In the State of California, backyard homes or secondary dwellings are now legally named and classified as an “accessory dwelling unit” or simply ADU. These units are self-contained residential housing units that are built on the same lot as the primary dwelling or existing single-family or multi-family dwelling. They are also often called in-law units, granny flats, among other names.

An ADU by definition is smaller in size than the primary dwelling and must include most of the amenities commonly found in the primary dwelling, including a kitchen, bathroom, adequate storage, and a sleeping area. This in turn means that ADUs require utility connection either to the primary residence or the main utility lines, including water, waste, and electricity.

ADUs are regulated by both the state and local governing bodies. As a result, although most local city planning and code enforcement departments abide by state regulations like the CRC/CBC, California Building Energy Efficiency Standards (Title 24), and CALGreen Compliance, some have additional requirements that must be reviewed prior to submission of your plan. For more on your particular city, visit our City Guides.

Type 1: Detached ADU

As the name suggests, a detached ADU is a backyard building that is not attached to the primary dwelling. Although all detached ADUs vary in size, shape, and style, each requires at least a kitchen, bathroom, and a sleeping area.

Pros: faster to build (do not have to modify the primary dwelling), physical separation if used for separate family or renters, most control over the design/layout

Cons: require additional setbacks, requires new foundation, requires utilities to be run from the primary residence or main

Solo ADUs

Bay Modular’s Solo ADU Series is a family of detached ADUs that are designed for on-site assembly using pre-finished modules.

Additional Pros: faster construction with prefab building, limited neighborhood impact and noise pollution with prefab building, construction timeline and budgets are reliable, most affordable solution

Type 2: Attached ADU

The immediate alternative to the detached ADU is an attached additional unit. These units are added on to the existing primary structure, but are functionally separate. This process still requires new construction and are often restricted to match the existing home style and tend to be slightly more complicated as they depend on the existing condition of the existing main structure.

Pros: reduces footprint in backyard if space is already limited, attached to primary residence and easier to connect to utilities, more control over design/layout

Cons: more costly, more time consuming (100% on-site construction), disturbances during construction (contractor parking and construction noise), design limitations to meet existing style of main structure, requires new foundation

Type 3: Garage Conversion

Possibly the second most attractive option after the detached ADU is the garage conversion — a conversion of an existing garage that includes full ADU amenities. Similar to an attached ADU and interior conversions, garage conversions require plumbing, mechanical, and electrical connections to run to the proposed conversion space.

Pros: fast build out because it is already a part of the building envelope, does not take additional backyard area, a part of primary residence and easier to connect to utilities

Cons: lose garage parking space, still requires conditioning (insulation) and all utilities, oftentimes the ceiling height is low, must maintain existing building style

Type 4: Interior Conversion

Most commonly basements conversions, interior conversions are transformations of unconditioned spaces to conditioned, living spaces. This requires adding plumbing, mechanical, and electricity connections from the main home to this extension. It is often easier to convert unconditioned spaces because they are already a part of the building envelope and usually only require cosmetic remodeling.

Pros: fast build out because it is already a part of the building envelope, does not take additional backyard area, a part of primary residence and easier to connect to utilities

Cons: still requires conditioning (insulation) and all utilities, potential loss of storage, must maintain existing building style

Type 5: Accessory Structure Conversion

Not dissimilar from an interior conversion, an accessory structure conversion will transform either an attached or detached structure that was previously not intended for habitation - used for storage or greenhouse, etc. However, because these structures were not intended for living originally, it is typically more costly and time consuming to upgrade a small to mid-sized structure like this that is was not framed for living and is already permanently placed.

Pros: can make us of an existing structure

Cons: more costly as they are unconditioned and often not build on a permitted foundation, requires utilities to be run from the primary residence or main, limited by the structure shape and often easier to go with a new detached build