Montgomery County already allows accessory dwelling units, but its rules are being rewritten to comply with Maryland’s 2025 statewide ADU law. Zoning Text Amendment ZTA 26-06 would change the county’s size limits, eligible zones, setbacks, density treatment and parking standards before the state compliance deadline of October 1, 2026.
The key issue for homeowners is timing: Montgomery County’s 2019 framework is still the published baseline, while ZTA 26-06 remains in the legislative process. This guide separates the rules in force now from the 2026 changes proposed but not yet verified as enacted. For the statewide comparison, see our Maryland County ADU Rules Tracker.
Last verified: September 6, 2026. This page is informational and is not legal, zoning, architectural, engineering or permitting advice.
Montgomery County ADU status at a glance
| Question | Current published rule | ZTA 26-06 proposal |
|---|---|---|
| Status | 2019 ADU framework remains published and operational. | Introduced April 21, 2026; public hearing held June 9; PHP Committee reviewed it July 27. We have not verified final enactment as of September 6. |
| Detached ADU maximum size | Smallest of 10% of lot area, 50% of principal-dwelling footprint, or 1,200 sq ft. | Remove the three-part cap and use the state definition: ADU not greater than 75% of the size of the primary single-family detached dwelling. |
| Attached ADU maximum size | Generally 1,200 sq ft; an ADU using only a basement or cellar may be allowed up to that basement/cellar area under the current published requirements. | Remove the separate attached-size standard and use the same 75% definition for attached and detached ADUs. |
| Parking | One additional on-site space for the ADU outside the transit exemption; no additional ADU parking within one mile of Metrorail, Purple Line or MARC. A Hearing Examiner waiver is available where adequate on-street parking exists. | Retain the one-space baseline and transit exemption, reduce the burden where no on-site parking currently exists, and expand waiver eligibility where lot size or shape effectively prevents additional parking. |
| Approval path | Montgomery Planning describes ADU development as mostly by right through DHCA and DPS, but some cases can require a waiver or conditional-use review. | Proposal expands ADUs as a limited use to zones where a single-family detached dwelling is allowed, excluding Industrial zones. |
| Short-term rentals | An ADU cannot operate as a short-term residential rental, and the current framework bars other short-term rental use on a site that includes an ADU. | The proposed text retains the rule that an ADU cannot be on a lot where a Short-Term Residential Rental use exists or is licensed. |
Where ZTA 26-06 stands now
Montgomery County introduced ZTA 26-06 on April 21, 2026 at the request of the Planning Board. The Council held a public hearing on June 9, and the Planning, Housing and Parks Committee reviewed the amendment on July 27.
The County Council’s ZTA index still does not show an enacted ordinance for ZTA 26-06, and we have not located a final adoption record as of this review. We therefore classify Montgomery County as Transition, not fully aligned. Homeowners should not design a project around a proposed provision until the Council has adopted the final text and the effective date is known.
Official status sources: Montgomery County Council Zoning Text Amendments and ZTA 26-06 introduction packet and planning analysis.
The biggest change: Montgomery’s ADU size limits
Current detached-ADU formula
Under Montgomery County’s published 2019 framework, the maximum gross floor area of a detached ADU is the smallest of three numbers:
- 10% of the lot area
- 50% of the footprint of the principal dwelling
- 1,200 square feet
That formula can make the practical limit much smaller than 1,200 square feet. For example, a principal dwelling with a 1,400-square-foot footprint would produce a 700-square-foot limit under the 50% test, even if the lot-area test would allow more.
Montgomery Planning’s current ADU page continues to publish this three-part rule. Review the current Montgomery Planning ADU guidance.
What ZTA 26-06 would replace it with
ZTA 26-06 proposes deleting the separate attached and detached size caps and adopting Maryland’s statewide definition: an ADU must be not greater than 75% of the size of the primary single-family detached dwelling.
This is a major expansion for many detached ADU projects because it removes the legacy 10%-of-lot, 50%-of-footprint and 1,200-square-foot tests. It also removes Montgomery’s separate 1,200-square-foot attached-ADU cap in favor of the same statewide definition.
An unresolved question: what does “75% of the size” mean?
ZTA 26-06 copies the state-law phrase “75% of the size”, but the proposed zoning text does not itself create a detailed measurement formula such as exterior footprint, above-grade finished area or total gross floor area.
That matters in Montgomery County because the legacy detached rule explicitly uses the principal dwelling’s footprint, while the statewide language uses the broader word size. Until Montgomery County publishes a final measurement method or implementation guidance, homeowners should not assume that the Howard County footprint method applies here.
Parking: current transit relief and the proposed 2026 changes
Montgomery County’s current parking framework requires one additional on-site parking space for an ADU in locations outside the transit exemption. Montgomery Planning notes that a detached house typically already requires two spaces, so a property outside the exemption commonly needs capacity for three spaces in total.
The ADU-specific parking space is not required within one mile of a Metrorail, Purple Line or MARC Rail station. The existing system also allows a Hearing Examiner to waive the ADU parking requirement where adequate on-street parking is available.
For ZTA 26-06, Montgomery Planning completed the parking study required by state law and recommended keeping the one-space baseline and one-mile transit exemption. The proposal also addresses two edge cases: where the principal dwelling currently has no on-site parking, only the ADU’s one space would be required rather than forcing creation of two spaces; and the Hearing Examiner could also waive parking where the size or shape of the lot effectively prevents additional on-site parking.
Is a Montgomery County ADU by right?
Montgomery Planning describes ADU development as mostly a by-right process facilitated through the Department of Housing and Community Affairs and Department of Permitting Services. That is more accurate than saying every ADU is automatically administrative.
Current county procedures still include cases that can reach the Office of Zoning and Administrative Hearings, including parking waivers, objections to DHCA decisions and certain conditional-use situations. ZTA 26-06 proposes broadening attached and detached ADUs as a limited use to all non-Industrial zones where a single-family detached dwelling is permitted.
See Montgomery County OZAH’s current ADU review and waiver procedures.
Owner occupancy remains part of the current licensing framework
Montgomery County’s current ADU requirements state that either the principal dwelling or the ADU must be the owner’s primary residence. This allows an owner to live in the ADU and rent the main house, or live in the main house and rent the ADU, subject to the applicable licensing rules.
The practical result is that the existing ADU rental-license framework is not designed for an absentee owner renting both dwellings simultaneously. Always confirm the current DHCA licensing requirements when your project involves rental income.
Montgomery County DHCA — current Class 3 ADU requirements.
Short-term rentals and Airbnb
A Montgomery County ADU cannot be operated as a short-term residential rental. County DHCA guidance expressly states that an ADU is not considered a short-term residential rental and cannot be operated as one.
The restriction is broader at the zoning level: the current ADU framework prohibits other rental uses such as Airbnb on a property that includes an ADU, and the proposed ZTA 26-06 text retains the rule that an ADU cannot be located on a lot where a Short-Term Residential Rental use exists or is licensed.
Montgomery County DHCA — Short-Term Rental FAQ.
What homeowners should do during the transition
- Identify the rule set you are designing under. If you are applying before ZTA 26-06 takes effect, confirm whether the current 2019 standards still control your application.
- Calculate the legacy detached limit. For a detached ADU, run all three current tests: 10% of lot area, 50% of principal-dwelling footprint and 1,200 sq ft.
- Check transit proximity. Confirm whether the property is within one mile of Metrorail, Purple Line or MARC because that changes the current ADU parking requirement.
- Do not assume a 75% measurement method. Ask DPS or DHCA how the principal dwelling will be measured once the new ordinance is effective.
- Confirm owner-occupancy and licensing. Rental plans should be tested against DHCA’s current license requirements.
- Re-check ZTA 26-06 immediately before spending. The final adopted text can differ from the introduced draft or committee version.
Primary sources used for this guide
- Montgomery County Council — Zoning Text Amendment status
- Montgomery County — ZTA 26-06 introduction packet, planning analysis and parking study
- Montgomery Planning — current ADU rules
- DHCA — Class 3 ADU requirements
- DHCA — Short-Term Rental FAQ
For Maryland’s statewide baseline, read our Maryland ADU Guide 2026. For county-by-county implementation status, use the Maryland County ADU Rules Tracker. Maryland ADU Guide is maintained by Gijo John; source-backed corrections can be sent to gijo.j@marylandaduguide.com.