Carroll County already allows accessory dwelling units today, but the rules are not simple—and they differ depending on whether the property is in an Agricultural/Conservation district, a Residential district, or an incorporated municipality. At the same time, the County is rewriting its ADU framework to comply with Maryland’s HB 1466 before the October 1, 2026 deadline. A public hearing on amendments to Chapters 155, 156 and 158 was held September 3, 2026, but Maryland ADU Guide has not yet verified a final adopted ordinance.
Current status
🟠 TRANSITION — Carroll County’s 2026 ADU amendments are still pending verification of final adoption.
What applies now: the currently published Carroll County Code. Latest verified legislative milestone: September 3, 2026 public hearing on proposed changes to Chapters 155, 156 and 158. Last editorial review: September 12, 2026.
Important correction to generic summaries: Carroll County is not starting from a place where ADUs are broadly prohibited. Current County code already contains detailed attached and detached ADU provisions. The real 2026 story is how those existing rules will be reconciled with Maryland’s newer statewide framework.
Start with the Maryland ADU Jurisdiction Finder if you are not certain whether Carroll County zoning actually controls your property. Municipal addresses can follow different local rules.
1. Quick answer: can you build an ADU in Carroll County?
Yes, in many parts of unincorporated Carroll County. But the current standards are district-specific.
| Question | Current Carroll County answer |
|---|---|
| Are ADUs already allowed? | Yes. Current Chapter 158 contains attached and detached ADU rules. |
| Is there one Countywide size rule? | No. Agricultural/Conservation and Residential districts use different current standards. |
| Owner occupancy? | Current code requires the property owner to occupy either the principal dwelling or the ADU. |
| Parking? | Current code imposes additional ADU parking, but Maryland’s 2025 law now requires a parking-study/waiver framework if a local government keeps additional ADU parking mandates in its post-2026 law. |
| Detached ADUs? | Allowed in some circumstances, but current requirements differ sharply by zoning district. |
| Building permit? | Yes. Carroll County also now requires a surveyed plot plan for new dwelling permit applications, including accessory dwellings. |
| 2026 amendment adopted? | Not yet verified by Maryland ADU Guide as finally adopted. |
2. Carroll County’s current ADU rules are already detailed
Carroll County’s current zoning code defines an accessory dwelling unit as an additional attached or detached dwelling unit on a lot with a principal dwelling unit. It separately defines attached and detached ADUs and then regulates them differently depending on zoning district.
This matters because a homeowner who reads only a statewide Maryland summary can easily miss major local differences in:
- maximum size;
- bedroom count;
- parking;
- detached-unit eligibility;
- lot size;
- distance from the main house;
- subdivision eligibility.
3. Agricultural and Conservation districts: current ADU rules
Under current §158.071.02, accessory dwelling units are allowed as accessory uses in Agricultural and Conservation districts subject to specific conditions.
| Current requirement | Agricultural / Conservation districts |
|---|---|
| Owner occupancy | Owner must occupy either the principal dwelling or the ADU. |
| Maximum size | 1,000 sq ft of livable floor area. |
| Bedrooms | Maximum two bedrooms. |
| Parking | One additional off-street space for each ADU bedroom. |
| Number of ADUs | One attached or detached ADU per lot. |
| Detached ADU lot size | At least 3 acres. |
| Detached ADU location | No more than 50 feet from the principal dwelling. |
| Building/health | Must meet applicable building-construction and health/environmental rules. |
4. Residential districts: attached and detached ADUs follow different current rules
Attached ADUs
Current §158.075.02 allows attached ADUs in Residential districts subject to the following major standards:
- direct access from outside;
- one attached ADU on the principal dwelling;
- owner occupancy in either unit;
- maximum size of 800 sq ft or one-third of the principal dwelling’s total livable floor area, whichever is greater;
- maximum two bedrooms;
- two additional off-street parking spaces;
- applicable building and health/environmental requirements.
Detached ADUs
The current Residential-district rule is very different. A detached ADU is permitted only where the lot or parcel is eligible to be subdivided to separate the detached ADU. The current code also requires owner occupancy, two off-street parking spaces and applicable building/health standards. Unlike the attached rule, the current Residential-district detached-ADU provision states that detached ADUs are not subject to a size limit.
5. What Maryland’s 2026 law requires Carroll County to address
Maryland Land Use §4-504 requires covered counties and municipalities to adopt compliant local ADU legislation by October 1, 2026. Carroll County staff began the current text-amendment process specifically because of HB 1466, with presentations in April and May, Planning and Zoning Commission recommended text in July, and a Board public hearing on September 3.
Maryland’s current statewide framework requires local ADU laws to:
- authorize qualifying ADUs on land with a primary single-family detached dwelling;
- allow an ADU up to 75% of the size of the primary single-family detached dwelling under the state definition;
- exclude ADUs from density calculations;
- exclude ADUs from measures limiting residential growth on the property;
- not impose side/rear setbacks more restrictive than the jurisdiction’s existing accessory-structure setbacks;
- use a parking study and waiver process if the local law imposes additional off-street parking specifically for ADUs.
The Maryland Department of Planning says local governments may be more permissive, but they must at least authorize ADUs consistent with the state framework.
6. Why Chapters 155, 156 and 158 are all involved
The September 3 hearing covered proposed amendments to three separate Carroll County chapters because ADU feasibility is not just a zoning-use question.
| Chapter | Why it matters to ADUs |
|---|---|
| Chapter 155 — Development & Subdivision | Contains subdivision rules, including a specific accessory-dwelling subdivision process and other development procedures. |
| Chapter 156 — Adequate Public Facilities | Currently defines an accessory dwelling as residential development. Maryland’s new law requires ADUs to be excluded from measures limiting residential growth, while still allowing relevant public-health/safety/facility-capacity standards. |
| Chapter 158 — Zoning | Contains the current definitions, size, parking, owner-occupancy, attached/detached and district-specific ADU standards. |
Maryland ADU Guide is not treating the proposed September text as enforceable until a final adopted ordinance and effective date are verified.
7. Parking is one of the clearest 2026 issues to watch
Carroll County’s current ADU rules require additional off-street parking:
- Agricultural/Conservation: one additional space per bedroom;
- Residential attached ADU: two additional spaces;
- Residential detached ADU: two off-street spaces.
Maryland §4-504 now says a jurisdiction that wants additional ADU off-street parking in its compliant local law must first complete a parking study and must provide a waiver process. The study must consider costs, available curb area, impervious-surface impacts and lot-size variability.
Homeowner takeaway: do not assume Carroll County’s existing parking numbers will necessarily remain unchanged in the final 2026 ordinance. Until final adoption is verified, use the current code for today’s application and monitor the amendment.
See the Maryland ADU Parking Rules & Parking Study Tracker.
8. Size: current Carroll rules vs. Maryland’s 75% definition
Maryland defines a qualifying ADU as no greater than 75% of the size of the primary single-family detached dwelling. The Department of Planning says local governments cannot use a lower cap as their minimum compliant authorization framework, although a jurisdiction may allow more permissive housing types outside the state-defined ADU category.
That creates an obvious reconciliation issue with Carroll County’s current local standards:
- Agricultural/Conservation: current 1,000-sq-ft cap;
- Residential attached: current 800 sq ft or one-third of the principal dwelling, whichever is greater;
- Residential detached: current provision says no size limit, but only on subdivision-eligible parcels.
Use the Maryland 75% ADU Size Guide & Calculator for the state measurement issue.
9. Detached ADUs: the current rules can be much more restrictive than the yard suggests
A homeowner can look at a large backyard and assume a detached ADU is straightforward. In Carroll County, the zoning district matters first.
- Agricultural/Conservation: current code requires at least 3 acres and places the detached ADU within 50 feet of the principal dwelling.
- Residential districts: current code ties detached-ADU eligibility to whether the parcel is eligible to be subdivided to separate the detached dwelling.
These are materially different tests. Neither should be replaced by a generic “backyard ADUs are allowed” answer.
10. Septic and well properties still need environmental-health verification
Carroll County’s current zoning code expressly requires ADUs to comply with applicable health and Maryland environmental rules. State law also preserves public-health, safety and facility-capacity standards.
If the property relies on private well or septic, obtain the actual records before final design. At minimum, verify:
- approved septic system and bedroom capacity;
- reserve area;
- well location and setbacks;
- whether the proposed ADU adds bedrooms or wastewater load;
- whether a detached location conflicts with septic/reserve areas.
Use the Maryland ADU Septic & Well Rules if that URL is active; otherwise continue through the Property Feasibility Checker.
11. Carroll County permit path: what to prepare now
Carroll County’s current permit guidance says new dwelling permit applications—including accessory dwellings—must include a plot plan prepared and stamped by a registered surveyor.
| Step | What to verify |
|---|---|
| 1. Confirm jurisdiction | County zoning or an incorporated municipality? |
| 2. Identify zoning district | Agricultural, Conservation, Residential or municipal zoning. |
| 3. Confirm attached vs. detached pathway | Current standards differ significantly. |
| 4. Verify current size/parking rules | Do not design from proposed 2026 text until adopted. |
| 5. Pull septic/well records if applicable | Check bedroom/capacity and reserve-area constraints. |
| 6. Prepare surveyed plot plan | County requires a registered surveyor’s plot plan for new dwelling permits, including accessory dwellings. |
| 7. Submit building permit package | Use current County requirements and attach required outside-agency approvals. |
12. A Carroll County mailing address does not always mean County zoning controls
Carroll County contains incorporated municipalities with their own land-use rules. The County’s own housing research documents different ADU treatment in places such as Union Bridge, Mount Airy, New Windsor, Sykesville, Hampstead and Manchester.
That means the first question is not simply “Am I in Carroll County?” It is:
Which planning and zoning authority controls this parcel?
Use the Maryland ADU Jurisdiction Finder before relying on this County guide.
13. What remains unresolved after the September 3 hearing
Carroll County’s official zoning-text-amendment page shows the ADU project moving through an April 23 Board presentation, May 6 Planning and Zoning Commission review, July 30 recommended redlined text and the September 3 public hearing. The County Attorney’s recent materials have continued to identify the ADU package as proposed/upcoming rather than as a recorded adopted ordinance.
Until final action is verified, homeowners should not assume the final 2026 ordinance will:
- keep the current 3-acre detached-ADU rule;
- keep subdivision eligibility for detached Residential ADUs;
- keep the existing parking numbers;
- keep the existing 800/1,000-sq-ft local size formulas;
- change owner occupancy;
- change septic/well requirements.
Those are precisely the items to re-check after final adoption.
14. Questions to ask Carroll County before paying for plans
- Which zoning district controls this parcel?
- Is County zoning the correct authority, or is the property inside a municipality?
- Which current attached/detached ADU provision applies?
- Has the September 2026 ADU ordinance been finally adopted and become effective?
- What size rule is the County administering today for this parcel?
- What parking rule is being administered today?
- For a detached ADU, does the current 3-acre or subdivision-eligibility rule apply?
- What side/rear setbacks apply under the current accessory-structure framework?
- What septic/well approval is required before permit issuance?
- What surveyed plot-plan information should the surveyor include?
15. Bottom line
Carroll County already has an ADU framework, but it is in transition. Current rules can be quite restrictive in ways that depend on zoning district: Agricultural/Conservation detached ADUs currently need 3 acres and must stay within 50 feet of the main dwelling, while Residential detached ADUs are tied to subdivision eligibility. Attached Residential ADUs use a different size formula. Additional parking is currently required.
Maryland’s 2025 ADU law now requires Carroll County to reconcile those local standards with the statewide October 1, 2026 framework. The County has completed a September 3 public hearing, but until a final ordinance is verified, the safest planning approach is:
confirm jurisdiction → identify zoning district → use current County code → verify septic/well and survey constraints → monitor final 2026 adoption before paying for a design that depends on proposed changes.
Next step: use the Maryland ADU Property Feasibility Checker, then review the Maryland County & City Rules Tracker and October 1, 2026 ADU Deadline Guide.
Primary sources used for this Carroll County guide
- Carroll County — September 3, 2026 ADU Public Hearing Notice
- Carroll County — Zoning Text Amendments / Accessory Dwelling Units
- Carroll County Code §158.071.02 — Agricultural and Conservation Accessory Uses
- Carroll County Code §158.075.02 — Residential Accessory Uses
- Carroll County Code §155.038 — Accessory Dwelling Unit Subdivision
- Carroll County — New Dwelling / Accessory Dwelling Permit Requirements
- Maryland Department of Planning — HB 1466 ADU FAQ
- Maryland Land Use §4-501 — ADU Definition
- Maryland Land Use §4-504 — Local ADU Requirements
Maryland ADU Guide is maintained by Gijo John as an independent research publication. We prioritize enacted code, official legislative records and current government guidance. Proposed legislation is not treated as law.