Mount Airy MD ADU Rules 2026: 30% Family Apartment Rule, Pre-1951 Accessory Apartments & State-Law Conflict

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Last researched: September 19, 2026. Mount Airy already has zoning pathways that resemble accessory dwelling units, but they come from older code sections written long before Maryland’s 2025 statewide ADU law. The Town currently distinguishes between a “family apartment” and an “accessory apartment”, both handled through the Board of Appeals special-exception process.

Homeowner takeaway: Do not assume Mount Airy’s old family-apartment or accessory-apartment rules are the final 2026 ADU standards. One uses a 30% floor-area cap and two additional parking spaces; the other is limited to conversion of older houses that existed before October 8, 1951 and contain at least 1,800 square feet. Maryland Planning now says covered jurisdictions must allow ADUs under a broader 75% framework. Ask the Town whether Chapter 112 has been amended or is being administered differently for HB 1466 compliance.

Mount Airy ADU planning snapshot

Question What we verified
Current ADU-type pathways Town Code § 112-62F(17) “Family apartment” and § 112-62F(22) “Accessory apartment.”
Family apartment size No more than 30% of the gross floor area of the principal structure.
Family apartment parking Two additional parking spaces.
Family apartment occupancy Owner-occupied principal dwelling; owner residency must continue.
Accessory apartment location Defined as a separate living unit created from existing internal space within a single-family residence.
Accessory apartment building-age rule The structure to be converted must have existed before October 8, 1951 and contain at least 1,800 sq ft of gross floor area.
Accessory apartment quantity One accessory unit, with no minimum floor area, for each 900 sq ft of gross floor area in the existing structure.
Garage/accessory-structure residence Current § 112-34 says any use of a garage or accessory structure as a residence is treated as an accessory apartment and routed through § 112-62F(22).
Accessory-structure setbacks Structures over 100 sq ft generally use 6-ft side and rear setbacks.
Building permits Carroll County processes building permits for the entire incorporated Town, including Mount Airy properties located in Frederick County.
2026 state-law issue Maryland Planning says covered jurisdictions may not limit ADUs below 75% of the primary detached dwelling and must follow the state parking/setback framework.

Mount Airy’s own Master Plan says its current code uses two ADU-type pathways

Mount Airy’s adopted planning materials specifically acknowledge that the Town already has code pathways for ADU-type housing through:

  • the family apartment special exception in § 112-62F(17); and
  • the accessory apartment special exception in § 112-62F(22).

The Master Plan also discusses ADUs as a potential way to provide smaller-scale affordable housing without the same growth impacts as large new subdivisions.

That is useful context, but the two existing code pathways were written decades before Maryland’s current ADU statute and are much narrower than the statewide definition.

The “family apartment” rule uses a 30% size cap

Mount Airy’s family-apartment special exception applies in residential districts and requires the family apartment to be inside an owner-occupied dwelling.

The current code says:

  • only one family apartment may be created per dwelling;
  • no more than 30% of the gross floor area of the principal structure may be used for the apartment;
  • two additional parking spaces must be provided;
  • at least one owner must continue to reside in the principal dwelling; and
  • a separate entrance, if used, must be on the side or rear to preserve the appearance of a single-family home.

The owner-occupancy rule is strict: if owner residency stops for more than 60 consecutive days, the family-apartment use must cease under the existing code.

The 30% cap conflicts with Maryland’s current 75% framework

Maryland’s current ADU definition permits an ADU up to 75% of the size of the primary single-family detached dwelling. Maryland Planning says local governments covered by HB 1466 may not impose a lower local maximum.

For a 2,000-square-foot primary dwelling:

  • Mount Airy’s legacy family-apartment formula gives 600 square feet; and
  • Maryland’s 75% ceiling is 1,500 square feet.

That does not mean every 1,500-square-foot ADU is automatically approvable. It means the legacy 30% family-apartment ceiling should not be treated as the final 2026 ADU maximum without asking the Town how it is implementing HB 1466.

Use our Maryland ADU size guide to understand the state rule, then ask Mount Airy which measurement method currently controls.

The separate “accessory apartment” rule is even narrower

Mount Airy separately defines an accessory apartment as a living unit created from existing internal space within a single-family residence with its own kitchen and bathroom.

Under § 112-62F(22), the existing structure must:

  • be owner-occupied;
  • have existed before October 8, 1951;
  • contain at least 1,800 square feet of gross floor area before conversion; and
  • generally avoid external structural changes except minor appurtenances such as fire escapes or dormer windows.

The section also says one accessory unit, with no minimum floor area, is permitted for each 900 square feet of gross floor area in the existing structure.

That is an unusual legacy rule. A modern 2026 homeowner should not assume that an ADU is only possible in a house built before 1951 simply because the old accessory-apartment section says so.

The parking rules differ between the two legacy pathways

The family-apartment section expressly requires two additional parking spaces.

The accessory-apartment section instead says off-street parking must be provided according to the zoning requirements for each dwelling unit or apartment.

Mount Airy’s general residential parking rule requires three spaces for each residential unit, subject to detailed lot-width and layout provisions.

Maryland’s current ADU law adds another constraint: if a jurisdiction imposes additional off-street parking specifically for ADUs, it must complete the required parking study and provide a waiver process.

Ask the Town which parking standard applies to a new HB 1466 ADU and whether the older two-extra-space rule remains enforceable.

The garage rule creates an important code tension

Mount Airy’s current accessory-structure section says that any use of a garage or accessory structure as a residence is considered an accessory apartment and must follow the special-exception process and standards in § 112-62F(22).

But the Town’s formal definition of “accessory apartment” says it is created from existing internal space within a single-family residence.

Those provisions do not read neatly together. A detached garage is an accessory structure, not internal space within the main residence.

For a garage conversion or detached ADU, do not try to resolve that tension yourself. Ask Mount Airy Planning & Zoning whether the Town now treats a detached structure under a newer ADU interpretation, the old accessory-apartment special exception, or a later HB 1466 amendment.

Current accessory-structure setbacks are 6 ft for structures over 100 sq ft

Mount Airy’s current § 112-34 says accessory structures are not allowed in front yards.

For setbacks:

  • an accessory structure 100 square feet or less must generally be at least 3 feet from side and rear lot lines;
  • an accessory structure over 100 square feet must generally be at least 6 feet from side and rear lot lines.

The same section limits total structural coverage to 30% of the rear-yard area and generally caps an accessory structure at one story / 10 feet, with a roof peak no higher than 15 feet. Garages can have up to two stories but must satisfy additional rules.

Maryland now says local ADU side and rear setbacks may not be more restrictive than existing accessory-structure setbacks. For a detached ADU, Mount Airy’s 6-foot benchmark is therefore important to discuss with Town staff.

Mount Airy spans two counties, but Carroll County processes all Town building permits

Mount Airy is unusual because the municipality extends into both Carroll County and Frederick County.

However, the Town’s current permit guidance states that Carroll County Government processes building permits for the entire incorporated limits of Mount Airy, including properties located in the Frederick County portion of Town.

That means the workflow is different from simply using whichever county appears in the parcel’s mailing or tax information.

Mount Airy zoning still comes first, while Carroll County handles most building-permit processing. The Town retains permits for items such as signs, fences, driveways, banners and zoning certificates.

Large accessory structures also go through Carroll County

Mount Airy’s current accessory-structure permit page says accessory structures over 150 square feet are applied for through Carroll County Permits, and electrical work also requires Carroll County permitting.

For a detached ADU, expect both Town zoning review and County building/trade review unless the Town tells you otherwise.

Water availability is a Mount Airy-specific planning constraint

Mount Airy’s Master Plan repeatedly identifies available water resources as a major constraint on new development. The same housing discussion that mentions ADUs notes that smaller-scale ADUs could potentially add housing without the same water-resource impact as large development projects.

That does not mean every existing lot has automatic utility capacity. Before final design, ask whether the ADU triggers:

  • water-allocation review;
  • a separate meter;
  • connection or capacity charges;
  • service-line upgrades;
  • sewer review; or
  • other utility approvals.

We did not find a separately identified 2026 Mount Airy ADU amendment

For this September 19, 2026 research pass, we checked Mount Airy’s current eCode zoning sections, the Town permit pages, the 2025 Master Plan discussion of ADUs, and 2026 Town Council / Planning Commission materials surfaced through the Town’s agenda center.

We did not find a separately identified 2026 ordinance clearly described as the Town’s final HB 1466 ADU amendment in the materials reviewed.

That does not prove no amendment is pending, newly adopted or not yet indexed. With Maryland’s October 1 deadline approaching, ask the Planning & Zoning Department directly whether Chapter 112 has been amended or whether an ADU ordinance is scheduled for action.

What to ask Mount Airy before paying for ADU plans

  • Has Mount Airy adopted an HB 1466-specific ADU ordinance?
  • Which current section governs a new ADU: family apartment, accessory apartment, a newer ADU section, or another pathway?
  • Does the 30% family-apartment limit still apply?
  • Does the pre-October-8-1951 accessory-apartment rule still apply to new ADUs?
  • How does the Town calculate the state 75% size limit?
  • Are internal, attached and detached ADUs all allowed?
  • Can a detached garage be converted?
  • How does the Town reconcile § 112-34 with the internal-space definition of accessory apartment?
  • What side and rear setbacks apply to a detached ADU?
  • What parking requirement and waiver process currently apply?
  • Has the Town completed the required ADU parking study?
  • Is owner occupancy required?
  • What water and sewer capacity approvals apply?
  • Which Town zoning approvals are needed before the Carroll County building permit?

Mount Airy homeowner checklist

  1. Confirm the parcel is inside Mount Airy municipal limits.
  2. Identify the zoning district.
  3. Ask Town Planning & Zoning for the current ADU ordinance or controlling special-exception section.
  4. Do not assume the old 30% or pre-1951 rules are the final 2026 standards.
  5. Determine whether the project is internal, attached, detached or a garage conversion.
  6. Confirm the legal size and measurement method.
  7. Verify accessory-structure setbacks, height and rear-yard coverage.
  8. Confirm parking and any waiver procedure.
  9. Check water/sewer capacity.
  10. Obtain Town zoning approval.
  11. Use Carroll County for the building and applicable trade permits even if the parcel is on the Frederick County side of Mount Airy.
  12. Do not make a non-refundable construction commitment until the Town confirms the current rules.

Mount Airy MD ADU FAQs

Does Mount Airy allow ADUs?

Mount Airy’s current code has two older ADU-type pathways: “family apartment” and “accessory apartment,” both handled as special exceptions. The Town’s Master Plan expressly describes those as existing ADU-type processes. Ask whether a newer HB 1466 ADU ordinance now supersedes or supplements them.

What is the Mount Airy family-apartment size limit?

The current legacy family-apartment section says no more than 30% of the principal structure’s gross floor area may be used for the apartment. Maryland Planning now says covered jurisdictions may not impose an ADU maximum below 75% of the primary detached dwelling, so the current controlling rule should be confirmed.

What is Mount Airy’s accessory-apartment rule?

The existing accessory-apartment special exception is written for owner-occupied houses that existed before October 8, 1951 and contain at least 1,800 square feet of gross floor area. It is an old conversion rule and should not automatically be treated as Mount Airy’s complete 2026 ADU framework.

Can I convert a detached garage into an ADU?

Mount Airy’s current accessory-structure section says a garage or accessory structure used as a residence is treated as an accessory apartment and routed through the special-exception standards. Because the accessory-apartment definition itself refers to internal space within a single-family residence, ask the Town how it currently handles detached conversions.

What are Mount Airy’s accessory-structure setbacks?

Structures over 100 square feet generally use 6-foot side and rear setbacks; structures of 100 square feet or less use 3 feet. Confirm how the Town applies those standards to a detached ADU under Maryland’s current setback rule.

Which county issues Mount Airy building permits?

Carroll County processes building permits for the entire incorporated Town of Mount Airy, including properties located in the Frederick County portion of the municipality.

Primary sources checked

Maryland ADU Guide is an independent research publication, not the Town of Mount Airy, Carroll County, Frederick County, a law firm, architect or contractor. This page reports Mount Airy’s current legacy family-apartment and accessory-apartment provisions and compares them with Maryland’s current HB 1466 framework. Confirm any 2026 amendment and parcel-specific requirements with the Town before spending money on design or construction.

YOUR NEXT PRACTICAL STEP

What to do next

Ask Mount Airy which ADU pathway is controlling today before choosing a size, garage conversion or parking plan.

  1. Confirm who reviews the property.
  2. Ask Town Planning & Zoning for the current ADU ordinance or special-exception standard.
  3. Gather your zoning district, house age, floor area, survey and utility information.
  4. Continue to the Maryland ADU planning tools.

Planning guidance is not permit approval.