The ADU Glossary

When considering building an ADU, it can be difficult to know where to start. Understanding the ADU terminology helps the process move more smoothly. This glossary is filled with common terms in design and construction that you will need throughout your project. Feel free to bookmark this page and come back to it as a resource while moving through each step of your project.

Zoning Terms

Accessory Dwelling Unit: An accessory dwelling unit (ADU) is a second complete dwelling unit which is built within or on the same lot as an existing single-family residence. An ADU provides complete independent living facilities including a kitchen, bathroom, and its own entryway. ADUs are typically not allowed to be sold separate from the primary home.

Are you interested in building an ADU on your property? Check to see if your property is eligible by searching it on the Housable website using the property checker. It's free and no commitment is required. You can see where

Detached ADU: A detached ADU is established in a newly built or existing structure that is detached entirely from the primary dwelling. These units are most commonly referred to as backyard cottages, carriage homes, and laneway houses.

Attached ADU: An attached ADU is established within or connected to a primary home. For example, a basement conversion, garage apartment, or in an in-law unit connected to the primary home. These may be referred to as a "Granny Flat", but these terms may not be exclusive to each type of unit.

Junior ADU: A junior ADU is located entirely within the existing walls of an established single family home. In California, they are required to be less than 500 square feet of living space. A private bathroom is not required, provided that the JADU has full access to a bathroom in the principal residence and off-street parking is not required. Unlike an ADU however, these homes require owner occupancy, meaning the owner of the property must reside in either the JADU or the primary home if they are to rent out either one.

Setbacks: A setback is the distance which a dwelling or other structure is required to be set back from a front (sidewalks & streets), rear (alleyway), and adjacent properties lines (neighbors). For example, a four foot rear setback means that the structure cannot be built within four feet of the rear property line.

Floor Area Ratio: FAR is the ratio of a dwelling’s total square footage (livable floor area) to the square footage of the parcel of land on which it is built.

FAR = (total floor area of existing habitable structures) / (area of the parcel)

Architectural Compatibility: In many jurisdictions ADUs are required to be “architecturally compatible” with the primary home and sometimes the surrounding neighborhood. This typically means that exterior features including roof pitch, siding, windows, doors, and colors must match those of the primary dwelling. These are commonly referred to as “design requirements," and they may be reviewed by your city on a discretionary basis, so it is important to familiarize yourself before going too far along with the design process.

Lot Coverage: Lot coverage is calculated as the percentage of the total area of the lot which can be covered by dwellings and other structures. Calculating your existing lot coverage will help you determine if there are any additional restrictions that will determine the maximum size of your ADU. Due to new state laws, this may not restrict an ADU that is less than 750 square feet, and many properties are eligible for more. Check you property using Housable's free property checker. This is non committal and information is not shared with other parties.

Lot Coverage = (total area of building envelopes) / (area of the parcel)

Parking Requirements: Some ADUs may trigger a parking requirement, in which you may be required to construct some sort of off-street parking for the ADU. In many jurisdictions, these requirements may be waived entirely. Almost no ADUs will require more than one parking space, and most will have their requirements waived when they meet certain qualifications. You can use our property checker to see if your property would require parking.

Discretionary Review (DR): The authority of a planning department to review projects and determine compliance on a case by case basis and implies that even projects that meet all defined zoning requirements may still be denied. Under new state law, almost all ADU projects are not subject to discretionary review.

Ministerial Review (MR): A “by-right” process for streamlined approval of projects that are determined to be allowed based on the zoning compliance of the project. Jurisdictions in California have been moving to a MR process as housing production has become so necessary in the state.

Financing Terms

Home Equity Line of Credit: Often called a “HELOC,” a home equity line of credit is a loan in which the lender lends money to a homeowner, using the homeowner's equity in the home as the collateral. (Also known as a second mortgage).

Cash-Out Refinancing: A cash-out refinance is a replacement of a first mortgage where additional cash is lent to the homeowner typically based on their built equity and appreciation since the last mortgage was issued. The interest rates on a cash-out refinancing are typically lower than the interest rate on a HELOC. Closing costs are typically paid in a cash-out refinance.

Appraisal: Appraisals come into play when you are applying to finance your project or when you are in the process of selling your home. ADUs can contribute to the appraisal value of properties but only once they are completed.

Assessment: A home assessment is a value estimate assigned to your home through a standardized process for the purpose of determining property taxes for the property.

Property Tax: The tax to be paid by property owners to state or local government which is determined by an assessment. Typically these taxes are paid tax annually, semi-annually or as part of a monthly mortgage payment.

Are you interested in building an ADU on your property? Check to see if your property is eligible by searching it on the Housable website.