The short answer
Published figures from real Maryland-area builders put a new detached ADU at $150,000–$350,000. That is Makara ADU's published range, quoted in The Banner — their figure, not ours. Excell Homes publishes $150,000–$200,000 for a turnkey 600-square-foot detached unit. Garage conversions run less: Foster Remodeling publishes $50,000–$200,000 for conversion-type projects in Northern Virginia. Industry-wide, Powell Custom Homes cites $150–$300 per square foot.
A note on prices: material costs move up and down, and no two lots need the same work. Treat any numbers on this page as starting points, not quotes. Your written estimate after our free lot walk is the real number for your project.
Sources: Makara ADU (quoted in The Banner); Excell Homes (excellhomes.us/adu, published figures); Foster Remodeling (fosterremodeling.com, published figures); Powell Custom Homes (industry-wide figure).
Why the range is so wide
An ADU is not one product. A 400-square-foot garage conversion and a 1,000-square-foot detached cottage share a name and almost nothing else. Three things decide where you land in the range:
1. Type. Conversions reuse a structure you already own, so they start lower. Detached builds start from dirt — foundation, framing, roof, all new — so they start higher. See our detached vs. conversion comparison for the full breakdown.
2. Size. Bigger costs more in total but less per square foot — a kitchen and a bath cost nearly the same whether they serve 400 or 800 square feet. Our cost-by-size table shows how the math works at 400, 600, 800, and 1,000 square feet.
3. Your lot. This is the one articles skip. Septic or sewer? Flat or sloped? How far must water, sewer, and power run? A long utility trench or a new septic system can add tens of thousands before a single wall goes up. What drives ADU cost walks through all seven factors.
What the price includes (and what it does not)
A real ADU price has four buckets: design and engineering (plans the county will accept), permits and fees (the county's cut — see below), construction (labor and materials), and site work (grading, utility runs, and septic if you need it). When you compare builder numbers, ask which buckets are in. A low number that excludes site work is not a low number.
Charles County's own fees are public and modest. Under the FY2027 fee schedule: a building permit application is $120, residential plan review is $0.19 per square foot ($139 minimum), trade permits are $28 each, and inspections run $94–$328. A 1,000-square-foot detached ADU works out to about $190 in plan review plus the $120 application, before trades and inspections. One more state-law break worth knowing: Maryland bars development impact fees and building excise taxes on ADUs under 750 square feet.
Sources: Charles County FY2027 Fees & Charges (charlescountymd.gov); Chapter 197 of the 2025 Laws of Maryland (HB 1466/SB 891), §4-506.
How to get YOUR number
There are two honest ways. The slow way: collect three bids and compare bucket by bucket. The fast way: a free feasibility study — we walk your lot, check zoning and septic, and hand you a written price for your actual property. Either way, never sign from a per-square-foot guess. Your lot is not average, so your price should not be either.
Need to pay for it? See financing an ADU. Want the earning side? See ADU rental income and ROI math.
Check it yourself — or skip the homework
Don't take our word for any of this. The builder figures above are published on the builders' own sites and in The Banner — look them up. Zoning and permit questions: Charles County Planning & Growth Management, 301-645-0692. The state law is Chapter 197 of the 2025 Laws of Maryland (HB 1466/SB 891).
Or skip the homework: call (240) 222-5082 and we'll walk your lot free and give you a written price — before you spend a dollar.