Carroll County ADU Rules 2026: Ordinance 2026-11

Carroll County adopted Ordinance 2026-11 to establish a shared ADU framework for properties with an existing single-family detached home. The ordinance covers internal conversions, attached additions and detached units. This guide applies to County zoning; incorporated municipalities may administer separate rules.

Adopted ordinance reviewed: County staff supplied the signed text on October 4, 2026. We reviewed Ordinance 2026-11, PDF pages 28–36, including new §158.177.

Effective-date clarification received October 5, 2026: Chris Heyn confirmed that the Commissioners approved the amendment on September 17. He explained that whether the final adoption meeting counts as a hearing for the State’s ten-day waiting period is debatable, leaving September 17 versus September 27 unresolved. The signed copy’s Article III shows September 17; we do not select one date as a definitive legal conclusion. Both dates have passed; confirm the administered ordinance with Zoning Administration before submitting plans.

Confirm your zoning authority with the Maryland ADU Jurisdiction Finder, then use the Property Feasibility Checker to organize your property questions.

Can you build an ADU in Carroll County?

New §158.177 describes an ADU as a permitted use on a lot with an existing single-family detached dwelling, subject to the ordinance’s requirements and other applicable regulations. Permission to build still depends on dimensions, health approvals and property restrictions.

Number and bedrooms: one ADU per lot, with no more than three bedrooms (§158.177(A), (D)).
Size: floor area may not exceed 75% of the primary home’s above-ground living area. Attached and detached ADUs also have a separate 75% footprint limit (§158.177(K)–(L)).
Entrance: direct access from outside is required (§158.177(C)).
Parking: no separate ADU off-street parking requirement (§158.177(F)).
Owner occupancy: the owner must occupy the primary dwelling or the detached dwelling. The wording specifically says “detached”; owners proposing an attached or internal ADU should confirm how the County applies this clause (§158.177(B)).
Preservation land: ADUs are prohibited on lots allowed on Agricultural Preservation properties, including a child’s lot released from an easement (§158.177(Q)).

What Ordinance 2026-11 changes

The adopted text replaces the former district-specific ADU provisions with references to §158.177. The old 1,000-square-foot Agricultural/Conservation cap, 800-square-foot/one-third Residential attached formula, two-bedroom limits, additional ADU parking requirements and three-acre detached-unit condition should not be presented as the adopted ordinance’s requirements.

PDF pages 28–32 show the amended definitions and deleted provisions; pages 33–34 contain the new requirements. A code-library page that still displays the earlier wording needs to be checked against this adopted text.

Agricultural and Conservation properties

The amended §158.071.02 directs ADUs in these districts to §158.177. The former three-acre detached-ADU condition is deleted. That does not make every rural parcel eligible: §158.177(Q) expressly excludes lots allowed on Agricultural Preservation properties, including released child’s lots.

Ask the County to check the parcel’s preservation status and easement documents. On well and septic properties, also obtain the approved system records and reserve-area plan before selecting an ADU location.

Internal, attached and detached ADUs

The amended §§155.005 and 158.002 distinguish three types:

Internal: a conversion entirely within the home’s existing floor area, such as a basement or attic.
Attached: an addition sharing one or more walls with the primary dwelling, or a converted attached structure outside the home’s existing livable floor area.
Detached: a new stand-alone structure or conversion of an existing stand-alone structure.

The adopted Residential accessory-use provisions now refer to §158.177. They delete the earlier condition tying detached-ADU eligibility to subdivision eligibility. An ADU remains accessory to the main home and does not create a separate saleable parcel or additional development rights (§158.177(O)).

Local ordinance and statewide rules

Use the adopted Carroll County text for the parcel-level standards described here and the Maryland ADU law guide for the statewide framework. Adoption is not an independent finding that every provision complies with state law.

For example, §158.177(P) allows the County to establish residential building-permit caps, including ADUs, in areas with specified facility concerns. The text alone does not establish that a cap currently applies to your property. Ask the County about any applicable allocation restriction and its legal basis before paying for design work.

Parking: no separate ADU off-street requirement

Section 158.177(F) removes a separate off-street parking requirement for ADUs. It asks sites to accommodate anticipated parking through existing or additional lawful off-street parking where feasible, and/or available on-street parking. It does not authorize parking areas in required yards.

Do not budget for one space per ADU bedroom or two extra ADU spaces solely because those numbers appear in the older code. Show existing parking and proposed access on your plans, and ask about any other applicable site requirements.

See the Maryland ADU parking guide for other jurisdictions.

Size: floor area and footprint are separate tests

Under §158.177(I), the County starts with the primary dwelling’s SDAT Living Area (Above Grade). An owner may submit approved building plans, a certified survey or other documentation acceptable to the County to demonstrate a different area.

Internal ADU: livable floor area cannot exceed 75% of the primary home’s above-ground living area (§158.177(K)).
Attached or detached ADU: floor area cannot exceed 75% of that above-ground living area, and the ADU footprint cannot exceed 75% of the primary dwelling’s footprint (§158.177(L)).

For example, a home with 2,000 square feet of above-ground living area yields a 1,500-square-foot floor-area limit. If its footprint is 1,000 square feet, an attached or detached ADU also faces a 750-square-foot footprint limit. Passing one test does not establish compliance with the other.

Section 158.177(J) excludes open porches, decks, patios, balconies, exterior stairs and other unenclosed spaces from floor area. The amended building-footprint definition measures exterior walls and supporting columns and excludes listed detached structures and other features. Have the designer label both calculations on the plans.

Use the Maryland ADU size guide to prepare your measurements, then confirm them with County staff.

Placement: the 50-foot rule and neighbouring homes

Section 158.177(M) says an ADU cannot be more than 50 feet from the primary dwelling. It also cannot be closer to an existing primary dwelling on an adjacent lot than it is to the primary dwelling on its own property.

For a detached unit, mark both distances on the site plan. A proposed location 30 feet from your house but 20 feet from the neighbouring primary dwelling would fail the second test, even though it meets the 50-foot maximum.

In Agricultural, Conservation, Residential, Employment Campus and Heritage districts, the underlying zone’s accessory-structure bulk standards apply (§158.177(G)). Commercial and Industrial districts use the R-7,500 accessory-structure standards (§158.177(H)). Obtain the applicable setbacks, height and other dimensions from Zoning Administration.

For a separate storage-shed or detached-garage project, see our Carroll County shed setback guide for rear-yard placement and small-shed permit questions. The shed guide covers ordinary accessory buildings, not approval to use a structure as a dwelling.

Permit approval does not verify private-covenant or HOA compliance. Section 158.177(N) preserves applicable restrictions except where Maryland law provides otherwise.

Septic and well properties 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, then continue through the Property Feasibility Checker.

Carroll County permit path: what to prepare

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	Identify an internal conversion, attached addition or detached unit under the adopted definitions; use §158.177 for the shared requirements.
4. Verify current size/parking rules	Verify the effective revised 2026 text against the recorded County ordinance.
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.

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.

Questions to resolve before paying for plans

Does County zoning control this parcel, and what is its zoning district?
Is the property affected by Agricultural Preservation restrictions, including a released child’s lot?
What SDAT above-ground living area and primary-home footprint will the County accept?
Does the proposed ADU meet both applicable 75% size tests?
For a detached ADU, do the distances to your home and neighbouring primary dwellings meet §158.177(M)?
How does the County apply the owner-occupancy clause to an internal or attached ADU?
Which accessory-structure bulk standards and septic/well approvals apply?
Does an allocation or facility-capacity restriction affect this application?

Ask for written parcel-specific guidance under Ordinance 2026-11 and §158.177. The remaining effective-date discrepancy is identified at the top of this guide; County staff replied on October 5 but did not resolve September 17 versus September 27 as a definitive effective date.

Next step: use the Property Feasibility Checker to assemble your questions and review the County & City Rules Tracker for the correct authority.

Municipality ADU guides in Carroll County

Carroll County’s towns can have separate zoning or implementation details. Check the local guide where applicable:

Next steps for Carroll County homeowners

If this is your jurisdiction, stay focused on this county’s rules rather than comparing unrelated Maryland counties. Once the local rule path is clear, move through the project in this order:

Already ready to speak with a professional? Prepare a Maryland ADU project request →

Primary sources used for this Carroll County guide

Adopted-text evidence: Ordinance 2026-11, signed copy supplied by Chris Heyn, Carroll County, on October 4, 2026; PDF pages 28–36. Definitions: pp. 28–30; district amendments: pp. 30–32; §158.177: pp. 32–34; effective-date entry: p. 35. Chris Heyn replied on October 5: approval on September 17 was confirmed, but he left the ten-day effective-date interpretation unresolved. Signed text and correspondence were rechecked October 10, 2026.

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.

YOUR NEXT PRACTICAL STEP

What to do next

Start by finding the office that reviews your property. A mailing city or a county guide does not establish your parcel’s authority.

Find Who Reviews My Property.
Obtain your parcel identifier, survey, floor plans and utility records.
Ask the reviewing office which current rules, supporting documents and approvals apply.
Continue to your planning workspace.

Enable JavaScript to prepare a downloadable next-step brief. Planning guidance is not permit approval.