A published example to learn from
Excell Homes, a DMV-area ADU builder, publishes the clearest rental math in the region. Their example: a $150,000–$200,000 turnkey 600-square-foot detached ADU, rented as a two-bedroom at $2,384/month (their cited Annandale figure via Apartment List, February 2026). Their published conclusion: break-even in 5–7 years, with a 20-year net of $280,000–$380,000+. Those are Excell's figures for their market — study the method, not the numbers.
Sources: Excell Homes (excellhomes.us/adu, published figures).
The formula (run it on your market)
Step 1 — Monthly rent. Look up actual two-bedroom rents near you — not Zillow's optimism, real listings. Be conservative; use the lower third of what you see.
Step 2 — Annual income. Monthly rent × 12, then subtract vacancy (one empty month a year is honest math) and landlord costs: insurance bump, maintenance reserve, property tax increase, and property management if you will not self-manage.
Step 3 — Net annual income ÷ total project cost = your simple return.
Step 4 — Break-even. Total project cost ÷ net annual income = years to pay back.
Example with Excell's inputs: $2,384 × 12 = $28,608 gross. Subtract a month of vacancy ($2,384) and, say, $3,000 in landlord costs: roughly $23,200 net. Against a $175,000 build, that is about a 13% simple return and a 7.5-year payback — inside their published 5–7 year band once you tune the inputs. Your inputs will differ. That is the point.
The costs the glossy articles skip
Vacancy — budget it; every rental sits empty sometimes. Maintenance — a second dwelling doubles the things that break. Insurance — tell your carrier; landlord coverage costs more than homeowner's. Property tax — an ADU raises assessed value. Management — 8–10% of rent if you hire it, your weekends if you do not. Financing cost — if you borrowed, the interest is part of the payback 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.
What about Charles County rents?
We do not publish Charles County rent figures on this site — we have not verified a dataset we trust, and inventing one would be worse than showing none. Pull three current two-bedroom listings within a few miles of your home and use the lowest. That is a better input than any article's table.
For the building side, see rental-income ADUs and cost by size. For how to pay for it, see financing an ADU. For the rules on renting it out, see renting your ADU.