Maryland's Accessory Dwelling Units Act of 2025 (SB 891 / HB 1466) took effect October 1, 2025 and required every county, municipality, and Baltimore City to adopt a local ADU-authorizing law by October 1, 2026. The state definition allows an ADU up to 75 percent of the size of the primary house.
Last verified: October 18, 2026 against the codified Maryland Land Use Article Sections 4-501 and 4-504 and Real Property Section 2-126 (mgaleg.maryland.gov), the General Assembly records for SB 891 (2025) and HB 1538 (2026).
An accessory dwelling unit, or ADU, is a second, smaller home on a lot that already has a single-family house. Under Maryland law an ADU is a secondary dwelling unit that is not greater than 75 percent of the size of, and subordinate in use to, the primary single-family detached dwelling. This page explains Maryland’s statewide ADU law, the October 1, 2026 local deadline it created, and what the state floor requires.
What does Maryland ADU law allow?
Maryland enacted a statewide ADU mandate in 2025. The Accessory Dwelling Units Act of 2025 was passed as two identical cross-filed bills, SB 891 and HB 1466, and approved by the Governor on April 22, 2025 as Chapters 196 and 197. It took effect October 1, 2025. The General Assembly’s own record for SB 891 lists the Land Use sections it touches as 4-501 through 4-504 (plus 1-401 for charter counties and 10-103 for Baltimore City), along with Real Property sections 2-126 and 11B-111.11.
Maryland is no longer purely local on ADUs. The state now sets a floor that every local ADU ordinance has to meet, while ADUs are still permitted through those local ordinances. Home-rule zoning is not eliminated, it is constrained: the bill text frames the subtitle as a State policy to promote and encourage ADUs, while leaving local zoning authority in place and directing local jurisdictions to adopt policies that further that intent.
| Rule | Maryland standard | Statute | Source |
|---|---|---|---|
| Local adoption deadline | Each legislative body must adopt a local law authorizing ADUs on or before October 1, 2026 | Land Use Section 4-504(a)(1) | Codified Section 4-504 |
| What counts as an ADU | A secondary dwelling unit on the same lot as a primary single-family detached house, detached or attached as an addition | Land Use Section 4-501(b) | Codified Section 4-501 |
| Maximum size | Not greater than 75 percent of the size of the primary single-family detached dwelling, and subordinate in use to it | Land Use Section 4-501(b)(1)(ii) | Codified Section 4-501 |
| Density | The ADU must be excluded from density calculations and from local growth-limiting measures | Land Use Section 4-504(c) | Codified Section 4-504 |
| Setbacks | Side and rear setbacks may not exceed the setbacks that already apply to accessory structures | Land Use Section 4-504(d) | Codified Section 4-504 |
| Parking | A local law may add off-street parking requirements, but only after a parking study, and it must include a waiver process | Land Use Section 4-504(e) | Codified Section 4-504 |
| HOAs and deed restrictions | May not impose an unreasonable limitation on developing or renting out an ADU | Real Property Section 2-126 | Codified Section 2-126 |
What changed: the October 1, 2026 deadline
Section 4-504(a)(1) of the Land Use Article required each Maryland legislative body to adopt, on or before October 1, 2026, a local law authorizing the development of accessory dwelling units in line with the statute. It applies to every county, every municipal corporation, and Baltimore City. That date has now passed.
The deadline is what drove the surge of public interest in Maryland ADU law during 2026, as counties drafted and debated local ordinances to comply. Section 4-504 itself does not spell out a penalty or a fallback state ordinance for a jurisdiction that missed the date, so the practical question for a homeowner is simply whether your county or town has adopted its ordinance yet, and what it says.
How big can an ADU be in Maryland?
Under the state definition in Section 4-501(b), an ADU may not be greater than 75 percent of the size of the primary single-family detached dwelling, and it must be subordinate in use to that primary dwelling. It can be a separate structure or an addition attached to the primary house. Your local ordinance can set its own size rules, but it is writing them inside that state definition.
Does Maryland require owner occupancy?
Not at the state level. Neither the ADU definition in Section 4-501 nor the local-law requirements in Section 4-504 impose an owner-occupancy requirement. What the Act does address on the rental side is private restrictions. Under Real Property Section 2-126, a restriction on use, which includes a deed, a declaration, a covenant and the bylaws or rules of a homeowners association, may not impose an unreasonable limitation on a property owner’s ability to develop an ADU or offer it for rent. An unreasonable limitation includes one that prohibits an ADU, either explicitly or by the effect of the restrictions. Limits on short-term rental of an ADU are not unreasonable under the section, and historic properties on or eligible for the Maryland Register of Historic Properties are excepted.
What can local governments still regulate?
Quite a lot, which is the part most summaries get wrong. Under Section 4-504, a local ADU law must provide for units that meet public health, safety and welfare standards, including building codes and adequate public facilities provisions. It must exclude the ADU from density calculations and growth limits. And it may not set side or rear setbacks stricter than those already applied to accessory structures.
Within those limits, Section 4-504 expressly lets a local law do three things:
- Set its own ADU safety standards.
- Prohibit converting an existing accessory structure into an ADU when the only vehicle access to that structure is from an alley.
- Add off-street parking requirements, as long as the jurisdiction first completes a parking study, weighs factors such as construction cost, curb space, impervious surface and lot size, and provides a waiver process.
That parking rule matters if you have read elsewhere that Maryland bans ADU parking mandates near transit. The enacted, codified Section 4-504 does not say that. Earlier versions of the bill and a 2026 follow-up bill proposed tighter limits, but neither is the law today.
What about individual counties, such as Montgomery County?
Every Maryland county, municipality, and Baltimore City was subject to the October 1, 2026 deadline under Section 4-504. For this page we have not confirmed any individual county’s adopted ordinance against a primary source, including Montgomery County, Prince George’s County, Baltimore County, and Baltimore City, so we do not state their size caps, parking rules or fees here. Check your local planning department for the ordinance that applies to your lot.
How Maryland compares to other states
Maryland now joins a growing group of states that set a statewide floor for ADUs while leaving details to local governments. For comparison, see our Massachusetts ADU law page, which covers that state’s by-right ADU rule, and our Florida ADU law page. If you are weighing a smaller structure rather than a full ADU, our tiny house in a backyard guide explains how those rules differ. You can browse every state and topic in the ADU laws library.
What changed in 2026?
The operative change for 2026 was the local-adoption deadline of October 1, 2026, set by Section 4-504(a)(1). On the legislative side, HB 1538, the Maryland Generational Housing Act of 2026, proposed going further: at least one internal and one external ADU per lot, limits on parking requirements, and limits on utility fees. Its General Assembly record shows a committee hearing on March 13, 2026 and no further action, and the codified Section 4-504 does not contain its changes. It did not become law.
What changes in 2027?
No further statewide Maryland ADU changes are scheduled as of October 18, 2026. The Accessory Dwelling Units Act of 2025 is the current state of the law. A successor to HB 1538 in the 2027 session is the thing to watch, and we will update this page if one is chaptered.
Frequently Asked Questions
Does Maryland allow ADUs?
Yes. Under the Accessory Dwelling Units Act of 2025, which took effect October 1, 2025, every county, municipality and Baltimore City had to adopt a local law authorizing ADUs by October 1, 2026. ADUs are permitted through those local ordinances, which must meet the state floor in Land Use Sections 4-501 through 4-504.
What is the new Maryland ADU law?
The Accessory Dwelling Units Act of 2025, passed as SB 891 and HB 1466 and enacted as Chapters 196 and 197. The Governor approved it on April 22, 2025, and it took effect October 1, 2025.
When did Maryland’s ADU law take effect, and what was the October 1, 2026 deadline?
The Act took effect October 1, 2025. Separately, Land Use Section 4-504(a)(1) required every county, municipality, and Baltimore City to adopt a local ADU-authorizing law on or before October 1, 2026. That deadline has passed, so check whether your jurisdiction has adopted its ordinance.
Does the state law override county zoning?
No. Local zoning authority stays in place. What changed is that each local ADU law has to meet the state floor: the 75 percent size definition, exclusion from density limits, no setbacks stricter than accessory-structure setbacks, and the parking-study and waiver conditions in Section 4-504.
How big can an ADU be in Maryland under state law?
Not greater than 75 percent of the size of the primary single-family detached dwelling, and subordinate in use to it, under Land Use Section 4-501(b). Your local ordinance can add its own size rules within that definition.
Does Maryland require extra parking for an ADU?
It depends on your local law. Section 4-504(e) lets a local government add off-street parking requirements for ADUs, but only after it completes a parking study, and only if the law includes a way to get a waiver.
Can an HOA or deed restriction block an ADU in Maryland?
Generally no. Under Real Property Section 2-126, a deed restriction, covenant or HOA rule may not impose an unreasonable limitation on developing or renting out an ADU, and an outright prohibition counts as unreasonable. Limits on short-term rentals are allowed, and historic properties are excepted.
Sources
- Maryland General Assembly, SB 891 (2025), status and history: https://mgaleg.maryland.gov/mgawebsite/Legislation/Details/SB0891?ys=2025RS
- Maryland General Assembly, HB 1466 (2025), status and history: https://mgaleg.maryland.gov/mgawebsite/Legislation/Details/hb1466?ys=2025RS
- Maryland General Assembly, Chapter 196 (SB 891) chapter text: https://mgaleg.maryland.gov/2025RS/Chapters_noln/CH_196_sb0891t.pdf
- Maryland Code, Land Use Article Section 4-501 (codified): https://mgaleg.maryland.gov/mgawebsite/Laws/StatuteText?article=glu§ion=4-501&enactments=false
- Maryland Code, Land Use Article Section 4-504 (codified): https://mgaleg.maryland.gov/mgawebsite/Laws/StatuteText?article=glu§ion=4-504&enactments=false
- Maryland Code, Real Property Article Section 2-126 (codified): https://mgaleg.maryland.gov/mgawebsite/Laws/StatuteText?article=grp§ion=2-126&enactments=false
- Maryland General Assembly, HB 1538 (2026), status and history: https://mgaleg.maryland.gov/mgawebsite/Legislation/Details/hb1538?ys=2026RS
Disclaimer
This page is general information, not legal advice. Maryland ADU law is implemented through local ordinances adopted around the October 1, 2026 deadline, and details vary by jurisdiction. Confirm the current rules with your local planning department before you build.