Room Addition vs. ADU: Which Fits Your Home?

The difference in one line

A room addition makes your home bigger. An ADU is a second home on the same lot. The law defines an ADU as a unit that “provides complete independent living facilities for one or more persons” and “shall include permanent provisions for living, sleeping, eating, cooking, and sanitation” (Gov. Code §66313). A bedroom and bath added onto the house, sharing its kitchen, is an addition.

In between is the junior ADU (JADU): “no more than 500 square feet of interior livable space in size and contained entirely within a single-family residence” (Gov. Code §66313). It needs an efficiency kitchen, which includes “A cooking facility with appliances” (Gov. Code §66333), and it can share a bathroom with the house.

Questions that usually decide it

Who will use the space? A growing family that shares meals usually fits an addition. A relative who wants their own kitchen and door, a caregiver or a tenant usually fits an ADU.
Will you rent it? An ADU “may be rented separate from the primary residence” (Gov. Code §66314). An addition is just more of your house.
Will you want to sell it on its own? Generally you can’t: an ADU “shall not be sold or otherwise conveyed separate from the primary residence,” with limited exceptions in state law and any local condo ordinance (Gov. Code §66314).
How much room is on the lot? An addition generally has to meet your zone’s normal setbacks. For an ADU, state law caps side and rear setbacks at 4 feet (see ADUs on the Central Coast).

Renting and living on site

A city or county can’t add its own standards beyond state law, “including an owner-occupant requirement, except that a local agency may require that the property may be used for rentals of terms 30 days or longer” (Gov. Code §66315).
A JADU is different: if it shares a bathroom with the house, the owner has to live on the property, and any rental must “be for a term longer than 30 days” (Gov. Code §66333).

Size

Attached ADU: “If there is an existing primary dwelling, the total floor area of an attached accessory dwelling unit shall not exceed 50 percent of the existing primary dwelling” (Gov. Code §66314), though local limits must still allow certain minimum sizes.
Detached ADU: up to 1,200 square feet under state law, or more if the local ordinance allows.
Addition: no ADU size cap; your zone’s rules for the main house apply, such as setbacks, height and lot coverage. Check with your city or county.

Permits and fees

Both need a building permit. CSLB summarizes the state code: no structure may be “erected, constructed, enlarged, altered” without a permit, and “Code requirements vary in different cities and counties around the state” (CSLB).
ADU permits have a state deadline. An ADU application is approved ministerially, and once complete it must be approved or denied within 60 days when there’s already a home on the lot, or it’s “deemed approved” (Gov. Code §66317). This 60-day rule is part of ADU law; ask your building department how long an addition usually takes to review.
Impact fees: none on an ADU of 750 square feet or less; above that, they’re charged in proportion to the main home’s size (Gov. Code §66311.5). Plan check and permit fees apply to both and vary locally.

Property tax

Either way, only the new part is reassessed. The assessor sets a new value “for the portion of any taxable real property which has been newly constructed,” and “The base year value of the remainder of the property assessed, which did not undergo new construction, shall not be changed” (Rev. & Tax. Code §71).

The Board of Equalization lists both “Increasing the square footage of a residence” and “Conversion of a single-family residence to a residence of two or more units” as new construction (BOE). Your county assessor can tell you what it means for your home.

What affects the cost

For either project, cost depends mostly on size, foundation and site work, whether plumbing and electrical have to be extended, and finishes. An ADU usually adds a kitchen, its own bathroom and its own entry, which an addition may not need. See room additions and ADU design & construction, or the rules in ADUs on the Central Coast and Converting a garage to an ADU.

Room addition or ADU: which fits?