How small is legal
Smaller than most people think. Maryland state law sets no minimum ADU size at all. And Charles County's published draft deleted the old 300-square-foot floor — the minimum is now whatever the building code requires. A compact, well-drawn 400-square-foot cottage is legal.
Sources: Chapter 197 of the 2025 Laws of Maryland (HB 1466/SB 891), §§4-501, 4-504(C)(1); Charles County Bill 2026-06 / ZTA 25-189 (published draft), §§3.01.700.F, G.
Why small is smart
- Lower total price. Less foundation, less roof, less everything — the fastest way to a smaller number.
- Fits tight lots. A 400-square-foot footprint tucks into side yards and small backyards where a big cottage cannot.
- Skips impact fees. ADUs under 750 square feet pay no development impact fee and no building excise tax under state law.
- Simpler everything. Smaller footprint, simpler foundation questions, easier to heat and cool.
Sources: Chapter 197 of the 2025 Laws of Maryland (HB 1466/SB 891), §4-506.
Layouts that work
At 400–600 square feet you have two honest choices. A studio keeps everything open — one great room with a sleeping alcove — and feels bigger than it is. A one-bedroom gives a real door between sleep and life, which renters and in-laws both appreciate.
Either way, the tricks are the same: storage walls instead of closets, a kitchen that fits real cooking in eight feet, big windows for light, and the outdoors as your extra room — a porch or patio doubles the living space for free. A small footprint also makes a great home office. See the charm-forward take at our backyard cottages page.
Two bedrooms is the cap
Under the county's published draft rules, an ADU holds at most 2 bedrooms — and never more bedrooms than the main house has. One ADU per lot. Plan the bedroom count early; it shapes the septic questions too, since the health department treats added bedrooms as added load.
Sources: Charles County Bill 2026-06 / ZTA 25-189 (published draft), §§3.01.700.F, G.
What a small ADU costs
Powell Custom Homes publishes $150–$300 per square foot as an industry-typical range — their figure, not our quote. Here is the honest catch: kitchens and bathrooms cost nearly the same whether the home is 400 or 1,000 square feet, so the per-foot price on a small ADU looks high. Judge it by the total, not the per-foot math.
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 (published, industry-wide).
For the full builder's picture, see our detached ADU builders guide and our service areas.
If your neighborhood has an HOA, bring the CC&Rs to the first visit — the county's staff position is "HOA guidelines apply," so the HOA's architectural rules still matter. Get their approval in writing early.
Check it yourself — or skip the homework
Don't take our word for any of this. 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. The county's published draft rules are Bill 2026-06 / ZTA 25-189 ("Flexibility for Accessory Dwelling Units"); 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 tell you what your county allows — before you spend a dollar.