What does an ADU cost in Montgomery County?

Why square footage is the wrong starting point

A 700-square-foot detached ADU on a flat lot with public water and sewer at the street is a fundamentally different project from the same 700 square feet on a sloped Potomac lot with a septic system and mature trees. Same size, very different cost. That's why a per-square-foot number is only a rough sanity check — the site conditions and approvals are what actually move a Montgomery County ADU budget.

The budget lines that decide the number

A realistic ADU budget is a stack, not a single figure. Here's what sits on top of the base structure cost:

• Design & feasibility. Architectural drawings, and often a feasibility study before you commit — cheap insurance against buying plans that won't fit your lot.
• Survey & engineering. A current survey, plus structural and sometimes civil engineering, especially for detached builds and grading.
• County & municipal approvals. DHCA Class 3 ADU licensing and DPS permits — and separate review if you're in Rockville, Gaithersburg, or Takoma Park, or in a historic district.
• Site preparation. Clearing, grading, tree protection, retaining walls, and drainage. This is where sloped or wooded lots get expensive fast.
• Utility connections. Water, sewer or septic, electric, and gas. Septic properties (common in Potomac) can face significant added cost or feasibility limits.
• Fire sprinklers. Detached ADUs generally require a residential fire-sprinkler system — plan on roughly $5,000–$15,000+ depending on size and water supply. Homeowners routinely miss this.
• Structure & finishes. The build itself, where your finish level swings the per-square-foot number the most.
• Landscaping & restoration. Putting the yard back together after construction.
• Financing costs. Interest and fees if you're borrowing against equity or using a renovation loan.
• Contingency. A sensible 10–15% buffer for the surprises that older properties reliably produce.

The three big cost drivers in Montgomery County specifically

Sprinklers on detached units. Because detached ADUs generally require sprinklers, a detached build carries a cost floor that a basement conversion doesn't. If budget is tight, an attached or basement ADU is usually the more economical path.

Septic vs. public sewer. Large-lot areas like Potomac often run on septic. Adding a dwelling unit to a septic property can trigger system upgrades or capacity questions that reshape both cost and feasibility — worth checking early.

Setbacks shrinking the buildable size. Detached ADUs face greater setbacks, and on smaller R-60 lots the buildable footprint can shrink to around 576 square feet. A smaller unit isn't always cheaper per foot — fixed costs like design, permits, utilities, and sprinklers don't shrink with it.