ADU Glossary
Confused by the jargon? Here are the key terms you need to know for your ADU project.
ADU (Accessory Dwelling Unit)
A secondary dwelling unit with complete independent living facilities, located on a lot with a proposed or existing single-family or multifamily dwelling. In California, ADUs can be detached, attached, or created by converting existing space (e.g., within a home or accessory structure).
JADU (Junior Accessory Dwelling Unit)
A unit of no more than 500 square feet created within the walls of a proposed or existing single-family residence (enclosed attached areas like an attached garage count). Local agencies may require a separate entrance and must require an efficiency kitchen; a JADU may share a bathroom with the primary dwelling or have its own.
Impact Fees
Development fees charged by a local agency to help fund public facilities (e.g., parks, roads). For California ADUs, impact fees are generally not allowed for ADUs up to 750 sq ft of interior livable space; for larger ADUs, any impact fee must be charged proportionally based on the ADU’s size relative to the primary dwelling.
Prefab / Modular ADU
An ADU that is built partially or fully off-site and then delivered for on-site installation. “Modular” typically means factory-built sections assembled on a permanent foundation and installed to meet applicable building standards; even with factory construction, the project still usually requires local permitting for site work (foundation, utilities, setbacks, etc.).
SB 9
California’s Senate Bill 9 requires ministerial (no-hearing) approval for qualifying projects in single-family zones: (1) a lot split into two parcels, and/or (2) a housing development with up to two primary units on a parcel. Used together, SB 9 can enable up to four total units across the two resulting lots, subject to eligibility limits and anti-displacement and environmental constraints.
Utility Connection / Separate Meters
Rules about whether an ADU must have its own direct utility connection (and, in some cases, its own meter). For water/sewer, ADUs/JADUs created from existing space generally cannot be required to add a new or separate connection (unless built at the same time as a new single-family home). For other ADUs, a separate direct connection may be required, and any connection/capacity charges must be proportional to the ADU’s burden (e.g., size or fixture count). Electricity and gas metering policies are often utility-specific.
Pre-approved Plans
ADU plan sets reviewed and approved in advance by a local agency so future applicants can reuse them. California law requires local agencies to run a pre-approval program (and publish approved plans and applicant contact info online). When a detached ADU application uses a qualifying pre-approved (or identical previously-approved) plan, the local agency must approve or deny it within a shorter timeline (commonly 30 days).
Fire Sprinkler Requirements
For ADUs, sprinklers are generally required only if the primary dwelling already has a sprinkler system; adding an ADU cannot trigger a requirement to retrofit sprinklers into the existing primary dwelling. Separate fire protection rules (like fire department access and water supply) can still apply, especially for newly constructed ADUs.
Condominium Conversion (AB 1033 Pathway)
A state-authorized pathway where a local agency may adopt an ordinance allowing a primary home and its ADU(s) to be separately conveyed (sold) as condominiums. This typically requires compliance with condo and subdivision rules, a safety inspection before recording the condo plan, and additional protections like lienholder consent and consumer notices (and may require HOA authorization in planned developments).
Ministerial Review
A non-discretionary approval process that checks a project only against objective standards (typically via checklists), without subjective judgment or a public hearing. For many ADU/JADU applications, a permitting agency must approve or deny a complete application within 60 days; otherwise it may be deemed approved.
FAR (Floor Area Ratio)
The ratio of a building’s total floor area to the lot area. In California ADU law, FAR/lot-coverage type limits generally cannot be used to preclude an ADU of at least 800 sq ft that maintains four-foot side and rear setbacks (subject to statutory conditions).
Setback
The distance a structure must be set back from property lines. For many attached/detached ADUs, side and rear setbacks generally may not be required to exceed 4 feet. No setback is required for an ADU/JADU created within existing living area or accessory structures, or for certain replacement structures in the same location and dimensions; coastal areas may have additional requirements.
Title 24
California’s Building Energy Efficiency Standards (Energy Code). ADUs must comply with the Energy Code. Solar PV requirements generally apply to newly constructed detached ADUs (with exceptions), while many ADUs created within existing space or as additions to existing homes are not subject to PV requirements.
Objective Standards
Standards that involve no personal or subjective judgment by a public official and are uniformly verifiable by reference to an external and uniform benchmark or criterion available and knowable before submittal.
Multifamily ADU
ADUs on lots with multifamily dwellings. State law generally requires allowing at least one ADU conversion within existing non-livable space (and up to 25% of existing multifamily units). It also allows detached ADUs on multifamily lots—commonly up to 2 on lots with a proposed multifamily dwelling, and up to 8 on lots with an existing multifamily dwelling (not exceeding the number of existing units), subject to statutory conditions.
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