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What Is An ADU

Accessory Dwelling Units

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. While the full name is a mouthful, the shorthand “ADU” is better.

It is an additional, self contained housing unit that is secondary to the main residence. ADU’s are sometimes referred to as “Granny Units” or “Mother-In-Law units” since many ADU’s were initially constructed to provide housing for family members. They can take many forms. In some cases, an ADU can be attached as an addition to the house, or as a second story over a garage. The garage itself may be converted to an ADU or the ADU may occupy a basement. An ADU can be a section of the main house that is separated from the main living space, or an ADU can be a stand-alone unit like a small house or cottage. The City of Santa Cruz only requires that an ADU have a kitchen, bathroom, and place to sleep.

How Many ADUs Are There?

In the twelve academic studies and professionally funded surveys that have been conducted on the presence of informal ADUs, they have all found that a whopping 10-20% of all the housing units in their study area are informal ADUs. Granted, these studies were generally conducted in populated areas, such as LA, San Francisco, Portland, and Vancouver, BC, but studies have also been conducted more broadly in metropolitan areas such as the Bay Area and the Boston Metropolitan Area, and the results are the same. Could 1/10th of all residential housing stock be informal ADU type development? That means there are more than thirteen million ADUs out there.


ADU’s Offer Flexibility & Extra Space

The flexibility of accessory dwelling units allows them to serve a variety of functions as a homeowner’s needs change over time. An ADU can be a home office, a private space for guests, or an opportunity for additional rental income. It can also be a home to a family member with special needs, or an elderly relative who wants to age in place, allowing them to receive the support and care they need while retaining as much independence as possible. As with any new build or remodeling project, an accessory dwelling unit is a significant investment of both your time and money. However, after the initial build has been completed, ADUs are less costly to operate and maintain per square foot, and rental income can help to supplement mortgage costs or retirement savings. Homeowners can also benefit from increased property value and the flexibility and security that comes with having a secondary and adaptable structure that can meet their future environmental, lifestyle or financial needs. For those looking to optimize a traditional home, an accessory dwelling unit is a great way to take advantage of existing resources, while increasing the efficiency of a property and providing extra income. We recommend working with a qualified design, construction and project management team who can help you to understand local development constraints and navigate the permit process. This trusted team can also ensure your ADU is exactly the way you imagined, delivered on time and on budget.

Detached ADU

Detached new construction ADUs, also sometimes called backyard cottages, granny flats, laneway houses, or DADUs, depending on the jurisdiction:

Garage Conversion ADUs

ADUs above a garage or workshop, or attached to it. In some areas, these may be called garage apartments or carriage houses

Bump-Out ADUs

the bump-out extends into the setback, the city would not allow it to actually meet the ground. On the other hand, if it didn't meet the ground, the bump-out would not be considered a part of the footprint and could therefore exist within the setback.

Basement Conversion ADUs

also commonly called basement apartments, mother-in-law units, in law units, secondary suites, English basements, accessory apartments, and a host of other names.

Internal ADUs

where part of the primary house besides the basement is converted to an ADU.

Types Of Accessory Dwelling Units

There are three types of Accessory Dwelling Units (ADUs): interior, attached, and detached ADUs.

Interior ADU – Located in the primary dwelling, an interior ADU is built from existing converted space, usually an attic or basement.
Attached ADU – These additions adjoin the primary dwelling – to the side or rear of the home, or constructed on top of an attached garage.
Detached ADU – A stand-alone structure separate from the primary dwelling. A detached ADU can be built as entirely separate unit or constructed over an existing accessory structure, such as a detached garage.


Under new California state laws, the term “accessory dwelling unit” describes a small, self-contained residential unit located on the same lot as an existing single-family home. An ADU is generally smaller in size while still including all of the basic amenities found in a primary dwelling, such as a kitchen, bathroom and sleeping area.

Today, ADUs go by several names: granny unit, granny flat, in-law suite, in-law cottage, mother-in-law apartment, or secondary dwelling unit. Historically, each of these terms had a unique meaning depending upon the city or county in which the primary dwelling was located. Nowadays, however, they are often used interchangeably.


Existing California law permits the construction of ADUs. However, local ordinances have – perhaps unintentionally – made it difficult for homeowners to build new units by imposing standards at the city and county level to regulate parking, construction, and lot coverage.
Fortunately, in September 2016, Governor Jerry Brown signed three ADU-related bills in an effort to address California’s housing crisis. These new bills also make it easier for California homeowners to obtain permission for ADU construction from local governing bodies.

Senate Bill 1069 reduces or eliminates parking requirements and utility hook-up fees. It also speeds up the approval process for second units attached to a primary residence.
Assembly Bill 2299 goes further than SB1069, requiring local second-unit ordinances to include ministerial, non-discretionary approval for any second unit that meets city requirements.
Assembly Bill 2406 streamlines the process for homeowners converting an existing bedroom into an attached studio living unit.
All of these laws intend to limit the ability of local government to regulate ADUs and local governments are required to adopt an ADU ordinance in accordance with the new state requirements by January 1, 2017. Additionally, these laws will speed up the approval process of ADU construction. They will also remove secondary utility hook-up fees that have been paid for by the primary dwelling owners. Furthermore, the laws will eliminate parking requirements for ADUs located in proximity to public transit. Visit here for the most updated information on the ADU Laws from the California Legislature. Hauser Construction is staying on top of the local zoning ordinances in Santa Clara County, with an eye on those successfully promoting the construction of ADUs. Check out the latest on ADU development in your city

What ADUs Have In Common

While their structural forms vary, ADUs share some common traits and face common design and development challenges. For one thing, the fact that they’re secondary housing units on single family residentially zoned lots places ADUs into a unique category of housing. And ADUs also have some other distinguishing characteristics that help further define, differentiate, and distinguish them from other housing types.

• ADUs are accessory and adjacent to a primary housing unit.
• ADUs are significantly smaller than the average US house.
• ADUs tend to be one of two units owned by one owner on a single family residential lot.
• ADUs tend to be primarily developed asynchronously from the primary house by homeowner developers.
• A large range of municipal land use and zoning regulations differentiate ADU types and styles, and dramatically affect their allowed uses
• Vast numbers of informal ADUs exist compared to permitted ADUs.

These differentiating characteristics make ADUs a distinct type of housing and have led to a lack of common understanding around the language and best practices of ADU development. This site is going to help change that by providing some clarity about ADUs, and how average homeowners can build them.