ADU rules in Montgomery County

In force since 2019, by right

ZTA 19-01 made accessory dwelling units a by right use in 2019. The Department of Permitting Services publishes Guidelines for Accessory Dwelling Units, revised December 11, 2025.

ZTA 26-06 and Bill 41-26 were pending before the county council on October 31, 2026.

The DPS guidelines say a detached ADU is permitted the same way as a new one family dwelling, and an addition or conversion ADU the same way as an addition.

Read in the DPS Guidelines for Accessory Dwelling Units (revised December 11, 2025), October 31 and November 1, 2026.

• By right: no special exception or hearing for a conforming ADU.
• The zoning ordinance limits the size of the ADU and where its entrance door can go; DPS zoning review checks both. Ask the builder for the current size limit against your lot and house.
• Owner occupied: the DPS guidelines define the ADU as subordinate to the owner occupied main one family dwelling on the lot.
• The ADU is licensed through the Department of Housing and Community Affairs accessory dwelling program after the permits close out.
• The state law behind it (Chapter 196 of 2025) sets 75 percent of the main house as the ceiling a county cannot go below.

1. 1

   Zoning

   DPS zoning review against the ADU standards (size, entrance location, setbacks). The guidelines point to a separate ADU zoning requirements sheet and to DPS staff for a pre application talk.

2. 2

   Building permit

   A DPS residential building permit. A detached ADU files like a new one family dwelling with full plans; an addition or conversion files like an addition. A non refundable filing fee is paid at application, and a Residential Use and Occupancy Certificate fee is charged for a detached ADU.

3. 3

   Fire protection permit

   A detached ADU gets a DPS fire protection permit for its 13D sprinkler system. An attached or basement unit needs one only when the house is already sprinklered.

4. 4

   Health (well and septic lots)

   On a septic lot the Well and Septic Section requires a separate septic reserve area and a new septic system permit for the ADU, after soils testing and an engineered plan; a separate well may be needed case by case. The building permit is released only after the septic permit issues.

5. 5

   Utilities

   WSSC water and gas permits are separate from the DPS permits, and WSSC checks the pipe size and meter for sprinkler flow on a detached unit. A right of way permit applies at 1,000 square feet of new floor area, and a sediment control permit at 5,000 square feet of disturbance.

The whole list, statewide, in the order a homeowner meets it:

No builder and no county page publishes a Montgomery County specific ADU price. The ranges below are Maryland wide, from one named builder page. A septic lot adds the new septic system and soils testing on top; ask the builder for a Montgomery number off your lot and plans.

Source for the table:

County fees

County permit fees come from the DPS residential building permit fee schedule the ADU guidelines point to. We did not quote dollar lines from it here:

• A non refundable filing fee is due at application, in person or online.
• A Residential Use and Occupancy Certificate fee is charged for a detached ADU.
• Electrical, fire protection, mechanical, right of way and drainage permits each carry their own fee when they apply.
• WSSC water and gas permits are billed by WSSC, not the county.

Source for the fees:

Sprinkler cost

A detached unit adds the NFPA 13D sprinkler system. On public water that runs about $1.35 to $1.60 per sprinklered square foot (Maryland Fire Sprinklers cost guide, marylandfiresprinklers.com/cost/). A well lot adds a storage tank and pump, priced on the same guide.

Statewide ranges and how the types compare:

Montgomery County writes the rule down: a detached ADU must be supplied with an approved 13D sprinkler system and DPS issues a fire protection permit for it. An attached or included ADU gets one only if the house already has sprinklers. The county cannot waive the state rule (Public Safety Article 12-504). Our sister company, Maryland Fire Sprinklers, designs, permits and installs them.