What an ADU costs in Maryland
Nobody publishes a Maryland county by county ADU price, and we will not invent one. What exists is a published statewide range by unit type from one Maryland builder, the county permit fee schedules, and the sprinkler cost guide from our sister company. Those three are on this page, each with its source named.
The real number comes from a builder who has walked your lot. The form at the bottom gets you one.
Published Maryland ranges
| Type | Published range |
|---|---|
| Detached new build | $120,000 to $300,000 and up |
| Attached addition | $100,000 to $250,000 |
| Basement conversion | $80,000 to $200,000 |
| Garage conversion | $75,000 to $150,000 |
Source for the table:
Clarksville Construction Services, "How much does it cost to build an ADU in Maryland"Why the spread is so wide: a detached unit is a whole small house, with a foundation, a roof, its own utility connections and a fire sprinkler system. A basement conversion reuses the shell and the roof, so the money goes to egress, plumbing and finishes. Size inside the county cap moves the number more than anything else.
Permit fees on top of construction
Two counties publish fee schedules a homeowner can read straight off the county site. The others set theirs at application; the county page on this site says what we found.
Howard County
County permit fees on top of construction, from the FY2027 Building Permit Fee Schedule (effective July 1, 2026):
- New residential construction filing fee: $0.25 per new gross square foot.
- Additions to residential construction: $50 filing fee.
- Alterations and repairs to a single family dwelling: $25 filing fee, then $50 for a single room or area up to 200 square feet or $100 over 200 square feet.
- A 10 percent technology fee on every permit. Excise tax and the school facilities surcharge are separate schedules on the same county page.
Anne Arundel County
County permit fees on top of construction, from the Inspections and Permits fee page (effective October 1, 2024):
- Building permit: $86 minimum, $242 at an estimated cost of $25,000, then $242 plus 1 percent of the estimated cost above $25,000.
- A $43 application fee on every building permit, non refundable.
- Plumbing permit for a new detached single family dwelling: $173. Electrical permit, 200 amps or less: $147.
- Additions and alterations: electrical $95 without a service change; plumbing $51 for the first fixture and $8 for each additional fixture.
State law (Chapter 196 of 2025) bars an impact fee on a unit under 750 square feet. School surcharges and excise taxes are separate county schedules; ask the permit office for yours before you budget.
Add this for a detached unit
A detached ADU is a new dwelling under the state building code and gets an NFPA 13D sprinkler system in every county. On public water the system runs about $1.35 to $1.60 per sprinklered square foot on the Maryland Fire Sprinklers cost guide. On a well, add a storage tank and booster pump at the unit, priced on the same guide.
An attached, basement or garage unit inside a house that has no sprinklers does not carry this line.
Who does the sprinkler work
Maryland Fire Sprinklers is our sister company. It designs, permits and installs NFPA 13D systems for ADUs across Maryland. The builder you are matched with handles the house, and the sprinkler side goes to them.
The rule behind the line:
ADU fire sprinklers in MarylandHow to read a builder's number
- Ask for the construction price and the permit fees on separate lines. The county fee is public; the construction price is the builder’s.
- On a detached unit, ask whether the sprinkler system is in the number or carried as its own line. Our sister company prices it either way.
- On a well lot, ask about the health review, the sprinkler tank and pump, and whether the septic system carries a second kitchen and bath.
- Design and engineering, utility taps and the electric service are the lines that get left off a first number most often.
Which unit type fits the lot and the budget:
Detached vs attached ADUCost and fees by county
Each county page carries these ranges next to the county fee schedule and the local rule.
A real number from a licensed builder
Send the address and the kind of unit. We check the county rule and the sprinkler question, then connect you with a licensed Maryland builder who prices off your lot. We do not build, and we do not quote. The builder does.
Sources read for this page
- Clarksville Construction Services, "How much does it cost to build an ADU in Maryland"
- Howard County FY2027 Building Permit Fee Schedule
- Anne Arundel County Inspections and Permits, permit fees
- Chapter 196 of 2025 (SB 891, cross filed HB 1466), Maryland General Assembly
- Maryland Fire Sprinklers cost guide