What "attached" actually means
An attached ADU is new living space joined to your house: a full addition with its own kitchen, bathroom, bedroom, and entrance — or finished space carved out of an existing part of the home, like an oversized laundry wing. Either way, it shares at least one wall with the main house, and it is a legal second dwelling, not just an extra bedroom.
Who it fits best
- A parent who wants closeness without crowding. Twenty steps from your back door, with her own kitchen and a door that locks — independence with family next door.
- Rental income without a second building. One structure, one roof to maintain, two front doors collecting rent.
- Lots where a detached cottage won't fit. Tight setbacks or a small backyard can rule out a separate building while still leaving room to push out one wall.
Why it costs less than a detached cottage
You are not paying for a second foundation, a second roof, or a utility trench across the yard — the big-ticket items are shared with the house you already own. That typically puts an attached ADU between a garage conversion and a detached cottage in cost. But it still costs more than a conversion, because the square footage itself is brand new: foundation work, framing, roofing tie-in, and finishes.
A note on prices: material costs move and no two houses tie in the same way, so treat any numbers you see on this site as starting points, not quotes. Your written estimate after the free walk-through is the real number for your project.
How the county treats attached ADUs
Under Charles County's published draft rules (Bill 2026-06), the house plus the addition together must meet all of your zoning district's setbacks — the addition can't push you closer to the property line than the rules allow. The draft also asks that the ADU's entry and stairs not be visible from the street, and it caps size at 1,000 square feet or 75% of the main house's floor area, whichever is smaller, on lots under 3 acres — with a 2-bedroom maximum and one ADU per lot. (These are the county's published intended rules; final adoption is being confirmed.) A dedicated ADU permit from the county comes before any building permit, and the county's fee schedule starts at a $120 application plus plan review at $0.19 per square foot.
Sources: Charles County Bill 2026-06 / ZTA 25-189 (published draft, §§3.01.700.E–I); Charles County FY2027 Fees & Charges.
What we handle
- Design that matches your house — siding, roofline, and trim drawn to look original, not bolted on.
- The structural tie-in — foundation, framing, and roof connection engineered so the addition moves with the house, not against it.
- Fire separation and sound — code-required separation at the shared wall, plus insulation so both homes stay quiet.
- Permits — the county ADU permit first, then building, electrical, plumbing, and mechanical permits, filed and followed up.
How long it takes
Design usually runs a few weeks and county permit review adds time on the front end — one established DMV builder publishes 30–60 days for permits on its projects. Construction on an attached addition often runs a few months once permits are in hand. The county's review speed is the wildcard we can't control, which is why we file early and follow up often.