ADUs in Montgomery County: Rules, Costs & ROI
Accessory dwelling units — ADUs, in-law suites, basement apartments — are one of the fastest-growing requests I get. With Montgomery County's rules loosening in recent years, more homeowners are adding a legal second unit for rental income or aging parents. Here's what you need to know before you start.
What Counts as an ADU
An ADU is a complete, independent living space on the same lot as your main home — its own kitchen, bathroom, and entrance. In Montgomery County they generally come in three forms:
- Interior/basement conversion: turning existing space into a unit — the most cost-effective option.
- Attached addition: a new unit built onto the house.
- Detached ADU: a standalone structure or garage conversion in the yard.
The Rules You Need to Know
Montgomery County allows ADUs in most residential zones, but there are conditions: a licensing process through the Department of Permitting Services, size caps, parking requirements, and owner-occupancy rules (you generally have to live on the property). Setbacks and lot size affect whether a detached unit is feasible. The rules change periodically, so the first thing I do is confirm current requirements for your specific zone before we design anything.
What an ADU Costs in 2025
- Basement/interior conversion: roughly $60,000–$120,000, depending on whether plumbing for a kitchen and bath already exists nearby.
- Attached or detached new build: typically $150,000–$300,000+ with foundation, utilities, and full finishes.
The biggest cost drivers are running new plumbing and electrical service, egress requirements, and site work for detached units. I scope all of it during the estimate.
The ROI: Why Owners Build Them
In the Bethesda and greater Montgomery County rental market, a legal ADU can command meaningful monthly rent, which often covers the financing on the build. Beyond income, a permitted ADU adds resale value because buyers see flexible use — rental, multigenerational living, or a separate home office. The key word is permitted: an unpermitted unit can't be advertised legally and becomes a liability at sale, which is exactly why I build them by the book.
Thinking About an ADU in Montgomery County?
I'll confirm what's allowed on your lot, handle the licensing and permits, and build a legal unit that adds real value. Call or text me at (240) 387-8186 for a free consultation.