Maryland counties are changing their ADU rules in real time ahead of the October 1, 2026 state compliance deadline. This tracker separates rules that are already in force from legacy rules and 2026 ordinances that are still moving through the local legislative process.
Last verified: September 6, 2026. We use county legislation, planning departments, zoning code materials and Maryland state guidance as primary sources. A status marked Transition means we found an active local update but have not verified final enactment. A status marked Legacy framework means older county rules are still published but a complete HB 1466 alignment measure has not yet been verified.
Maryland county ADU status at a glance
| County | Verified status | Size rule / transition | Key local overlays | Official source |
|---|---|---|---|---|
| Howard | Active — aligned CB 3-2026 effective April 8, 2026 | ADU may not exceed 75% of the exterior footprint of the principal dwelling. | Owner must live in the principal dwelling or ADU. Short-term rentals are prohibited. Parking sources conflict: Howard County’s ADU webpage summarizes a three-space rule, but the final amended CB 3-2026 text requires 1.0 additional off-street space per ADU, and Council minutes state the three-space Amendment 10 was not moved. Verify current enforcement with DPZ before relying on the webpage summary. | Howard County ADU portal |
| Montgomery | Transition Existing 2019 framework remains published; ZTA 26-06 is the state-law alignment measure | Legacy detached rule: smallest of 10% of lot area, 50% of principal-dwelling footprint, or 1,200 sq ft. ZTA 26-06 proposes replacing those caps with the state 75% standard. | Existing framework is mostly by right and has transit-related parking relief. Official 2026 materials identify zones, size, density, setbacks and parking as areas needing state-law alignment. | Montgomery Planning ADU page |
| Prince George’s | Transition CB-074-2026 presented and referred July 7, 2026 and reviewed by PHED September 3; companion CB-092-2026 was presented and referred September 1 for non-zoning ADU requirements | Final local size and implementation rules are not yet treated as enacted in this tracker. | County implementation is now split across at least two active bills: CB-074-2026 for zoning and CB-092-2026 for non-zoning requirements such as short-term rentals, rental rules, impervious surface and individual sewage disposal. Neither should be treated as final enacted law yet. | Prince George’s County ADU Task Force |
| Baltimore County | Legacy framework Existing “accessory apartment” rules remain published; no HB 1466 conforming ordinance verified by us yet | Legacy rules include internal units capped at one-third of the dwelling or 2,000 sq ft, whichever is less; detached accessory apartments have separate 800/1,200 sq ft caps depending on lot size. | Owner-occupancy remains part of the published legacy framework. Detached accessory apartments have historically required a Special Hearing and Use Permit. | Baltimore County Council legislation |
| Anne Arundel | Transition Bill 6-23 framework is active; Article 18 comprehensive update is pending 2026 Council review | Current public page still lists detached ADUs as the lesser of 800 sq ft or 50% of the principal dwelling. The 2026 Article 18 draft adds a 75% principal-dwelling floor-area definition, but its staff note says the stricter detached cap can remain. Maryland Department of Planning separately says local jurisdictions may not limit ADU size below the 75% limit, so we treat this point as unresolved pending final local action and legal interpretation. | No additional parking is required under the current public ADU page. ADUs are allowed in residential districts except R22 and land designated RCA in the Chesapeake Bay Critical Area. | Anne Arundel County ADU facts |
Howard County ADU rules — CB 3-2026
Howard County currently provides the clearest major-county example of a completed 2026 alignment. CB 3-2026 was signed in February and became effective April 8, 2026. The county allows attached and detached ADUs and renovation of existing space for an attached ADU in qualifying residential districts.
The county calculates maximum ADU size from the exterior footprint of the principal dwelling, with the ADU limited to 75% of that footprint. Howard County also requires the property owner to reside in either the principal dwelling or the ADU.
CB 3-2026 prohibits short-term rental use of ADUs. Parking requires extra care: Howard County’s current ADU webpage says parking was increased to three spaces with exceptions, but the final amended CB 3-2026 text still shows 1.0 additional off-street space per ADU, and the Council’s February 2, 2026 minutes state that Amendment 10—the proposed three-space change—was not moved. Because the county’s summary page and enacted-text record conflict, homeowners should confirm the currently enforced parking requirement with DPZ before filing. The county also applies lot-coverage and stormwater requirements to new detached ADUs. Read our complete Howard County ADU rules guide. Official Howard County ADU portal.
Montgomery County ADU rules — ZTA 26-06 transition
Montgomery County has had a comparatively broad ADU framework since 2019, but the County Council’s 2026 materials state that several existing provisions are inconsistent with HB 1466. ZTA 26-06 is intended to bring the local code into compliance.
The key distinction is current published rule versus proposed 2026 replacement. Montgomery Planning still publishes the 2019 detached-ADU formula: the smallest of 10% of lot area, 50% of the principal dwelling footprint, or 1,200 square feet. ZTA 26-06 proposes removing that cap and using the state 75% standard instead.
Official 2026 Council material also identifies allowable zones, minimum size, density calculations, setbacks and off-street parking as areas requiring alignment. The Council’s Planning, Housing and Parks Committee reviewed ZTA 26-06 on July 27, 2026. Until final enactment is verified, this tracker labels Montgomery County Transition, not fully aligned. Read our Montgomery County ADU Rules: ZTA 26-06 Transition Guide, then verify the legacy baseline on the official Montgomery Planning ADU page.
Prince George’s County ADU rules — CB-074-2026 transition
Prince George’s County is one of the most important jurisdictions to watch during the final weeks before the state deadline. The County Council presented and referred CB-074-2026 on July 7, 2026. The bill is specifically designed to add ADU definitions, designate areas where ADUs may be allowed and establish local regulations consistent with state law.
The Planning, Housing and Economic Development Committee considered CB-074-2026 on September 3, 2026. Because we have not yet verified final County Council enactment, we do not publish the proposed bill’s details as final homeowner requirements. Read our Prince George’s County ADU transition guide, then check the official Prince George’s County ADU Task Force.
Baltimore County ADU rules — legacy accessory-apartment framework
Baltimore County’s published zoning materials use the term accessory apartment. The legacy framework allows a second living quarter within a principal single-family detached dwelling or within an accessory building on the same owner-occupied lot, subject to detailed conditions.
Published legacy size limits include an internal accessory apartment of no more than one-third of the dwelling’s floor area or 2,000 square feet, whichever is less. For an accessory building, the older framework uses a maximum of 1,200 square feet on a lot of at least one acre and 800 square feet on a smaller lot. Detached units have historically required a Special Hearing and Use Permit.
Those legacy limits should not be assumed to be Baltimore County’s final HB 1466 framework. As of this review, the County Council’s current pending-legislation page did not show an ADU conforming bill that we could verify. We therefore label the county Legacy framework and will update this entry when a 2026 conforming measure is identified. Check Baltimore County pending legislation.
Anne Arundel County ADU rules — Article 18 update
Anne Arundel County already allows ADUs under its 2023 framework. The county’s current public guidance says an ADU may be within, attached to or detached from a single-family detached dwelling, with no minimum lot size and no additional parking requirement.
The current public page still lists a detached ADU size of the lesser of 800 square feet or 50% of the principal dwelling’s floor area. Anne Arundel’s 2026 Article 18 draft adds a definition stating that an ADU may not exceed 75% of the floor area of the principal dwelling, but the same county draft says the stricter detached-ADU cap can remain. Maryland Department of Planning guidance says jurisdictions may not limit ADU size below the 75% limit. Because those official sources are not fully consistent, homeowners should treat the detached-size question as unresolved until final County Council action and legal interpretation. The county says the comprehensive update was introduced in July 2026 and will be heard by the County Council in September and October.
Because the current public ADU page and the pending 2026 update are not yet identical, this tracker labels Anne Arundel Transition. Review the official Article 18 update.
How to use this tracker
- Do not treat a proposed ordinance as final. We distinguish proposals, committee actions and enacted law.
- Check your exact jurisdiction. Municipal zoning may differ from county rules.
- Verify before spending. Confirm the current ordinance with the local planning or permitting office before ordering plans, signing a prefab contract or paying a contractor deposit.
- Use the review date. Maryland is in an unusually fast regulatory transition through October 1, 2026.
Research methodology
This tracker is maintained by Gijo John for Maryland ADU Guide. We prioritize official county legislation, planning and zoning pages, staff reports, adopted code and Maryland state guidance. Third-party articles may help identify an issue to investigate, but they are not used as the final authority for a county status.
For the statewide baseline, see our Maryland ADU Guide 2026. For private-covenant questions, see Maryland ADU HOA Rules. Corrections supported by official sources can be sent to gijo.j@marylandaduguide.com.