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Accessory Dwelling Unit Dictionary

Welcome to The ADU Guide’s Accessory Dwelling Unit Dictionary! This comprehensive glossary will help you navigate the world of Accessory Dwelling Units (ADUs) and home additions. Whether you’re a homeowner, renter, or industry professional, you’ll find the terms you need to better understand this fast-growing segment of the housing market.


Accessory Dwelling Unit (ADU):
A smaller, self-contained residential unit located on the same property as a primary dwelling. ADUs can be attached to or detached from the main house and typically include a separate entrance, kitchen, bathroom, and living space.
A construction project that expands the size of an existing building by adding new square footage. Additions can be single or multi-story, and can include new rooms, extensions, or expansions.


Basement ADU:
An accessory dwelling unit created within an existing basement or by converting a basement into living space. Basement ADUs often require the installation of egress windows, insulation, and proper ventilation.
Building Code:
A set of regulations governing the design, construction, alteration, and maintenance of buildings to ensure the health, safety, and welfare of occupants.


Carriage House:
A detached ADU, often located above a garage or in a separate outbuilding on the property. Carriage houses can provide additional living space or rental income for homeowners.
Certificate of Occupancy (CO):
A document issued by local government agencies that certifies a building’s compliance with applicable building codes and other regulations, allowing it to be legally occupied.


Detached ADU:
An accessory dwelling unit that is a separate, freestanding structure from the main house. Detached ADUs can be built on the property as new construction or can be converted from an existing structure like a garage or shed.
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Egress Window:
A window that provides a safe and accessible exit in case of an emergency, as required by building codes for living spaces such as bedrooms and basement ADUs.
Energy Efficiency:
The goal of reducing energy consumption and using resources more effectively in a building’s design, construction, and operation.


Floor Area Ratio (FAR): A zoning regulation that limits the total square footage of a building in relation to the size of the property. FAR helps control building density, size, and scale in a neighborhood.


Granny Flat:
Another term for an accessory dwelling unit, often used to describe a small, self-contained living space for an aging family member or an elderly relative.
Green Building:
A building designed, constructed, and operated to minimize its environmental impact, reduce energy consumption, and create a healthier living environment for occupants.
Living room space of a garage conversion in Los Angeles, CA


Home Addition:
The process of expanding an existing home by adding new square footage, rooms, or living spaces. Home additions can be single or multi-story and can include ADUs, extensions, or expansions.


In-law Suite:
A private living space within a primary residence designed to accommodate an aging family member or relative. In-law suites can be a separate, self-contained unit or can share some amenities with the main house.
Internal ADU:
An accessory dwelling unit created within the existing structure of a primary residence, such as a basement or attic conversion, or by dividing the existing living space.


Junior Accessory Dwelling Unit (JADU):
A smaller accessory dwelling unit, usually up to 500 square feet, located within the walls of an existing primary dwelling. JADUs often share some amenities with the main house, such as a bathroom or kitchen facilities.


Lot Coverage:
The percentage of a property’s total area that is occupied by all buildings and structures, including the main dwelling, ADUs, and accessory buildings. Lot coverage limits are established by local zoning regulations to control building density and preserve open space.


Modular Construction:
A building method that involves the assembly of prefabricated sections, or modules, off-site and then transporting them to the building site for installation. Modular construction can offer significant time and cost savings compared to traditional construction methods.
Multigenerational Housing:
A living arrangement that accommodates multiple generations of family members under one roof or on the same property. ADUs and in-law suites can provide private, self-contained spaces for multigenerational living.


Net-Zero Energy Building:
A building designed and constructed to produce as much renewable energy on-site as it consumes, resulting in a net-zero energy balance. Net-zero energy buildings often incorporate solar panels, energy-efficient technologies, and other sustainable features.


A property where the owner resides in either the primary dwelling or the accessory dwelling unit. Many jurisdictions require at least one of the units to be owner-occupied for ADUs to be allowed.
Detached new construction ADU in Vermont Square, Los Angeles, CA. Built by Soul Builders INC


Passive House:
A building standard that focuses on energy efficiency, indoor air quality, and comfort by incorporating airtight construction, high-performance insulation, and heat recovery ventilation systems.
A document issued by local government agencies that grants permission to construct, alter, or demolish a building or structure, provided it complies with applicable building codes and zoning regulations.


Rentable ADU:
An accessory dwelling unit that can be legally rented to tenants. Rentable ADUs can provide additional income for homeowners and increase the supply of affordable housing in a community.
Residential Zoning: A category of land use that regulates the development of residential properties, including single-family homes, multi-family dwellings, and accessory dwelling units. Residential zoning typically establishes density limits, building height restrictions, and other development standards.


The minimum distance required by zoning regulations between a building or structure and the property line. Setbacks help preserve open space, maintain privacy, and ensure adequate access to light and air.
Site Plan:
A detailed drawing that shows the layout of a property, including the location of buildings, structures, parking, landscaping, and other site features. Site plans are typically required as part of the permitting process for ADUs and home additions.
An ADU living and kitchen was built by Create Construction in Burbank, CA.


Tiny Home:
A small, compact living space, typically less than 500 square feet, designed to maximize efficiency and functionality. Tiny homes can be built on wheels or a foundation and can sometimes be used as an ADU, depending on local zoning regulations.
Transferable Development Rights (TDR):
A land use policy that allows property owners to transfer the development potential from one property to another, often as a means of preserving open space or historic resources.


Universal Design:
A design approach that focuses on creating spaces that are accessible, usable, and adaptable for all people, regardless of age, ability, or mobility. Universal design principles can be applied to ADUs and home additions to accommodate changing needs over time.
Urban Infill:
The development of vacant or underutilized land within existing urban areas, as opposed to expanding outward. ADUs and home additions can contribute to urban infill by increasing the density and diversity of housing options in established neighborhoods.


A system of land use regulations that governs the type, size, and location of development on a property. Zoning ordinances can impact the feasibility of building ADUs and home additions, by establishing limits on density, height, setbacks, and other development standards.