What Is an ADU?

An accessory dwelling unit, or ADU, is a second self-contained home on a lot that already holds a single-family house. It has its own kitchen, its own bathroom, its own sleeping space, and its own entrance, and it stays legally tied to the main house rather than becoming a separate property.

Homeowners typically arrive with a layout drawn and a budget set aside, then learn the parcel sits in a district that does not permit a second dwelling, or that the septic field cannot carry another bathroom group. Across metro Atlanta, the answer changes at the county line, and inside a single city it can change from one zoning district to the next. Feasibility comes first, and design follows it.

An ADU Is a Second Dwelling, Not an Extra Room

Four features separate an ADU from finished square footage. The space has permanent cooking facilities, a full bathroom, a sleeping area, and independent access that does not require walking through the primary house. Strip out the kitchen, and it becomes a bonus room or a guest suite. Add all four, and local code treats it as a dwelling unit, with everything that follows in permitting, inspection, and utility service.

The word accessory carries weight too. The unit stays subordinate to the main house, sits on the same parcel, and is almost always capped at a share of the primary dwelling or a fixed square footage, whichever comes out smaller. Homeowners use a dozen names for the same thing. Granny flat, in-law suite, carriage house, backyard cottage, and secondary suite all describe an ADU. ADU stands for accessory dwelling unit, and that is the term the permit office will use.

Four Build Types Cover Nearly Every ADU

Every ADU falls into one of four construction paths, and the path drives the schedule far more than the finish selections do.

Type	What it is	Typical location	Main constraint
Detached	A freestanding structure built new	Rear yard	Setbacks, height limits, and utility trenching from the house
Attached	New conditioned space sharing a wall with the house	Side or rear elevation	Fire separation, roof tie-in, and footprint against setbacks
Internal conversion	Existing interior space made independent	Basement or attic	Ceiling height, egress windows, and a separate entrance
Garage conversion	An existing garage turned into living space	Attached or detached garage	Slab height and moisture, insulation, and replacing lost parking

Garage conversions are the most common entry point because the slab, walls, and roof already exist, which removes the foundation and shell work from the schedule. The tradeoff is everything the original structure was never built to do, from moisture control at the slab to insulation in walls that were never conditioned.

Detached units behave like small houses. They need their own foundation, their own shell, and their own service runs, which puts them closest to ground-up home construction on a compressed footprint. Attached units follow the logic of a conventional home addition, with the added requirement of a code-compliant separation between the two dwellings.

Zoning Decides Feasibility Before Design Does

The first question is never how big or how much. It is whether the parcel permits a second dwelling at all. Six items settle that, and every one of them is answered locally.

What to verify	Where the answer lives
Whether a second dwelling is permitted	The zoning district assigned to the specific parcel
Maximum size	Local ordinance, usually a percentage of the primary house or a fixed square footage cap, whichever is smaller
Placement and height	Accessory structure setbacks, rear yard requirements, and story or height limits
Owner occupancy	Local ordinance, which commonly requires the owner to live in one of the two units
Number permitted	Local ordinance, typically one per lot
Private restrictions	HOA covenants and recorded deed restrictions, which apply on top of zoning

Covenants are enforced by the neighborhood, not the county, and they can prohibit what the zoning code allows. A parcel can clear every public requirement and still be blocked by a subdivision restriction recorded decades ago. Pull the covenants at the same time as the plat.

Metro Atlanta Rules Are Set Jurisdiction by Jurisdiction

Georgia has no statewide ADU allowance. House Bill 1166, introduced in the 2025 to 2026 session, would have kept local zoning decisions from applying to dwellings of 400 square feet or less. It cleared the House in March 2026 and died in the Senate, so local control remains the rule.

What that produces is a patchwork. The City of Atlanta permits detached accessory dwelling units in several single-family districts, including R-4, R-4A, and R-5, and the Department of City Planning has floated changes that would allow attached units and widen the eligible districts. Cobb County went the other direction. A countywide ADU ordinance was drafted in 2024, the planning commission recommended denial in a unanimous vote, and the county adopted its zoning code updates without the ADU provisions.

Two homeowners fifteen minutes apart can therefore get opposite answers to the same question. Before anything else, confirm which jurisdiction has authority over the parcel, then confirm the district within it. County and city planning departments will answer both by parcel number.

Utilities and Site Work Drive the Schedule More Than Square Footage Does

A second dwelling adds load to systems sized for one house. Four items decide whether that is straightforward or the long pole in the project.

• Wastewater. On sewer, the question is whether the ADU ties into the existing lateral or needs its own connection and tap. On septic, a system sized for a three-bedroom house may not have the capacity or field area for another dwelling, which routes the project through county health department review before permitting moves.
• Water. The existing service line may need upsizing to carry simultaneous demand, and some jurisdictions require a separate meter for a unit that will be rented.
• Electrical. Existing panel and service capacity determines whether a subpanel is sufficient or a service upgrade is required. Where a separate meter is wanted, the utility sets the requirements for the second service.
• Site access and grading. Rear yard construction needs a path wide enough for equipment and material delivery. On a sloped lot, a simple slab can turn into retaining and drainage work, and detached units still need trenching for water, sewer, power, and gas.

None of this shows up in a floor plan. All of it shows up in the schedule, which is why the site walk should happen before the design is finalized rather than after.

The Build Runs in a Fixed Order

An ADU follows the same sequence as any permitted dwelling, compressed into a smaller footprint.

1. Parcel and zoning verification, including covenants
2. Survey and site plan showing setbacks, easements, and utility locations
3. Design drawn to the constraints already confirmed
4. Permit submittal and plan review
5. Site work, utility trenching, and foundation
6. Framing, roof, and dry-in
7. Mechanical, electrical, and plumbing rough-in with inspections
8. Insulation, drywall, and interior finish
9. Final inspections and certificate of occupancy

Steps one through three are where projects are won or lost. A design produced before the zoning check has to be redrawn, and a redraw after permit submittal restarts the review clock. Holding design, permitting, and construction under one licensed builder removes the handoffs where that usually happens. The same discipline applies to any structural project on an existing house, including adding a second story instead of building outward.

Frequently Asked Questions

What does ADU stand for?

Accessory dwelling unit. It describes a second, smaller, self-contained home on the same lot as a single-family house. Granny flat, in-law suite, carriage house, and backyard cottage all refer to the same thing in everyday use.

What is the difference between an ADU and a guest house?

Permanent cooking facilities, usually. A guest house or pool house with a bedroom and a bathroom but no kitchen is an accessory structure, not a dwelling unit. Add a kitchen and the same building becomes an ADU, which changes the zoning approval, the permit path, and often the utility requirements.

What is one drawback of an ADU?

The approval path is longer and less predictable than a renovation of the same size. A kitchen remodel is governed by building code. An ADU is governed by building code plus zoning, plus any covenants on the property, and any one of those can stop the project outright. Homeowners who plan for a straightforward addition schedule are usually surprised by the front end, not the construction.

How big can an ADU be?

Local ordinance sets it, and the common structure is a percentage of the primary dwelling or a fixed square footage cap, whichever produces the smaller number. Because the cap is tied to the main house in most jurisdictions, two identical lots can support different sized units.

Does an ADU need its own utility meters?

Not always. Some jurisdictions allow the unit to run off the primary house meters, and some require separate service when the unit will be rented. The utility provider has its own requirements on top of the local code, so both need to be confirmed during feasibility rather than at rough-in.

Can a garage be converted into an ADU?

Yes, where zoning permits a second dwelling and the structure can be brought up to residential code. The work usually involves raising or sealing the slab, insulating walls and roof, adding egress, and running full mechanical, electrical, and plumbing systems. Replacing the parking the garage provided is a frequent requirement, which sometimes means a new detached garage build as part of the same project.

Find Out What Your Lot Allows

JMJ Construction Group is a licensed general contractor serving homeowners throughout Atlanta and the North Metro. Tell us the address and what you want the space to do, and we will tell you what the parcel supports. Schedule a consultation today.