ADU Rules in Cobb County, GA: What Homeowners Need to Know

A Complete 2026 Guide for Cobb County Homeowners | Garages for Atlanta

Cobb County is the most nuanced of the three major Metro Atlanta counties covered in this blog series, for a reason that is both specific and significant: as of 2026, unincorporated Cobb County does not have a broadly adopted countywide ADU ordinance for detached units. A proposed ordinance that would have permitted detached ADUs county-wide was drafted in 2024, went through public hearings in November 2024, and was withdrawn by the Cobb County Board of Commissioners after significant public opposition — particularly in East Cobb neighborhoods concerned about neighborhood character and density.

This matters enormously for Cobb County homeowners. It means that in most of unincorporated Cobb County in 2026, a detached backyard cottage ADU is not permitted by right. However, three important alternatives remain available: interior conversions (finished basements, in-law suites within the primary home structure), attached ADU additions that expand the primary home's footprint, and the City of Marietta's own ADU framework that governs properties within Marietta city limits separately from the county. This guide covers all of it in plain language.

Unincorporated Cobb County: The 2024 ADU Withdrawal and Current Status

As of early 2025, Cobb County has not adopted a countywide ordinance that broadly legalizes detached ADUs in unincorporated areas. The Board of Commissioners voted 5-0 to withdraw the proposed 2024 ADU code amendment after public opposition at hearings in November 2024. The withdrawn draft had proposed allowing ADUs in most residential zoning categories, capped at the lesser of 50 percent of the gross square footage of the primary home or 850 square feet, with mandatory owner occupancy and a prohibition on short-term rental use. Without an adopted ordinance, detached backyard ADUs are not broadly permitted in unincorporated Cobb County residential zones.

Expert Insight: What the Cobb County ADU Withdrawal Means for Homeowners in 2026

The November 2024 withdrawal of Cobb County's proposed ADU ordinance is the most important fact for any Cobb County homeowner researching ADU construction in 2026. Online research frequently surfaces information about the proposed ordinance — what it would have allowed, what the proposed size limits were, what the owner occupancy requirements were going to be — without clearly noting that this ordinance was never adopted. The rules described in these articles do not currently apply in unincorporated Cobb County. Homeowners who proceed with a detached ADU project in unincorporated Cobb County based on this information will encounter permit denials. The practical path for Cobb County homeowners interested in accessory dwelling space is interior conversions, attached additions, or properties within the City of Marietta where the city's own ADU rules apply. Garages for Atlanta confirms current Cobb County ADU permissibility at the first site evaluation using current county code, not withdrawn proposal documents.

What Is Permitted in Unincorporated Cobb County in 2026

While detached ADUs are not broadly permitted in unincorporated Cobb County, in-law suites and accessory dwelling spaces within or attached to the primary home structure remain an available option in most residential zones. Interior conversions — finished basements, converted garages, and in-law suite additions within the existing building footprint — are permitted as residential improvements under standard building permits. Attached additions that expand the primary home's footprint and create accessory living space are also permitted subject to setback and lot coverage requirements. These configurations provide meaningful accessory living space within the current Cobb County regulatory framework.

ADU Type	Currently Permitted in Unincorporated Cobb County?	Permit Pathway	Key Requirement
Detached ADU / Backyard Cottage	Not broadly permitted (2024 ordinance withdrawn)	No standard permit pathway for detached ADU	Consult Cobb County Community Development before any planning
Interior Conversion (garage, basement, attic)	Yes — as residential improvement	Residential building permit + trade permits	Must meet habitability code; CO required for change of occupancy
Attached Addition (in-law suite expanding footprint)	Yes — as residential addition	Residential addition permit + trade permits	Setbacks, lot coverage, and FAR must be confirmed
ADU within primary structure (no new footprint)	Yes — as interior renovation	Residential building permit	Fire separation, egress, habitability standards apply

City of Marietta: ADU Rules Within Marietta City Limits

The City of Marietta operates its own building department and zoning code, separate from Cobb County. Properties within Marietta city limits follow the City of Marietta's regulations, not the unincorporated Cobb County code. Marietta's approach to ADU permissibility requires the property owner to occupy either the primary dwelling or the ADU, and the city's zoning code governs what configurations are permitted in specific residential districts. A Marietta mailing address does not guarantee that a property falls within Marietta city limits — some Marietta addresses are in unincorporated Cobb County.

The City of Marietta has historically been more progressive than unincorporated Cobb County on accessory dwelling structures, recognizing the city's strong multi-generational household demographics and the housing needs of the community that surrounds WellStar Kennestone Hospital and Marietta Square. Garages for Atlanta's Marietta location page covers the city's specific permit process, tree ordinance requirements, and architectural matching standards in detail.

The Practical Path for Cobb County Homeowners: Interior Conversion and Attached Additions

For Cobb County homeowners who want accessory living space in 2026, the practical path runs through interior conversions and attached additions rather than detached backyard ADUs. A garage conversion to an in-law suite, a finished basement conversion with independent access, or an attached wing addition that creates a self-contained suite within the primary structure all deliver meaningful accessory living space under the current Cobb County regulatory framework. Each approach has different cost profiles, construction timelines, and design considerations.

Garage Conversion to In-Law Suite in Cobb County

Converting an existing garage to a self-contained in-law suite within the primary structure is the most cost-efficient path to accessory living space in Cobb County. Because it uses the existing structure's footprint and does not create a new detached dwelling unit, it falls within the standard residential improvement permit pathway rather than the ADU framework. The conversion scope includes insulation to 2025 Georgia Energy Code standards, electrical upgrade to 2023 NEC, plumbing rough-in for kitchen and bath, HVAC, egress windows, and interior finishes.

• Zero new lot coverage: uses existing permitted garage footprint

• Standard residential building permit — no ADU ordinance required

• Certificate of occupancy required for habitable change of use

• Cobb County Community Development or Marietta Building Department depending on jurisdiction

• Cost range: $40,000 to $80,000. See garage construction pricing.

Attached In-Law Suite Addition in Cobb County

An attached addition that creates a self-contained suite within the expanded primary home structure — with its own entrance, kitchen, and bath but sharing at least one wall with the main house — is permitted as a residential addition in Cobb County. This configuration provides more space and design freedom than a garage conversion while remaining within the permitted framework. The roofline integration and exterior matching requirements for an attached garage addition apply equally to an attached in-law suite addition.

• Permitted as residential addition under standard Cobb County building permit

• Roofline tie-in and exterior material matching resolved at concept design stage

• Fire-rated wall assembly between primary home and in-law suite recommended for sound and safety

• Independent HVAC zone for the in-law suite: mini-split or zone-controlled extension

• Cost range: $120,000 to $175,000. See attached garage builders page.

Looking Forward: Cobb County ADU Ordinance Status

The November 2024 withdrawal of Cobb County's proposed ADU ordinance does not necessarily mean the county will never adopt an ADU framework. The Chairwoman who proposed the 2024 ordinance cited ADUs as an important tool for addressing housing affordability and multi-generational family needs. The county's median home sales price surpassed $500,000 for the first time in 2024, and demand for accessory housing options continues to grow.

Garages for Atlanta monitors Cobb County's zoning updates and will update this guide when and if the county adopts new ADU regulations. Any homeowner planning a project based on current Cobb County rules should confirm current permissibility with the Cobb County Community Development Department before committing to a design or budget.

Costs for Accessory Living Space in Cobb County in 2026

Project Type	2026 Cost Range	Timeline	Cobb County Status
Garage Conversion to In-Law Suite	$40,000 – $80,000	8–14 Weeks	Permitted — standard residential improvement permit
Attached In-Law Suite Addition	$120,000 – $175,000	18–26 Weeks	Permitted — residential addition permit
Basement Conversion to In-Law Suite	$55,000 – $95,000	10–18 Weeks	Permitted — interior renovation permit
Detached ADU / Backyard Cottage	Not currently permitted	N/A	2024 ordinance withdrawn — confirm with Cobb County before planning
City of Marietta In-Law Suite / ADU	$120,000 – $200,000	18–28 Weeks	Marietta's own framework — confirm with City of Marietta Building Dept

For Cobb County homeowners in Marietta, the city-specific location page covers local permit requirements, tree ordinance, and architectural standards. For the broader Metro Atlanta ADU framework, the complete Georgia ADU guide covers all jurisdictions. Contact Garages for Atlanta at 404-509-5526 or online for a no-cost site evaluation.

Frequently Asked Questions

1. Are ADUs allowed in Cobb County, GA?

Answer: As of 2026, unincorporated Cobb County does not have a broadly adopted ordinance permitting detached ADUs. The Board of Commissioners withdrew a proposed 2024 ADU ordinance after public opposition in November 2024. Interior conversions (garages, basements) and attached in-law suite additions are permitted under standard residential building permits. The City of Marietta has its own framework that may differ from unincorporated Cobb County rules.

2. What happened to Cobb County's proposed ADU ordinance?

Answer: Cobb County staff drafted an ADU code amendment in 2024 that would have allowed detached ADUs capped at the lesser of 50% of the primary home or 850 square feet in most residential zones. After public hearings in November 2024, the Board of Commissioners voted 5-0 to withdraw the proposal following significant public opposition from East Cobb residents concerned about neighborhood character and density impacts.

3. Can I build an in-law suite in my garage in Cobb County?

Answer: Yes. Converting an existing garage to an in-law suite within the primary structure is permitted in Cobb County under a standard residential improvement building permit. It does not require an ADU ordinance because it is an interior renovation rather than a new detached dwelling unit. The converted space must meet habitability code standards and receive a certificate of occupancy for change of use.

4. Can I build a detached backyard cottage in Cobb County?

Answer: As of 2026, detached backyard ADUs are not broadly permitted in unincorporated Cobb County. The 2024 proposed ordinance that would have allowed this was withdrawn. Confirm current permissibility with the Cobb County Community Development Department before planning any detached ADU project.

5. What are the ADU rules in the City of Marietta?

Answer: The City of Marietta operates its own building department and zoning code, separate from Cobb County. Properties within Marietta city limits follow city ADU rules, which require property owner occupancy and govern configurations by residential zoning district. A Marietta mailing address does not guarantee the property is within Marietta city limits — some Marietta addresses are in unincorporated Cobb County. Confirm your jurisdiction by parcel data.

6. What types of accessory living space are permitted in Cobb County in 2026?

Answer: Interior conversions (garages, basements, attics converted to in-law suites within the primary structure) and attached additions (in-law suite additions that expand the primary home's footprint) are permitted under standard residential building permits in Cobb County. Detached ADUs are not broadly permitted in unincorporated Cobb County as of 2026.

7. How much does a garage conversion to in-law suite cost in Cobb County?

Answer: A garage conversion to an in-law suite in Cobb County runs $40,000 to $80,000 in 2026 depending on the existing garage's condition, the scope of insulation and HVAC work required, and the finish level. An attached in-law suite addition runs $120,000 to $175,000. These are the two practical paths to accessory living space under the current Cobb County regulatory framework.

8. What permits are required for a garage conversion in Cobb County?

Answer: A garage conversion to an in-law suite in Cobb County requires a residential building permit for change of use, an electrical permit for panel upgrades and circuit additions, a plumbing permit for kitchen and bath rough-in, and a mechanical permit for HVAC installation. A certificate of occupancy must be issued after all final inspections pass before the converted space can be legally occupied.

9. Will Cobb County adopt an ADU ordinance in the future?

Answer: Unknown. The Cobb County Board of Commissioners withdrew the 2024 proposed ADU ordinance, but the county's Chairwoman has cited ADUs as an important housing affordability tool. Whether and when a revised ordinance is proposed and adopted depends on future county leadership decisions. Garages for Atlanta monitors Cobb County zoning updates and will advise clients on current permissibility at each project's site evaluation.

10. How do I start an accessory living space project in Cobb County?

Answer: Start with a no-cost site evaluation from Garages for Atlanta that confirms which jurisdiction governs your property (Cobb County unincorporated or City of Marietta), identifies the permitted configuration types under current rules, and produces a detailed cost proposal for the practical path available on your property. Contact Garages for Atlanta at 404-509-5526.