The bull case
Income. A rented ADU is monthly cash flow on land you already own. Excell Homes publishes the clearest example in the region: a $150,000–$200,000 turnkey 600-square-foot detached unit, a $2,384/month two-bedroom rent (their Annandale figure), break-even in 5–7 years, 20-year net of $280,000–$380,000+. Those are Excell's published figures for their market — the method transfers; the numbers are yours to fill in.
Family economics. Housing a parent in a backyard cottage instead of a care facility, or an adult child instead of a rental across town, is a return measured in dollars and in life.
Option value. Family now, rental later, home office in between — one building serves three chapters. Few investments reconfigure like that.
Sources: Excell Homes (excellhomes.us/adu, published figures).
The bear case
Concentration. It is a five- or six-figure bet on one asset in one zip code. Diversification this is not.
Illiquidity. You cannot sell half an ADU. The money is in the ground until the property sells or the rents accumulate.
Landlord work. Tenants, toilets, turnover — budget the costs (vacancy, maintenance, insurance, taxes, management) or the math lies. Our ROI math page lists every line item the glossy articles skip.
Regulatory risk. Rules are mid-rewrite across Maryland — today's draft is tomorrow's ordinance. Build to the verified rules and keep margin for change.
Financing cost. Borrowed money has a price; interest stretches every payback calculation.
The questions that decide it
1. What is my all-in cost? Not a per-square-foot guess — a written estimate on your lot. Get one free.
2. What is realistic rent, here? Three current listings near you, take the lowest. Not a national article's number.
3. What is my alternative? The money could index-fund instead. The ADU wins when the local math beats the market math after costs — or when the family need has no market substitute.
4. Can I carry the downside? Vacancy, a big repair, a rate rise — if the project only works in the sunny case, it does not work.
5. Am I allowed? Zoning, septic, HOA — the unglamorous gates. Check your county's rules before the spreadsheet.
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 we will not do: invent appreciation statistics or ROI guarantees. Anyone who does is selling, not advising.