The size table
These bands come from an industry-wide figure of $150–$300 per square foot (Powell Custom Homes, industry-typical), applied to common ADU sizes. They are planning math, not quotes — your lot decides the real number.
400 sq ft (studio or one-bed): roughly $90,000–$120,000. Expect the high end of per-square-foot cost — the kitchen and bath do not shrink with the floor plan.
600 sq ft (one-bed or compact two-bed): roughly $120,000–$180,000. Excell Homes publishes $150,000–$200,000 for a turnkey 600-square-foot detached unit — their figure lands inside this band.
800 sq ft (comfortable two-bed): roughly $150,000–$240,000.
1,000 sq ft (full two-bed, the Charles County draft maximum on smaller lots): roughly $180,000–$300,000. Makara ADU's published $150,000–$350,000 range for a detached ADU brackets this end.
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: Powell Custom Homes ($150–$300/sq ft, industry-typical); Excell Homes (excellhomes.us/adu, published figures); Makara ADU (quoted in The Banner).
The line items people forget
Per-square-foot math covers the building. It does not cover the county fees: $120 permit application, plan review at $0.19 per square foot ($139 minimum), $28 per trade permit, $94–$328 per inspection (Charles County FY2027 schedule). It does not cover utility runs — trenching water, sewer, and power from the main house to a backyard unit. And it does not cover septic: if you are on well and septic, the health department must approve your system for the added bedrooms before any permit issues, and a new or expanded system is its own project with its own price.
One state-law break: Maryland bars development impact fees and building excise taxes on ADUs under 750 square feet — so a 600-square-foot unit skips a fee category the 1,000-square-foot unit pays.
Sources: Charles County FY2027 Fees & Charges (charlescountymd.gov); Chapter 197 of the 2025 Laws of Maryland (HB 1466/SB 891), §4-506; Charles County Bill 2026-06 / ZTA 25-189 (published draft, health-department sign-off).
Size limits in Charles County
Size is not just a budget choice — the county caps it. Under Charles County's published draft rules (not yet confirmed as final law): on lots under 3 acres, an ADU may be up to 1,000 square feet or 75% of the main home's floor area, whichever is smaller; on lots of 3 acres or more, up to 75% with no absolute cap. Maximum 2 bedrooms, one ADU per lot. State law independently caps every ADU at 75% of the primary dwelling's size.
Sources: Charles County Bill 2026-06 / ZTA 25-189 (published draft, §§3.01.700.F, G); Chapter 197 of the 2025 Laws of Maryland (HB 1466/SB 891), §§4-501, 4-504(C)(1).
Which size is right for you?
Match size to the job. A studio or one-bed (400–600 sq ft) suits a single renter, a home office with a guest room, or a tight budget. A two-bed (800–1,000 sq ft) suits aging parents who need space, a small family tenant, or strong rental income. Bigger is not better — bigger is just more to heat, clean, and pay for. See ADU for family vs. rental income to match the size to the mission, and small ADUs for the compact end done well.
Check it yourself — or skip the homework
Zoning and permit questions: Charles County Planning & Growth Management, 301-645-0692. Well and septic questions: Health Department, Division of Environmental Health, 301-609-6751.
Or skip the homework: call (240) 222-5082 — we will walk your lot, size the ADU honestly, and hand you a written price.