Montgomery County ADU permits and rules, in plain English.

Two departments, one project

The single most useful thing to understand up front: a Montgomery County ADU runs through two county departments, not one.

• DHCA — Department of Housing and Community Affairs. This is where you apply for the Class 3 Accessory Dwelling Unit license. ADUs must be licensed whether or not you intend to rent them, and before being advertised for rent.
• DPS — Department of Permitting Services. This is where the traditional building permits come from — structural, electrical, plumbing, mechanical — with inspections through construction.

Most projects involve a DHCA review and preliminary inspection alongside the DPS permit process. Your builder or architect usually coordinates both, but you should know both exist so nothing surprises you.

Who's eligible

• One ADU per lot. One accessory dwelling unit is allowed on a single-family property, subordinate to the principal dwelling.
• Owner-occupancy is required. The property must be your primary residence — you may live in either the main house or the ADU. This rules out pure investment properties.
• No short-term rentals. Airbnb-style short-term rentals are not permitted on a property that includes an ADU.

Detached ADU size and placement

For a detached ADU, the maximum size is the smallest of three limits:

• 10% of the lot size,
• 50% of the principal dwelling's footprint, or
• 1,200 square feet.

In practice, setbacks are frequently the real limiter, not the size formula. Detached ADUs face the greater side and rear setbacks required for accessory structures, and once a unit exceeds certain length, width, or height thresholds the required setbacks increase. On smaller R-60 lots — much of lower Montgomery County — that can constrain a detached unit to roughly 576 square feet. Height limits, maximum lot coverage, and stormwater rules for accessory structures all still apply.

Attached & basement ADUs

An attached or basement ADU can generally run up to 1,200 square feet — and may be larger if the existing basement already exceeds that. These are often the most cost-efficient path because the shell already exists, but they still need code-compliant egress, fire separation, and a DHCA license. A note that catches people out: a full second kitchen (a range/cooktop) in a basement gets flagged at DPS and won't be approved without the DHCA certification. A wet bar or kitchenette — sink, fridge, no range — is treated differently.

Garage conversions

Converting an existing, legally built garage into an ADU is expressly allowed and is often faster and cheaper than a new detached build — when access, parking, and utilities cooperate. It still runs the same DHCA license + DPS permit path, and the same owner-occupancy and size rules apply to the converted space.

Sprinklers, parking, and the details that add cost

• Fire sprinklers. Detached ADUs generally require a residential fire-sprinkler system — a real budget line, typically $5,000–$15,000+.
• Parking. Generally one additional off-street space is required, unless the property is within one mile of a Metrorail, Purple Line, or MARC station, where that requirement may be reduced or waived. Farther out, more spaces can be required.
• Stormwater & grading. Detached units trigger stormwater management and often grading work.

Municipal differences — check this first

This is the part homeowners most often get wrong: Montgomery County rules are not automatically the whole story for your property. Several incorporated municipalities administer parts of the process themselves.

• Takoma Park issues its own rental license, separate from the county.
• Rockville and Gaithersburg administer certain planning, permitting, and zoning matters under their own codes.
• Historic districts require a Historic Area Work Permit before other approvals.

The practical takeaway: confirm which jurisdiction governs your specific address before you design anything. It's the first question our project review asks, for exactly this reason.

The permit path, step by step

1. 1
   Pre-screen the property.

   Confirm jurisdiction, zoning, lot shape, footprint, access, setbacks, parking, HOA issues, and whether detached, attached, or conversion is realistic.

2. 2
   Start DHCA review.

   Begin the Class 3 ADU license process with DHCA, typically including a preliminary inspection.

3. 3
   Submit plans to DPS.

   Architectural, site, and structural drawings move through DPS discipline review before permit issuance.

4. 4
   Build, inspect, close out.

   Construction inspections and final approvals follow. Detached builds run longer because of utilities, sprinklers, grading, and site work.