limit.

Before you Start

It’s worth the time to research ADU options, evaluate your property carefully (for lot size, setbacks, water and septic), calculate building costs (or purchase costs for a prefab), and consider how much rental income you would need to cover the cost of adding an ADU to your property. When it comes to financing, you’ll most likely have more luck – and get a better deal – if you work with a local or regional bank rather than a national chain. That’s because the local bankers recognize our need for housing and truly want to serve their own community.

No matter how you add an ADU to your property, it’s a win for Anderson Valley!