1. Site work and grading
A flat, clear backyard and a sloped, wooded one are different projects wearing the same floor plan. Grading, tree removal, retaining walls, and access for equipment all bill before the foundation is poured. Walk the lot first; price second.
2. Septic vs. sewer
On public sewer, you tie in and move on. On septic, the health department must approve your water supply and sewage disposal before any ADU or building permit issues — and adding bedrooms counts as new load on the system. A new or expanded septic system is its own project with its own price, and it can be the single biggest line item on a rural lot. Our ADUs on septic and well guide covers the approval order.
Sources: Charles County Bill 2026-06 / ZTA 25-189 (published draft, §§3.01.700.D, J); Health Department BRF guidance (adding bedrooms = new construction).
3. Utility run distance
Water, sewer, power, and gas must travel from the main house (or the street) to the ADU. A 20-foot trench and a 200-foot trench are different jobs. Detached backyard units pay this; attached units and conversions mostly do not.
4. Finishes
Cabinets, counters, flooring, fixtures, tile — the same choices that swing any remodel's price swing an ADU's. Builder-grade throughout vs. custom everything can move the total by tens of thousands on the same floor plan. Pick your lane early and the budget holds.
5. Size
Bigger costs more overall, less per square foot. And size interacts with fees: Maryland bars impact fees and excise taxes on ADUs under 750 square feet, and Charles County's plan review bills at $0.19 per square foot. See the cost-by-size table.
Sources: Chapter 197 of the 2025 Laws of Maryland (HB 1466/SB 891), §4-506; Charles County FY2027 Fees & Charges.
6. Conversion vs. new build
Reusing a garage or basement skips foundation, framing, and roof — the most expensive phases. New detached construction buys design freedom and a blank slate. The crossover point is the existing structure's condition; our detached vs. conversion comparison shows where each wins.
7. Design complexity
Simple rectangle, simple roof: cheap to engineer and fast to frame. Cantilevers, complex rooflines, and tricky lots need custom engineering and slower labor. Custom design earns its keep on difficult lots and wastes money on easy ones — see custom ADUs.
The one-line version
Land first, structure second, finishes third, floor plan last. Budget in that order and you will not be surprised. 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.
Ready for numbers? Start with what an ADU costs in Maryland, or book a free feasibility study and get your lot's real answer.