Detached, attached or basement ADU: which one fits your lot
The three kinds of ADU are not three prices for the same thing. In Maryland they land on different sides of the fire sprinkler rule, meet different permits, and cost very different money. The sprinkler line alone decides a lot of budgets.
Here is how they compare on the six things that matter, then the county pages where the cap is set.
Detached gets sprinklers, a conversion does not
A detached ADU is a new one family dwelling under the Maryland building code (COMAR 09.12.51.04, IRC R313.2) and gets an NFPA 13D sprinkler system in every county. An attached addition, a basement or a garage conversion inside a house that has no sprinklers is an addition or alteration, and the state code does not force a system on it. The only common exceptions are a county addition test and a house that already has sprinklers.
The full sprinkler rule, with the sourcesSix things that differ
Fire sprinklers
- Detached
- Yes, a full NFPA 13D system, in every Maryland county. The state building code treats the cottage as a new one family dwelling and the county cannot waive it.
- Attached
- No, if the house has no sprinklers and the addition stays under the county addition test (Howard and Anne Arundel use 50 percent of the house). A sprinklered house puts heads in the new space.
- Basement
- No, inside an unsprinklered house. It is an alteration under the state code.
Size cap
- Detached
- The state ceiling is 75 percent of the main house. Howard, Charles and the state law use that number; Anne Arundel caps at the lesser of 800 square feet or half the house.
- Attached
- Same cap, measured on the new space. An addition that is large against the house can trip the county sprinkler test before it trips the cap.
- Basement
- Rarely the limit. The basement is already inside the footprint; ceiling height and a way out decide it.
Permits
- Detached
- Zoning, a new residential construction building permit, a fire protection permit for the sprinklers, health review on a well or septic lot, then utility taps or a new service.
- Attached
- Zoning, an addition permit, health review on a well or septic lot, and electric or plumbing permits for the new fixtures.
- Basement
- Zoning, an alteration permit, egress and ceiling height at inspection, plumbing and electric for the kitchen and bath.
Published cost
- Detached
- $120,000 to $300,000 and up on the Maryland range, plus the sprinkler line at about $1.35 to $1.60 per square foot on public water, more on a well.
- Attached
- $100,000 to $250,000 on the same range. No sprinkler line in an unsprinklered house.
- Basement
- $80,000 to $200,000 on the same range. A garage conversion is listed at $75,000 to $150,000.
Privacy and use
- Detached
- Its own front door, its own yard line, no shared wall. The one that works best as a rental or for a parent who wants a real separate home.
- Attached
- Shares a wall and usually the water and sewer line. Close enough for care, separate enough for a kitchen of its own.
- Basement
- Under the same roof. Light, noise and a second exit are the questions, and a walkout grade solves most of them.
Utilities
- Detached
- A tap off the house line or a new service, sized for sprinkler flow. On a well, a storage tank and booster pump at the unit. A sub panel or a second meter for electric.
- Attached
- Usually extends the house water, sewer and electric. A sub panel if the house panel is full.
- Basement
- Extends the house systems. An ejector pump if the drain sits below the sewer line.
Where the ranges come from:
ADU cost in MarylandThree questions that usually settle it
- 1
Who lives in it and for how long?
A parent for the next ten years and a tenant for the next twenty want a detached unit. An adult kid for three years is a basement.
- 2
Is the lot on a well?
A well adds the health review and a sprinkler tank and pump to a detached unit. On a well lot a basement or attached unit skips both.
- 3
How big is the house?
The cap is a share of the main house. A small house with a big yard still gets a small cottage; an attached suite on a small house trips the addition test sooner.
The permits each type meets, in order:
ADU permit checklistThe cap in your county
Each county page says which types its bill allows and the size cap it sets.
A builder who has done all three
Send the address and which type you are leaning toward, or say you are not sure. We check the county rule and the sprinkler question, then connect you with a licensed Maryland builder who already does ADUs there.