ADU Glossary

Above-Garage ADU

An above-garage ADU is an accessory dwelling unit built as a second story on top of a garage, so the garage stays usable below and the home sits above it.

Accessible ADU: see ADU Accessibility

Accessory Dwelling: see Accessory Dwelling Unit

Accessory Dwelling Unit

An accessory dwelling unit (ADU) is a second, self-contained home on a lot that already has, or will have, a main house or apartment building, with its own space to live, sleep, cook, eat and wash.

ADU: see Accessory Dwelling Unit

ADU Accessibility

ADU accessibility is how easily a person with limited mobility, sight or strength can reach, enter and use an accessory dwelling unit, through features such as a step-free entry, wider doors and a roll-in shower.

ADU Architect

An ADU architect is a state-licensed architect who designs an accessory dwelling unit and prepares the drawings the building department reviews, often managing plan-check corrections as well.

ADU Architect Cost: see ADU Architect

ADU Bathroom

An ADU bathroom is the toilet, sink and tub or shower that serve an accessory dwelling unit, which California law calls its sanitation facilities.

ADU Bedroom

An ADU bedroom is a sleeping room inside an accessory dwelling unit that meets the building code for sleeping space, including a minimum floor area, ceiling height and an emergency escape opening.

ADU Blueprints: see ADU Plans

ADU Budget: see ADU Cost Estimate

ADU Builder: see ADU Contractor

ADU Construction

Building an ADU, from site work and foundation through framing, systems, finishes and final inspection.

ADU Construction Budget: see ADU Cost Estimate

ADU Construction Cost: see ADU Hard Costs

ADU Construction Process

The sequence of building an ADU: feasibility, design, permits, site work, foundation, framing, rough-in inspections, finishes and final inspection.

ADU Construction Timeline

How long an ADU takes from first design to move-in, split into design, permitting and construction stages.

ADU Contractor

An ADU contractor is a licensed construction business that contracts to build an accessory dwelling unit, either doing the work itself or running the subcontractors who do the trades.

ADU Contractor License: see ADU Contractor

ADU Cost

The total price of creating an ADU: construction (hard costs), design, engineering, permits and fees (soft costs), site work and utility connections.

ADU Cost Calculator

A tool that estimates an ADU's total cost from its type, size, location and site conditions.

ADU Cost Estimate

An ADU cost estimate is a written, itemized prediction of what it will cost to build a specific accessory dwelling unit on a specific lot, listing the work, materials, allowances, exclusions and assumptions behind the total.

ADU Cost Per Square Foot

ADU cost per square foot is the cost of an accessory dwelling unit divided by its floor area, a shorthand for comparing units of different sizes.

ADU Design

The layout, look and specification of an ADU: room plan, height, windows, finishes and how it sits on the lot.

ADU Design Fees: see ADU Soft Costs

ADU Design-Build

ADU design-build is a way of delivering an accessory dwelling unit in which one firm is responsible for both designing the unit and building it, usually under one contract or a design agreement that rolls into a construction contract.

ADU Designer: see ADU Architect

ADU Electrical Connection: see ADU Electrical Panel

ADU Electrical Panel

An ADU electrical panel is the breaker panel that distributes power to the circuits inside an accessory dwelling unit, usually a subpanel fed from the main house panel, or a main panel on its own utility service and meter.

ADU Engineer

An ADU engineer is a licensed engineer who designs the parts of an accessory dwelling unit that carry loads or deal with the ground, such as the foundation, beams and earthquake bracing, and signs the calculations the city reviews.

ADU Engineering Cost: see ADU Soft Costs

ADU Feasibility

ADU feasibility is the answer to whether a specific lot can take an accessory dwelling unit: legally under zoning and state law, physically on the site, with workable utilities, and within the owner’s budget.

ADU Financing

How homeowners pay for an ADU: cash, home equity loans and lines of credit, cash-out refinances, construction and renovation loans, and grants where they exist.

ADU Financing Program: see ADU Grant

ADU Floor Plan

The drawing of an ADU's rooms, walls, doors and windows seen from above. Floor plans are what most buyers browse; permit sets add a site plan, elevations and structural sheets.

ADU Foundation

An ADU foundation is the structure the accessory dwelling unit sits on, which carries its weight into the ground and ties it down.

ADU Framing

ADU framing is the structural skeleton of an accessory dwelling unit: the walls, floor and roof members, usually wood studs, joists and rafters or trusses, that hold the building up and resist wind and earthquakes.

ADU Gas Connection

A gas line to an ADU. Many California cities have adopted all-electric requirements for new construction, so a new ADU may not be allowed gas at all.

ADU General Contractor: see ADU Contractor

ADU Grant

An ADU grant is money a public agency or program gives a homeowner toward the cost of building an accessory dwelling unit, which does not have to be repaid as long as the program’s conditions are met.

ADU Hard Costs

ADU hard costs are the money spent physically building an accessory dwelling unit: site preparation, foundation, framing, roofing, windows, mechanical, electrical and plumbing systems, finishes, and the trenching that brings utilities to the unit, together with the contractor’s labor, overhead and profit.

ADU Housing: see Accessory Dwelling Unit

ADU HVAC

ADU HVAC is the heating, ventilation and air-conditioning system of an accessory dwelling unit.

ADU Incentive: see ADU Grant

ADU Infrastructure: see ADU Utility Connection

ADU Insulation

Thermal insulation in an ADU's walls, roof and floor. In California, new ADUs must meet the Title 24 energy code, which sets insulation levels by climate zone.

ADU Investment: see ADU ROI

ADU Kitchen

An ADU kitchen is the permanent cooking area that, together with a place to sleep and a bathroom, makes an accessory dwelling unit a separate home rather than an extra room.

ADU Layout: see ADU Plans

ADU Living Space: see ADU Size

ADU Loan

Any loan used to build an ADU. Lenders rarely offer a product called an "ADU loan"; it is usually a home equity, construction, renovation or refinance loan.

ADU Meter: see Separate Utility Meter

ADU Ordinance: see Local ADU Ordinance

ADU Owner-Occupancy Requirement

An ADU owner-occupancy requirement is a local rule that the property owner must live in either the main house or the accessory unit, so that both units cannot be rented to other people at the same time.

ADU Parking Requirement

An ADU parking requirement is a local zoning rule that a new accessory dwelling unit must come with a set number of off-street parking spaces, usually on the driveway or elsewhere on the lot.

ADU Permit

An ADU permit is the building permit a city or county issues to allow construction of an accessory dwelling unit, after checking the plans against local zoning standards and the building code.

ADU Permit Fees

ADU permit fees are the charges a city, county or utility district collects before and during the building of an accessory dwelling unit: plan review, the building permit, inspections, and in many places impact fees, school fees and utility connection charges.

ADU Permitting: see ADU Permit

ADU Plans

ADU plans are the set of scaled drawings and documents that describe an accessory dwelling unit well enough for a building department to check it against zoning and building codes and for a contractor to build it.

ADU Regulations

The state laws and local ordinances that decide whether, where and how big an ADU can be built.

ADU Rent: see Long-Term Rental ADU

ADU Rental

Renting out an ADU, long-term to a tenant or, where local rules allow, short-term to guests.

ADU Rental Agreement

An ADU rental agreement is the written lease between the owner of an accessory dwelling unit and the tenant who lives in it, setting the rent, the term and the house rules for a home that shares a lot with the owner’s house.

ADU Rental Income

The rent an ADU earns, before or after vacancy, maintenance, insurance and taxes.

ADU Return on Investment: see ADU ROI

ADU ROI

ADU ROI (return on investment) is the money an accessory dwelling unit earns or adds in value, expressed as a percentage of what it cost to build.

ADU Roofing

ADU roofing is the weather-tight covering on top of an accessory dwelling unit's roof frame, such as asphalt shingles, metal, tile or a low-slope membrane, together with the underlayment and flashing beneath and around it.

ADU Sewer Connection: see ADU Sewer Lateral

ADU Sewer Lateral

A sewer lateral is the private pipe that carries wastewater from a home to the public sewer main.

ADU Site Plan

An ADU site plan is a scaled, top-down drawing of the whole lot that shows where the accessory dwelling unit will sit, every existing structure, and the distances from each building to the property lines.

ADU Site Preparation

ADU site preparation is the work that readies a lot before the foundation goes in: locating property lines and buried utilities, clearing or demolishing what is in the way, grading the ground, digging utility trenches and opening a path for equipment.

ADU Site Preparation Cost: see ADU Hard Costs

ADU Size

ADU size is the floor area of an accessory dwelling unit, usually counted as interior livable square feet, and it is the number that state and local rules cap, protect and use to set fees.

ADU Soft Costs

ADU soft costs are the parts of an accessory dwelling unit budget that pay for paperwork, professional services and money rather than for physical construction: design, engineering, surveys, permits and government fees, utility charges, insurance and financing costs.

ADU Square Footage: see ADU Size

ADU Tax Incentive

Tax treatment that lowers an ADU's cost. There is no general federal ADU tax credit. In California, adding an ADU triggers reassessment of the new construction only, not the whole property.

ADU Utilities: see ADU Utility Connection

ADU Utility Connection

An ADU utility connection is the way an accessory dwelling unit gets water, sewer, electricity and, where used, gas: either as a branch off the main house's existing service, or as a new, separate connection running directly from the utility to the unit.

ADU Utility Connection Cost: see ADU Utility Connection

ADU Utility Connection Requirement: see ADU Utility Connection

ADU Water Connection

An ADU water connection is the pipe that brings drinking water to an accessory dwelling unit, usually as a branch off the main house's water line, or as a new service with its own meter where the water provider requires or allows one.

ADU Zoning

The local zoning rules that say where an ADU may go on a lot and how big and tall it may be. In California, state law requires cities to allow ADUs on lots zoned for residential use and limits how far local zoning can restrict them.

Attached ADU

An attached ADU is an accessory dwelling unit built onto the main home, sharing at least one wall with it, but with its own entrance, kitchen and bathroom so it works as a separate home.

Backyard ADU: see Detached ADU

Basement ADU

A basement ADU is an accessory dwelling unit made by converting a basement into a separate home, with its own entrance, kitchen, bathroom and sleeping space.

Buildable Area

Buildable area is the part of a lot where a building may legally and physically go, found by subtracting setbacks, easements and other no-build areas from the whole parcel.

Building Code

A building code is a set of minimum rules for how buildings must be designed and built so they are safe to live in, covering structure, fire safety, exits, plumbing, wiring, ventilation and energy use.

Building Department

A building department is the city or county office that reviews construction plans against the building code, issues building permits, inspects the work as it is built and gives the final sign-off that lets a new home be lived in.

Building Permit: see ADU Permit

California ADU Law

California ADU law is the set of state statutes, now in Chapter 13 of the Government Code (Sections 66310 to 66342), that tells every city and county in California what it must allow for accessory dwelling units and junior ADUs, and what it may not require.

California ADU Ordinance: see Local ADU Ordinance

California ADU Regulations: see California ADU Law

California Building Code

The California Building Code (CBC) is Part 2 of Title 24 of the California Code of Regulations: the state’s main construction code for buildings other than small houses, based on the International Building Code with California changes.

California Government Code Section 65852.2: see California ADU Law

California Government Code Section 65852.22: see Junior ADU

California Residential Code

The California Residential Code (CRC) is Part 2.5 of Title 24 of the California Code of Regulations: the state building code for detached one- and two-family dwellings and townhouses, which is the code most ADUs in California are designed under.

Cash-Out Refinance

A cash-out refinance is a new, larger mortgage that replaces your existing one and pays you the difference in cash, so the equity you have built becomes money you can spend, for example on an ADU.

Construction Loan

A construction loan is a short-term loan that pays for building or rehabilitating a home, releasing the money in a series of advances, often called draws, as construction progresses rather than as one lump sum.

Converted ADU

A converted ADU is an accessory dwelling unit made from space that already exists, such as a garage, basement, attic, spare wing or shed, instead of a new building.

Detached ADU

A detached ADU is an accessory dwelling unit built as its own freestanding building on the same lot as the main home, usually in the backyard, with its own kitchen, bathroom and entrance.

FAR: see Floor Area Ratio

Floor Area Ratio

Floor area ratio (FAR) is the total floor area of all buildings on a lot, on every story, divided by the lot’s area, so a FAR of 0.5 allows 3,000 square feet of building on a 6,000 square foot lot.

Front Setback: see Setback

Garage Conversion: see Garage Conversion ADU

Garage Conversion ADU

A garage conversion ADU is an accessory dwelling unit created by turning an existing garage, attached or detached, into a complete home with its own kitchen, bathroom, entrance and living space.

Height Limit

A height limit is the maximum vertical height a zoning code allows for a building, measured from a reference point on the ground to a reference point on the roof that the local code defines.

HELOC

A HELOC, or home equity line of credit, is an open-end line of credit secured by your home that lets you borrow repeatedly against your equity, up to a set limit, during a draw period, then repay what you borrowed over a repayment period.

Home Equity Loan

A home equity loan is a loan that uses the equity in your home as collateral and pays you the whole amount as one lump sum, usually at a fixed interest rate, which you repay in regular monthly payments from the start.

Interior ADU: see Converted ADU

JADU: see Junior ADU

Junior ADU

A junior ADU (JADU) is a small home of no more than 500 square feet carved out of an existing or proposed single-family house, with its own entrance and an efficiency kitchen, and permission to share the main home’s bathroom.

Local ADU Ordinance

A local ADU ordinance is the section of a city or county's zoning code that sets the rules for accessory dwelling units in that jurisdiction: where they are allowed, how big and tall they can be, setbacks, design standards and the application process.

Long-Term Rental ADU

A long-term rental ADU is an accessory dwelling unit leased to a tenant who lives in it as their home, usually for a month-to-month or fixed-term lease, rather than rented to short-stay guests.

Lot Coverage

Lot coverage is the percentage of a lot’s area covered by buildings, measured from above as the combined footprint of every roofed structure divided by the total lot area.

Lot Size: see Minimum Lot Size

Manufactured ADU

A manufactured ADU is a manufactured home, built in a factory to the federal HUD Code on a permanent steel chassis, that is placed on a lot as an accessory dwelling unit beside the main house.

Minimum Lot Size

A minimum lot size is the smallest parcel area, and sometimes width or depth, on which a zoning code allows a certain building or use.

Ministerial Approval

Ministerial approval is a permit decision in which an official checks an application against fixed, objective standards and must approve it if they are met, with no public hearing and no discretion to deny it on judgment.

Modular ADU: see Prefab ADU

Multigenerational ADU

An ADU built so relatives, often aging parents or adult children, can live on the same lot with their own front door.

New Construction ADU

An ADU built as new floor area, attached or detached, as opposed to a conversion of existing space.

Permit Expediting

Permit expediting is paying a specialist, called a permit expediter, to prepare, file and track a building permit application for you, and to answer the city’s questions until the permit is issued.

Permit Review

The city's check of permit drawings, also called plan check. Reviewers return corrections until the set complies.

Planning Approval

Planning approval is a city or county sign-off that a project meets the zoning code and land-use rules, given before or alongside the building permit.

Planning Department

A planning department is the city or county office that applies the zoning code and general plan, checking where a project goes, what it is used for and how big it is.

Prefab ADU

A prefab ADU is an accessory dwelling unit whose walls, floors or whole room modules are built in a factory and then shipped to the lot, where a crew sets them on a foundation and connects utilities.

Rear Setback: see Setback

Renting an ADU: see Long-Term Rental ADU

Residential ADU: see Accessory Dwelling Unit

Residential Building Code: see California Residential Code

Residential Zoning: see Zoning District

Secondary Dwelling Unit: see Accessory Dwelling Unit

Separate Utility Meter

A separate utility meter is a meter the utility installs and reads for the accessory dwelling unit alone, so the ADU's electricity, gas or water is billed on its own account instead of through the main house's meter.

Setback

A setback is the minimum distance a zoning code requires between a building and a property line, measured horizontally from the line to the nearest wall or other regulated part of the structure.

Shared Utility Meter: see Separate Utility Meter

Short-Term Rental ADU

A short-term rental ADU is an accessory dwelling unit rented to guests for short stays, typically nights or weeks, often through platforms such as Airbnb, rather than leased to a tenant who lives there.

Side Setback: see Setback

Statewide ADU Law: see California ADU Law

Tiny Home ADU

A tiny home ADU is a very small house, generally 400 square feet or less, that is permitted as an accessory dwelling unit on a lot with a main home.

Two-Story ADU

A two-story ADU is an accessory dwelling unit with living space on two levels, usually with the living area and kitchen below and bedrooms above, joined by an inside stair.

Universal Design ADU

An ADU designed to work for people of any age or ability: no-step entry, wide doors and halls, a curbless shower and reachable controls.

Zoning Code

A zoning code is a city or county law that sets what may be built on each piece of land, and how big, how tall and how far from the lot lines it may be.

Zoning District

A zoning district is an area on a city or county map where one set of land-use rules applies, shown by a short code such as R-1 for a single-family residential zone.