An attached ADU is a complete second dwelling joined to the main house: a lower level, a wing or an addition. As of October 2026, the City of Atlanta's zoning code does not allow attached ADUs, according to City Planning. Decatur and Sandy Springs do allow attached units, under their own limits.
Why can't I add an attached ADU in the City of Atlanta?
Atlanta's current ADU rules, adopted as part of the city's 2018 to 2019 zoning fixes, cover detached units only. City Planning has published a proposal to allow attached ADUs of up to 50% of the main house or 1,000 sq ft, but it is a proposal, not law. If you want a unit inside your Atlanta house today, the realistic options are a detached unit instead or waiting for the code to change. We will tell you which applies to your lot.
Where are attached units allowed nearby?
- Decatur. The city's ADU page lists the lower level of the main house as an example of an attached ADU. Units are capped at 800 sq ft and 40% of the main house, with up to two bedrooms, and the owner must live on the property. See Decatur.
- Sandy Springs. The development code, as published, lets a guest house be attached to or detached from the main home, with heated floor area under 1,500 sq ft. See Sandy Springs.
- Marietta. The zoning code defines an ADU as attached or detached, located to the rear of the main house. We confirm the use rules with the city. See Marietta.
What is the difference between an in-law suite and an ADU?
People use "in-law suite" for anything from a spare bedroom with a bath to a full apartment. Under most codes, the line is the kitchen and independence: a space with its own cooking facilities and entrance that could be lived in separately is a dwelling unit, and needs to be permitted as one. A bedroom suite without a kitchen is usually just part of the house. We help you decide which you actually need.
What does an attached unit involve?
- Fire-rated separation between the two units, as the building code requires for two dwellings in one structure.
- A separate exterior entrance.
- Ceiling height, egress windows and light that meet code, which is the usual challenge in a basement.
- Plumbing that drains to the sewer, which may need an ejector pump below grade.
- Heating and cooling for the unit, separate or zoned.
How much does an attached ADU cost?
Angi's national range for attached new construction is about $100,000 to $216,000; for basement conversions it is about $60,000 to $150,000. We did not find a reliable Atlanta-specific range as of October 2026; our cost guide collects what is published.