What this checker does
Pick your county, answer six quick questions about your lot, and the checker applies your county's published ADU rules — size caps, bedroom limits, owner-occupancy, HOA and septic flags — and hands you a plain-English verdict: what you can likely build, the key limits, and the permit steps in order.
The rules behind it
Charles County (enacted): up to 1,000 sq ft or 75% of the main home on lots under 3 acres; 75% with no cap on 3+ acres; max 2 bedrooms; 24 ft height; no parking required; no owner-occupancy rule. Full text: Charles County ADU rules.
St. Mary's County (existing by-right law): 300–900 sq ft, max 40% of the main home; owner-occupancy required.
Calvert County (proposed): draft rules propose a 75% relative-size cap; final vote not yet published.
Prince George's County (in committee): bill proposes up to 75% of the main home, owner-occupancy, 4-person max, no short-term rentals. Full text: Prince George's County ADU rules.
Montgomery County (current law + pending changes): attached up to 1,200 sq ft; detached limited by footprint, lot, and 1,200 sq ft cap; owner-occupancy currently required. Full text: Montgomery County ADU rules.
Not an official answer — and that is the point
No website can give you a binding determination; only your county's permit office can. What the checker does is better than a guess: it reads the actual published rules for you and tells you where you stand before you call — so your one call to the county is short and specific. Every pending or draft rule is labeled. When the checker is done, it gives you the county's real phone number and email next to your results.