Building an ADU (accessory apartment) in Montgomery County, Maryland

Where county rules apply

The county's zoning ordinance doesn't cover every address in Montgomery County. Montgomery Planning says zoning for "Rockville, Gaithersburg, Poolesville, Laytonsville, Barnesville, Brookeville and Washington Grove" is maintained independently and must be verified with those municipalities. If you live in one of them, check that city's or town's ADU rules.

Takoma Park isn't on that list, but the county says projects there need City of Takoma Park approval as well as county permits. DHCA says ADU rental licenses in Takoma Park come from the city. Several other towns and special taxing districts, such as Kensington, Chevy Chase Village and Garrett Park, also require their own approvals; the county's municipalities page lists which ones and in what order.

For Maryland's statewide ADU law, see our Maryland page. [3][4][5]

What changed in October 2026

Maryland's HB 1466 required each local legislative body to adopt a local law authorizing ADUs by October 1, 2026. The County Council adopted Ordinance 20-36 (ZTA 26-06) on September 15, 2026 "to amend the use standards for attached and detached accessory dwelling units consistent with State law." It took effect October 5, 2026. The ordinance:

• redefines an ADU as a second dwelling unit "not greater than 75% of the size of and subordinate in use to the primary single-family detached dwelling unit," matching state law;
• deletes the old size caps (1,200 sq ft for attached ADUs; for detached ADUs, the least of 50% of the house's footprint, 10% of the lot or 1,200 sq ft);
• deletes the extra setback for detached ADUs longer than 24 ft along a lot line, so ADUs follow the setback, height and lot coverage standards for accessory structures in the zone;
• excludes ADUs from density calculations in every zone; and
• eases parking when the house has no on-site parking, and adds a waiver when the lot's size or shape rules out more parking.

The ordinance also allows ADUs as a limited use in every zone except the industrial zones (IL, IM and IH). Before, they were limited to the agricultural, rural and residential detached zones. [1][2][6][7]

What you can build

An ADU has its own facilities for cooking, eating, sanitation and sleeping. It can be:

• attached: part of the house, such as a basement, a converted part of the house or an addition; or
• detached: in a separate accessory building on the same lot, new or converted.

Only one ADU is allowed per lot, and it must have the same street address as the house. An attached ADU needs a separate entrance on the side or rear of the house. A front entrance works only if it existed before May 20, 2013, or if it is a single door shared by the house and the ADU.

DHCA's requirements sheet says an ADU must be on a single-family detached property and is not permitted in townhouses, condominiums, duplexes or mobile homes. [1][2][5]

Size, height and setbacks

Size. An ADU may not be greater than 75% of the size of the primary single-family detached house. Its footprint, together with other buildings on the lot, is also limited by your zone's total lot coverage limit.

Setbacks and height. An ADU must meet the setback, height and building lot coverage standards for an accessory structure in your zone.

Older buildings. A structure built legally before May 31, 2012 can become a detached ADU without regard to setbacks or floor area, if you don't enlarge it, raise it, or add windows on the wall nearest a neighbor.

Note. DHCA's ADU requirements sheet still lists the older size caps and parking rules that Ordinance 20-36 removed. The Department of Permitting Services (DPS) guidelines, revised October 2026, reflect the new rules. If the two disagree for your project, confirm with DHCA. [1][2][5]

Parking

If your lot is within one mile of any Metrorail, Purple Line or MARC Rail station, no extra parking space is required. Elsewhere, you need one on-site space for the ADU in addition to the spaces required for the house. If the house has no on-site parking, the ADU only needs its own one space.

If you can't meet that, a Hearing Examiner at the Office of Zoning and Administrative Hearings (OZAH) can waive it after finding that there is adequate on-street parking, or that the size or shape of the lot effectively prevents more on-site parking. DHCA says you can still apply for the license and file the waiver request with OZAH. [1][2][5]

License, permits and fees

DHCA license first. Every ADU must be licensed by DHCA "regardless of whether they will be rented or not, and prior to being advertised for rent." The application fee is $615: a $260 filing fee, a $230 public notice sign fee and a $125 annual license fee. In the City of Takoma Park, you pay $490, because the rental license comes from the city. You need proof of residency, a signed affidavit, a drawing of the ADU and a drawing of the parking area.

DHCA checks the application for completeness within 5 business days. You must post the public notice sign within 5 days of acceptance and keep it up at least 30 days. Housing code inspectors inspect the property, the Director issues a report, and the license is approved 30 days after the Director's report. Construction can't start before DHCA's preliminary inspection report.

DPS building permit. Submit DHCA's stamped drawings and preliminary inspection report with your building permit application through DPS eServices. A non-refundable filing fee is due when you apply. Depending on the work you may also need electrical, mechanical, fire protection, sediment control, right-of-way and drainage approvals. A drainage plan is required for any detached ADU. Water and gas permits come from WSSC Water. DPS charges a Residential Use and Occupancy Certificate fee for detached ADUs. [2][5]

Fire sprinklers, septic and building code

A detached ADU must have an approved NFPA 13D fire sprinkler system. An attached or internal ADU needs one only if the existing house has sprinklers. Check with WSSC Water whether your water service or meter must change for the sprinklers.

If your property is on a septic system, you must establish a separate septic reserve area for the ADU and install a new septic system under a new permit. Soils testing comes first, and the water-table test generally runs February 1 to April 15. A separate well may also be needed. Contact the DPS Well & Septic Section at 240-777-0311.

DHCA inspects for housing code items such as ceiling height (at least 6 ft 8 in in a basement unit), egress windows in every bedroom, and a full kitchen with a stove at least 24 inches wide. [2][5]

Renting it out and HOAs

The ADU license allows you to rent the unit. No more than two people age 18 or older may live in the ADU, and total occupancy follows the county's housing code. DHCA says either the house or the ADU must be the owner's primary residence.

Short-term rentals. An ADU can't be on a lot where a short-term residential rental exists or is licensed.

HOAs. DHCA asks you to sign an affidavit that an ADU isn't prohibited by your homeowners or community association's governing documents. Maryland's Real Property Article § 2-126 says a restriction on use, including a homeowners association's bylaws or rules, may not impose "an unreasonable limitation" on developing or renting out an ADU, if the owner has the exclusive right to use the property and follows applicable laws. Limits on short-term rental of an ADU don't count as unreasonable, and the section doesn't apply to certain historic properties. DPS adds that private covenants are not enforced by the county. If your association's rules limit ADUs, confirm with DHCA how it handles the affidavit. [1][2][5][8]

Permit steps in Montgomery County

1. Confirm the county is your zoning authority

   If you are in Rockville, Gaithersburg or another municipality with its own zoning, follow its rules. In Takoma Park and some towns, you also need local approvals; see the county's municipalities page. [3][4]

2. Check your lot with DPS

   DPS staff can tell you whether an ADU is allowed on your property and explain zoning, building and fire requirements. Call MC311 at 240-777-0311. [2][10]

3. Apply for the DHCA ADU license

   Apply through DHCA's Class 3 ADU license page with the $615 fee, two drawings, proof of residency and the affidavit. If you lack the required parking, file a waiver with OZAH. [5][9]

4. Post the notice sign and pass DHCA's preliminary inspection

   Post the public notice sign within 5 days of acceptance for at least 30 days. Don't start construction until DHCA's preliminary inspection report is done. [5]

5. Apply for the DPS building permit

   Submit through DPS eServices with DHCA's stamped floor plan and preliminary inspection report, and pay the filing fee. [2]

6. Build and pass inspections

   Request DPS inspections as work progresses. After the final DPS inspection, DHCA inspects the ADU and issues the license. [2]

Who to call

• DHCA Licensing and Registration

  Class 3 ADU licenses, inspections and owner requirements

  240-777-0311

  1401 Rockville Pike, 4th Floor, Rockville, MD 20852

  [5][9]

• Montgomery County Department of Permitting Services

  ADU zoning questions, building permits, inspections, well and septic

  240-777-0311

  2425 Reedie Drive, 7th floor, Wheaton, MD 20902

  [2][10]

• Office of Zoning and Administrative Hearings (OZAH)

  ADU parking waivers and objections to an ADU license decision

  240-777-6660

  100 Maryland Avenue, Room 200, Rockville, MD 20850

  [5][11]

Questions people ask