ADU rules in Frederick County

In force

Section 1-19-8.212 allows a limited accessory dwelling unit of up to 1,000 square feet in the RC, A, R1, R3, R5, R8, R12, R16, VC and MXD zones by affidavit. An ADU over 1,000 square feet goes to the Board of Appeals as a special exception.

The Residential Building Permit Information sheet (revised September 1, 2025, fees only) lists four ADU types: within a single family dwelling, an addition, within an accessory structure, and a stand alone separate accessory structure.

The same sheet says a sprinkler fire suppression system application will be required for all accessory dwelling units within an accessory structure or built as an accessory structure on a single family lot.

Read in the Frederick County Residential Building Permit Information sheet (revised September 1, 2025) and the affidavit text of section 1-19-8.212, October 31 and November 1, 2026.

• Size: a limited ADU is 1,000 square feet or less, approved by affidavit. Larger units need a special exception from the Board of Appeals.
• One ADU per lot. The owner lives in the principal dwelling or in the ADU and files an annual statement with the Zoning Administrator.
• Parking: at least one additional space for the ADU; on street parking counts.
• Placement: not in the required front yard. An ADU in or as an accessory structure also meets the accessory structure limits in 1-19-8.240: footprint no more than half the principal dwelling footprint or 600 square feet, whichever is greater, with an R-1 exception up to 2,000 square feet on a lot of 20,000 square feet or more.
• Who files: a Registered Homebuilder for a stand alone accessory structure (or the Affirmation of Landowner form when the owner will live in it); an MHIC contractor for the other types when the owner will not live in the ADU.
• No short term rental of the ADU in the nature of an extended stay hotel or Airbnb.

1. 1

   Zoning

   Zoning review against 1-19-8.212: the 1,000 square foot line, one per lot, owner residence, the extra parking space, and the accessory structure footprint limits. Under 1,000 square feet you file the Affidavit for Accessory Dwelling Units Not Exceeding 1000 Square Feet with the application; over it, the Board of Appeals first.

2. 2

   Building permit

   A residential building permit through the Division of Permits and Inspections, with a plot plan showing property lines, the existing dwelling, the proposed construction, parking and the well and septic area. Fees are due at application, except impact fees and tap fees. The Certificate of Occupancy closes it out.

3. 3

   Fire protection permit

   A sprinkler fire suppression system application for every ADU within or built as an accessory structure, and for any ADU added to a house that already has sprinklers.

4. 4

   Health (well and septic lots)

   Well and septic lots, most of the county outside the municipalities, need Environmental Health to confirm the well and the septic system carry a second kitchen and bath. Moving a system more than five feet from the foundation takes a septic repair permit ($60), and a missed inspection carries a $75 reinspection fee.

5. 5

   Utilities

   Public water lots: a tap or a new service sized for sprinkler flow on a detached unit. Well lots: a storage tank and booster pump at the unit. Electric is a sub panel or a second Potomac Edison meter.

The whole list, statewide, in the order a homeowner meets it:

No builder and no county page publishes a Frederick County specific ADU price. The ranges below are Maryland wide, from one named builder page. The 1,000 square foot affidavit line keeps most detached units mid range; a well lot pushes it back up with the tank, pump and health review.

Source for the table:

County fees

County permit fees on top of construction. The current Fee Schedule (effective July 1, 2026) is a scanned table on the Permits and Inspections page, so we quote only the lines on the permit information sheet:

• Fees are due at the time of application, with the exception of impact fees and tap fees.
• Filing fees are non refundable; the fee to extend an application or permit is the minimum permit fee; a revision fee applies to every resubmission.
• Health Department: a $75 reinspection fee when a well or septic inspection cannot be completed, and a $60 septic repair permit when the system moves more than five feet from the foundation.
• Impact fees and tap fees are billed separately, and the state law waives the impact fee on an ADU under 750 square feet.

Source for the fees:

Sprinkler cost

A detached unit adds the NFPA 13D sprinkler system. On public water that runs about $1.35 to $1.60 per sprinklered square foot (Maryland Fire Sprinklers cost guide, marylandfiresprinklers.com/cost/). A well lot adds a storage tank and pump, priced on the same guide.

Statewide ranges and how the types compare:

Frederick County puts it on the permit sheet: a sprinkler fire suppression system application is required for all accessory dwelling units within an accessory structure or built as an accessory structure on a single family lot. The county cannot waive the state rule (Public Safety Article 12-504), and on a well lot the system needs its own tank and pump. Our sister company, Maryland Fire Sprinklers, designs, permits and installs them.