Accessory Dwelling Units 101

Everything You Need to Know Before You Build

An Accessory Dwelling Unit (ADU) is a legal residential unit that is added to a lot with an existing or proposed home. The unit can be realized through ground-up construction, a garage conversion or addition, an addition to the primary residence, or a conversion of part of the existing residence.


Across California there is a shortage of housing, especially affordable units. The State is encouraging homeowners and developers to utilize ADUs as a smart-growth plan to provide more needed housing.


The State of California updated the standard for Accessory Dwelling Units in January 2020. The new laws allow more flexibility for the construction of ADUs on single-family and multi-family properties.


ADUs provide homeowners with awesome opportunities for independence, with the most common motivation being the rental income potential, followed by the prospect of flexible living space for their multi-generational households.

Types Of Single Family Adus

California State Law allows for the following types of Single Family ADUS:

Detached ADU

An independent structure separate from the primary dwelling with the look and feel of a house, just on a smaller scale. Includes full amenities. It will need its own sewer, water, electrical and other components. Cordoning off an area of the yard where the ADU is built will create more separation and dedicated access for whomever is occupying the unit. Larger sized options of this type of ADU may result in higher rental income. Makes a great living space for extended family.

Attached ADU

A unit attached to your existing home or garage that share at least one wall with the primary structure. The ADU generally will make use of the water and energy connections of the primary house. Includes full amenities. It needs to have a completely separate entryway from the main dwelling. Attached ADUs typically cost a lot less to build and are ideal for smaller or compact lots where space is at a premium.

Garage Conversion ADU

Converting your existing attached or detached garage into livable space is an effective way to exploit your property's full potential. Although the size of the ADU is limited to the footprint of the existing garage, converting a garage into an ADU can be a big cost saver and is a win-win proposition that will allow you to turn underutilized space into long-term equity in your home.

Above Garage ADU

Depending on the condition of the garage, building an above-the-garage ADU can cost double the price of a simple garage conversion, but adding an ADU above a typical two-car garage is a great way to add habitable space and rental income potential to your property, while keeping your vehicles and other storage underneath.

Interior Conversion ADU

A converted existing interior space such as an attic or basement that is fully integrated into the existing structure. The walls of the ADU are all shared with the primary dwelling. They might require plumbing and ventilation for the separate kitchen and bath. Most often, they share utility service and mechanical appliances with the main unit. These units are the most common ADU homes.

Junior ADU (JADU)

Junior Accessory Dwelling Units (JADUs) are exactly as the name implies, a smaller version of a Single Family ADU with a few noted restrictions. It must be erected within a proposed or existing single family dwelling or accessory structure such as a detached garage or carport. Most often, they share utility service and mechanical appliances with the main unit. These units are the most common ADU homes.

Types Of Multi-Family ADUs

Existing building elements are not required to be brought up to current code to add ADUs to Multi-Family lots. Rental periods must be in excess of 30 days.

Detached ADU

California state law permits you to build two detached ADUs on the same multi family lot utilizing the minimal 4’ side and rear setback requirements. Normally, the two detached ADUs can be attached to each other. But there are jurisdictions that insist the detached ADUs be separate. Detached ADUs are not subject to Rent Stabilization Ordinances (RSO).

Attached ADU

In addition to the two detached ADUs all multi-family dwellings may convert existing non-livable interior spaces of a building [parking garages, recreation rooms, storage areas, attics, and basements] into one or more ADUs. Up to 25% of existing building units count. You cannot convert spaces that are currently living spaces into ADUs,

Over carport ADU

You cannot convert uncovered parking (like a parking lot), but you can convert over your building's car port to create more habitable units. Build up to develop a revenue-generating residential complex.

Parking Conversion

For multi-family properties located within a half-mile of public transit, the conversion of an existing parking garage to ADUs provides valuable rental income. We've employed strategic design solutions to repurpose parking structures. Construction of these ADUs comply with the mandatory soft-story requirements for these parking areas and can minimize overall costs by including the seismic retrofitting within the ADU construction scope.