The math, told straight
Start with build cost. Makara ADU, a Maryland builder quoted in The Banner, puts ADU construction at $150K–$350K. Excell Homes, a DMV builder, publishes $150K–$200K turnkey for a 600-square-foot detached unit. Those are their published figures, not our quote.
Excell also publishes the return side of their math: break-even in 5–7 years and a 20-year net of $280K–$380K or more. Again, their figures, from their market — useful as a shape of the investment, not a promise for your lot.
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: Makara ADU (quoted in The Banner); Excell Homes (published).
What about rents in Charles County?
Here is where we stay honest. Excell cites $2,384 per month for a 2-bedroom in Annandale (Apartment List, February 2026) — but Annandale is Northern Virginia, not Charles County. We publish no Charles County rent figure because we have not verified one.
Before you budget, check current rental listings for Waldorf, La Plata, and White Plains yourself. A builder who hands you a rent number without local comps is guessing with your money.
Sources: Excell Homes (published); Apartment List, February 2026 (via Excell Homes).
Long-term rent or short-term rental?
Two separate questions. First, owner-occupancy: we found no owner-occupancy requirement in Charles County's current rules or the published draft rules — the full text was searched and the language is not there. So long-term rental looks allowed, but confirm at permit time.
Second, short-term rental: your HOA may restrict or ban short-term rental of the ADU — state law explicitly lets HOAs do this. And Excell's published jurisdiction guide notes that Montgomery County bans ADU short-term rentals entirely. Charles County's own draft is quieter, but your covenants are the real gate. Read them before you count on Airbnb income.
Sources: Charles County Bill 2026-06 / ZTA 25-189 (published draft); Real Property §2-126 (Chapter 197 of the 2025 Laws of Maryland); Excell Homes (published).
What makes a good rental unit
Durable finishes that survive turnover. A private entrance so tenants feel independent. A full bath, laundry hookups, and sound insulation between the homes. These are design choices, not legal ones — and they decide your vacancy rate.
One legal perk: the county's published draft requires NO off-street parking spaces for ADUs. And state law caps any new parking requirement at one space outside transit areas. Your tenant's car is not your permit problem.
Sources: Charles County Bill 2026-06 / ZTA 25-189 (published draft), amending §297-335; Chapter 197 of the 2025 Laws of Maryland (HB 1466/SB 891), §4-504(D)(4).
The tax question
Adding a second home to your lot can change your property tax assessment. We cannot tell you by how much or in which direction — that is the assessor's call. Talk to your tax professional before you budget, not after the build.
For smaller footprints and the fee math, see our small ADUs page. For the cottage design angle, see backyard cottages. For the full builder's picture, see our detached ADU builders guide and service areas.
Your HOA gets a vote too
Maryland law says your HOA can't unreasonably limit your ADU — nothing that drives the cost way up or blocks the project in practice. But the HOA still has teeth: it can ban short-term rentals of the ADU, and it can treat the ADU as a separate lot for dues and voting. The county's own staff position is "HOA guidelines apply" — the county won't overrule your neighborhood's private covenants. Read your CC&Rs before you pay for plans, and get the HOA's architectural approval in writing early. Sources: Real Property §2-126 (Chapter 197 of the 2025 Laws of Maryland); Charles County staff briefing to the Board of County Commissioners, April 28, 2026.
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.