“An accessory dwelling unit, usually just called an ADU, is a secondary housing unit on a single-family residential lot. The term “accessory dwelling unit” is a institutional-sounding name, but it’s the most commonly-used term across the country to describe this type of housing.” (source: https://www.buildinganadu.com/what-is-an-adu)

In short, the fact that it’s a secondary housing unit is what defines an ADU.

Types of ADUs

1. Basement conversion ADUs: commonly called mother-in-law units, in law units, basement apartments, secondary suites, or accessory apartments

2. Detached new construction ADUs: sometimes referred to as ​ granny flats, backyard cottages, or DADUs

3. Garage conversion ADUs

4. ADUs above a garage or workshop, or attached to it: sometimes called garage apartments or carriage houses

5. Addition ADUs or “bump-out ADUs”

On January 1, 2017, new laws in California came into effect to streamline the approval process for ADUs. The City of Alameda adopted a new ADU ordinance that came into effect on August 7, 2017 to comply with state law.

City of Alameda ADU FAQs: https://www.alamedaca.gov/files/sharedassets/public/alameda/comm-services/formsandhandouts/planning/adu-faq.pdf

I would recommend you read the FAQs in their entirety. I’m going to share some excerpts here:

According to the City of Alameda, an ADU may be created via the following methods:

1. Building a detached accessory structure

2. Converting an existing accessory structure

3. Building an addition attached to the primary dwelling

4. Converting space in the primary dwelling (e.g. basement)

5. Junior ADU: Similar to repurposed space but meets specific requirements and specifications for a Junior ADU prescribed by state law.

Where are ADUs allowed?

ADUs are allowed on properties that are zoned residential with only one single-family home. If the property already has more than one dwelling unit, such as a duplex or apartments, then that property is not eligible for an ADU. If your property is part of a HomeOwners Association (HOA), please check with your HOA to see whether the HOA rules allow you to have an ADU.

Can I use my ADU as a short term rental (e.g., AirBnB)?

No, ADUs may not be rented out for periods of less than 30 days.

May I convert my garage into an ADU?

It is possible to convert an existing garage to an ADU as long as the parking space(s) in the garage can be relocated to another area of the property. When the parking space(s) are relocated, they must meet City requirements for parking space dimensions, backup area, and landscaping. The relocated parking spaces also cannot be located in any of the front yard or the street side yard on a corner lot. These rules ensure that the parking space is truly usable and provide adequate room for vehicle maneuverability. The California Building Code also has requirements for converting non-habitable space to habitable space. Refer to the California Building Code or consult a qualified building professional for more information about building code requirements. If the ADU is attached to a garage then fire resistive construction will be needed between the garage and the ADU.

May I build a second story above the garage in my backyard for an ADU?

No, detached ADUs are limited to a single story.

What is a Junior ADU?

State law allows a special type of ADU called a Junior ADU (JADU). A JADU must be created from an existing bedroom and located entirely within the existing walls of an existing single family home. The JADU must be less than 500 square feet in size. Although plug-in stove tops can be used, a JADU does not contain an actual kitchen. A private bathroom is not required as 4 long as that the JADU has full access to a bathroom in the primary dwelling. No off-street parking is required.

What are the steps to obtaining permits for an ADU?

1. Apply for ADU Clearance from the Planning Division

2. Record a Declaration of Restrictions at the County

3. Apply for building plan review in the Permit Center

4. Begin construction once a permit is issued

5. Call for inspections

6. Obtain a Certificate of Occupancy

How much are the permit costs for an ADU?

Cost of the permits vary depending on size, project valuation, and other factors. A detached ADU of 300 sf costs approximately $6,000. A number of factors can affect the cost of the permit, including but not limited to, other work that may be necessary to facilitate an ADU project, the quality of the plan submittal, and incomplete drawings that require multiple rounds of review. Please check with Permit Center for actual cost of your project.

Click here to download the City of Alameda’s FAQs about ADUs: https://www.alamedaca.gov/files/sharedassets/public/alameda/comm-services/formsandhandouts/planning/adu-faq.pdf