Mighty Buildings Blog

What is an ADU?

It is not the beauty of the building you should look at: 

it’s the construction of the foundation that will stand the test of time.” 

– David Allen Coe


       ADU stands for Accessory Dwelling Unit. An ADU is a secondary house, apartment or in-law unit that is constructed on any single family or multi-family lot with an existing residential dwelling. The term “accessory dwelling unit” is a bit legalese-sounding, but it’s the most commonly-used term across the country to describe this type of housing. Because the full name is a mouthful, the shorthand “ADU” is often used instead.

ADUs are known by a number of other names as well:
  • Granny flat
  • In-law Suite
  • Companion Unit
  • Ohana Unit
  • Backyard Cottages

       And moreThe Department of City Planning for L.A. describes an ADU as “a self-contained housing unit located on the same property as a single-family home,” though California State law has since expanded this to multi-family properties as well. UCLA’s CityLab states that it must include amenities such as a kitchen, bathroom, and living area as well as a separate entrance.

       California State law, overseen by the Department of Housing and Community Development, puts the maximum square footage of an ADU at “up to 1,200 square feet.” However, local jurisdictions are allowed to create their own requirements for ADUs, so it’s worth becoming acquainted with what your community allows.

       ADUs not only go by a number of different names, there are different types of ADUs depending on how it is built in relation to the existing structure(s) on a lot:
1) Attached ADU – an addition attached to the primary residence.
2) Conversion ADU – converting a garage, basement or other underutilized location in the existing floor area of the primary residence or other structure on the property.
3) Detached ADU - a separate, standalone structure.
4) Junior Accessory Dwelling Units – have to be at max 500 sq. ft conversion of space, typically a bedroom. There are a lot of them that are allowed to have a shared bathroom, but still need to have a separate entrance to the unit.

       There are a lot of mysteries about what is and what’s not an ADU. Let’s define those that are NOT an ADU:
·  an RV
·  Pool House (for some people they might have similar functionality but at the end of the day it’s two separate units with different requirements)
·  Guest House (primary difference is the lack of a kitchen in ADUs)

       There are other types of units which are allowed as ADUs in some locations but not others, including Tiny Homes & Yurts. Because of all the nuances of local regulations around ADUs it is important to understand what is and isn’t allowed in your area.

       On January 1, 2020, it became easier than ever to build Accessory Dwelling Units in California, thanks to a package of new bills from the Legislature. These bills invalidate local ADU ordinances across the state and replace them with state-mandated rules unless the local jurisdiction adopts an ADU ordinance that is at least as permissible as the state-mandated rules. The new laws in most cases allow1 ADU and 1 JADU, if you are in a multi-residential zone you are able to have 2 detached ADU’s and 1 JADU. You are pretty much allowed to have ADUs in any residential zone (single family residential zone, multi-family residential or mix use, even in some cases agricultural and commercial zones if there is residential housing in this area as well). 

      Some other important ADU regulations in California are: 
  • ADUs can be legally rented
  • ADUs cannot be sold separate from the primary residence but may be rented.
  • ADUs do not require owner occupancy of the ADU and primary residence to be rented
  • Where ADUs are Permitted
  • One ADU and one JADU (Junior ADU) can be added to lots zoned for single-family or multifamily use that contain an existing or proposed dwelling.
  • Max Size
  • Cities must allow a maximum floor area of at least 800 sq ft for Studios/1 Bedroom units & 1000 sq ft for 2 Bedroom units 
  • Some cities will allow up to 1200 sq ft
  • Approval Process
  • ADU applications must be approved/denied within 60 days without a public hearing or discretionary review
  • If the ADU is under 800 sq ft, no lot coverage requirements can be imposed
  • No Impact Fees may be assessed for units under 750 sq ft; proportional fees may be assessed to larger units
  • HOAs may no longer preclude ADUs, though they are permitted to place design restrictions and other requirements on them
  • Setbacks + Passageways
  • Side and Rear setbacks reduced to 4 feet statewide
  • Setbacks are not required for an existing garage that is converted to an accessory dwelling unit, and a setback of no more than four feet from the side and rear lot lines are required for a ground-up ADU or an ADU that is constructed above a garage.
  • Passageways are not required in conjunction with the construction of an accessory dwelling unit.
  • Parking
  • Parking requirements are one parking space per unit or per bedroom. These spaces may be provided as tandem parking on an existing driveway.
  • ADUs do not need to provide any parking if they are:
  • Within a half mile from public transit. 
  • Within an architecturally and historically significant historic district. 
  • Part of an existing primary residence or an existing accessory structure. 
  • In an area where on-street parking permits are required, but not offered to the occupant of the ADU. 
  • Located within one block of a car share area.
  • No replacement parking is required for the main residence when a garage, carport, etc, is demolished to create an ADU.
  • Fire sprinklers shall not be required in an accessory unit if they are not required in the primary residence.
  • ADUs shall not be considered new residential uses for the purpose of calculating utility connection fees or capacity charges, including water and sewer service. Local agencies are prohibited from requiring an ADU applicant to install a new or separate utility connection or impose a related connection fee or capacity charge for ADUs that are contained within an existing residence or accessory structure. For attached and detached ADUs, this fee or charge must be proportionate to the burden of the unit on the water or sewer system and may not exceed the reasonable cost of providing the service.

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