The ADU Glossary

When considering building an ADU, it can be difficult to know where to start. Having the right terminology at the top of your head can help the process move more smoothly. This glossary can help to familiarize yourself with the common terms in design and construction that you will need throughout your project. 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 and the homeowners are usually required to reside in one of the two units.

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.

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.

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).

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. Commonly these are referred to as “design requirements” 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.

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

Parking Requirements: Any additional requirements for parking spaces when adding an ADU. In many jurisdictions, these requirements mandate that additional spaces be created for the ADU and in nearly all cases at least mandate that any parking that is lost for the ADU must be replaced.

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.

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.