ADU fire sprinkler requirement in Anne Arundel County: the 800 square foot cottage and its 13D system.

Anne Arundel got to detached ADUs earlier than most of Maryland. Bill 6-23 passed the county council in April 2023, and since then a homeowner in Severna Park, Pasadena or Edgewater has been able to put a small separate unit on the lot for a parent, an adult child or a tenant. Three years in, the pattern of the calls is settled. The zoning office says yes, the builder draws an 800 square foot cottage, and then someone at Inspections and Permits asks where the sprinkler plans are. This page is the answer to that question, for this county.

What Anne Arundel allows now

Bill 6-23 allows one ADU per lot with a single family house, inside the house or detached. A detached unit is capped at the smaller of 800 square feet or half the floor area of the main house, and it cannot be a mobile home or a premanufactured house. The council dropped the owner occupancy requirement during the bill’s amendments. The state’s 2025 ADU law sets a 75 percent cap, larger than the county’s 800 square feet; as of November 1, 2026 the county had not raised its number, and the bill in front of the council that fall, Bill 71-26, still carried the 800 square foot definition. Ask the zoning office for the current cap before you draw, because a larger unit may be allowed by the time you file.

Does a detached ADU need fire sprinklers in Anne Arundel County

Yes. The county does not have its own ADU sprinkler rule because it does not need one. The residential code Maryland adopts requires a sprinkler system in every new one and two family dwelling, and Public Safety Article section 12-504 keeps a county from weakening that. A detached cottage is a new dwelling. It gets a full NFPA 13D system, designed from the plans, permitted with the building permit and tested at the final. Our statewide ADU guide covers the attached and basement cases; the short version is that an apartment inside an unsprinklered house is an alteration and stays unsprinklered, while a separate building never is. If the house already has sprinklers, and every house built here since 2015 does, the new unit inside it gets heads too.

What a 13D system looks like on an 800 square foot cottage

Six to ten heads, one riser with a control valve and a drain, CPVC pipe in the ceiling, and a cover plate on every head so the ceiling reads like a ceiling. Closets, small bathrooms and a garage bay with no living space above it can be left out under the standard. The design has to flow its two worst case heads for seven minutes on a one story unit, and that flow number is what the water supply has to meet. Our 13D guide covers the standard room by room; what changes on a cottage is only how much water it asks for.

Public water or a well

North county, Glen Burnie, Severna Park, Arnold and the Annapolis neck, is on public water. The ADU ties into the house service or takes its own tap, and we flow test at the street before design. Annapolis and the older neighborhoods have small meters and long runs, so the flow test decides whether the house line carries both buildings or the meter has to grow. Our public water versus well article shows what a failed test looks like.

South of Route 50 and out toward Davidsonville, Harwood, Lothian and the Pasadena peninsula, most lots are on wells. A well alone does not deliver sprinkler flow, so the cottage gets a 300 gallon tank and a booster pump in conditioned space where the well line enters, sized to the design and priced as its own line. A photo of the pressure tank gauge and the pump tag is enough for us to size it. Our well water guide has the details, including when one tank can serve the house and the ADU.

Two permit offices, one standard

This county has a wrinkle that catches ADU builders every month. Inside the City of Annapolis, the building permit files with the city permit office and the sprinkler plans get their own review from the Annapolis Fire Marshal. Everywhere else in the county, the permit files with Anne Arundel County Inspections and Permits and the county fire marshal reviews the sprinkler set. Same NFPA 13D standard, two offices, two queues, and a plan set addressed to the wrong one sits. The sequence is the same on both tracks: zoning approval for the ADU, then the building permit with the sprinkler drawings and hydraulic calculations under a Maryland State Fire Marshal fire protection license, rough in with the framing, and an acceptance test at the final before the unit gets its occupancy certificate. Our Anne Arundel permit guide walks both tracks.

What it costs

A cottage is priced as new construction, and the fixed parts of the system are a bigger share of a small job, so plan on the top of our published new construction range per sprinklered square foot. A well lot in south county adds the tank and pump package. Both ranges are on the cost page, and our Anne Arundel cost article shows how north and south county differ. The number we send is from the plans and it holds.

When to call us

Once the zoning office says yes and the plan set exists, and before anyone sizes the water line to the cottage. On a well, before the tank location is decided, because the tank goes where the well line comes in and has to fit through the door. Send the plans and the gauge photo, tell us which side of the Annapolis line the lot is on, and we flow test or size the tank and send a number that stands up at plan review.