Accessory dwelling units and fire sprinklers in Maryland: which ADUs need them.

Accessory dwelling units used to be a Montgomery County conversation. Now we get the call from Howard, Anne Arundel, Frederick and Baltimore County too: a homeowner wants a backyard cottage for a parent, a basement apartment for rent, or a room over the garage for an adult child, and somewhere in the permit process the word sprinklers comes up. The answer depends almost entirely on one thing. Is the ADU a new building, or is it space inside a house that already exists?

The detached ADU is a new house

Maryland has required sprinklers in every new one and two family home since 2015, statewide, with no county opt out. A detached ADU, whether it is a cottage on a slab or a garage with an apartment above it, is a new one family dwelling under the residential code. It gets a full NFPA 13D system, designed and permitted the same way a new house does, and the building permit will not close until the sprinkler final passes. Our 13D requirements guide covers what the standard asks for room by room.

Small buildings are easier to sprinkler than large ones in one way and harder in another. Fewer heads and short pipe runs keep the piping simple. But every system carries the same fixed pieces, a riser, a control valve, a backflow assembly where the water utility wants one, a permit and two inspections, and on a 600 or 800 square foot footprint those fixed pieces are a bigger share of the bill. Our published range for new construction is $1.35 to $1.60 per sprinklered square foot, and a small detached ADU should expect the top of that range. We quote each one from the plans.

Where the water comes from

This is the part that decides the budget on a backyard ADU. The system has to be able to run its design heads for a set time, and under 13D that is 10 minutes of stored water, or 7 minutes for a one story dwelling under 2,000 square feet, which describes almost every ADU built in Maryland.

• On public water. The ADU usually taps the main house service or gets its own tap. Either way we flow test the supply before design, because a long run of small pipe from the street to the back of a deep lot can lose enough pressure to change the head layout. Our article on low water pressure explains what a failed flow test looks like and how it gets fixed.
• On a well. A well alone rarely delivers the flow a sprinkler system needs, so the ADU gets the same stored water package a well fed house does: a tank and a pump sized for the design, adding $3,000 to $6,000 to the system price. Our well water guide covers sizing. On some lots one tank can serve both the main house and the ADU if the main house was already sprinklered off a tank; we check that before quoting a second one.

The attached ADU: it depends on the house

A basement apartment, a garage conversion or a suite added onto the side of the house is not a new building. It is an alteration or an addition to one that exists, and the residential code treats those differently. The model code exempts alterations and additions from the sprinkler rule when the existing house has no sprinklers. Maryland counties then layer their own rules on top, and this is where the answer moves.

• The house already has sprinklers. Coverage has to extend into the new unit. A finished basement apartment in a 2016 house means heads in every new room, permitted and inspected, exactly as our guides on finishing a basement in Montgomery and Charles County describe. A garage that becomes living space loses its garage exemption the day it gets drywall and a bed; see our piece on attics and garages.
• The house has no sprinklers and the ADU is inside the existing walls. In most of the counties we work in, converting a basement in a 1985 colonial into an apartment does not by itself force sprinklers into the whole house. It does bring the other life safety rules with it: a code egress window or door from the sleeping room, interconnected smoke alarms in both units, and fire separation between them.
• The house has no sprinklers and the ADU is an addition. Now the county addition rules apply. Howard County requires sprinklers when an addition reaches 50 percent of the existing gross floor area, Anne Arundel and Montgomery use their own 50 percent tests, and Baltimore County publishes no number and decides at plan review. Our home additions guide has the county by county detail. A large addition that crosses the line can trigger sprinklers for the addition, and in some counties the whole house.

Where sprinklers buy you something back

An ADU inside the main house makes the building a two family dwelling for code purposes, and the code wants a fire rated separation between the two units. The residential code allows that separation to drop from one hour to a half hour when the building is sprinklered throughout under 13D. On a basement apartment that can mean a single layer of drywall on the ceiling instead of two, and simpler detailing where ducts and pipes pass through. It does not pay for the system on its own, but it narrows the gap, and it is the kind of trade the plan reviewer expects you to know about.

Sprinklers also settle the question of how the ADU gets insured and, later, sold. A permitted 13D system with a passed final is a document, and a rental unit with a document is easier to license, easier to insure and easier to disclose than one without. Our article on unpermitted sprinkler work shows what happens at resale when that document is missing.

Zoning first, sprinklers second

None of this matters until the county says an ADU is allowed on your lot. Each Maryland county sets its own rules on where ADUs can go, how large they can be, whether the owner has to live on the property and whether the unit can be rented. Get that answer from the zoning office before you pay anyone for sprinkler design. Once the zoning letter is in hand, bring us the plans. We tell you which of the three situations above you are in, flow test the supply, and give you a number that holds.

The short version: a backyard cottage gets sprinklers, full stop. A basement apartment gets them when the house already has them or when the county says the project is big enough to count as new. Everything else is a permit question, and it is cheaper to ask it before the drywall goes up.