Building an ADU in Maryland

What state law does

Maryland passed HB 1466 in 2025. It took effect October 1, 2025. It says "it is the policy of the State to promote and encourage the creation of accessory dwelling units" on land with a single-family detached house as the main home.

The core rule: "On or before October 1, 2026, each legislative body shall adopt a local law authorizing the development of accessory dwelling units." The Maryland Department of Planning says this covers counties and municipalities with planning and zoning authority. The law names charter counties and Baltimore City among the places it applies.

The law "applies only to the development of accessory dwelling units on land with a single-family detached dwelling unit as the primary dwelling unit."

If your county or city already had an ADU law that meets every requirement, the Department of Planning says it does not need to change it. [1][2][4][5][6][7][8][9]

What counts as an ADU

State law defines an ADU as a secondary dwelling unit on the same lot as a single-family detached home. It must be "not greater than 75% of the size of" the main house and "subordinate in use" to it. It can be a separate structure or an addition to the house.

A dwelling unit means complete living facilities for at least one person, with at least sanitation, cooking, eating and sleeping. The Department of Planning says local governments may also allow ADUs that are fully inside the existing house.

The law doesn't say how to measure the house for the 75% limit. The Department of Planning recommends using the finished living area above grade. It also says local governments "may not limit the size of ADUs to anything below the 75% limit." Ask your local planning office how it measures. [5][10]

Is an ADU allowed by right?

Not necessarily. The Department of Planning says HB 1466 does not require by-right approval. A local government may allow ADUs by right, or it may require a conditional use or special exception to confirm its requirements are met. Local laws may be more generous than the state minimum, for example with by-right approval or lower impact fees.

So the process in your county or city depends on the local ADU law it adopted. Check with your local planning and zoning office. [5]

What your local law must and may include

A local ADU law must:

• provide for ADUs that meet public health, safety and welfare standards, including building codes and adequate public facilities rules;
• leave ADUs out of density calculations and any growth limits on the lot;
• not set side or rear setbacks for an ADU stricter than the existing setbacks for accessory structures.

A local law may:

• set safety standards for ADUs;
• ban converting an accessory structure (such as a garage) into an ADU if the only vehicle access to it is from an alley;
• add off-street parking requirements, but only after the local government completes a parking study. Those requirements must weigh the cost of building parking, on-street curb space, added paved surface and stormwater, and lot size, and must come with a waiver process.

The Department of Planning notes that a jurisdiction that does not require off-street parking for ADUs does not need a parking study. [2][5]

HOAs, deeds and covenants

If you have the exclusive right to use your property and follow all applicable laws, a "restriction on use" cannot impose "an unreasonable limitation" on your ability to build or rent out an ADU. That covers covenants and conditions in deeds, declarations, contracts, HOA bylaws or rules, security instruments and similar documents.

An unreasonable limitation includes one that prohibits an ADU, either outright or by the effect of the restrictions. It does not include a limit on short-term rental of an ADU. The rule does not apply to historic property listed in, or found eligible for, the Maryland Register of Historic Properties. The Department of Planning's FAQ describes this exemption as covering the National Register of Historic Places instead, but the statute names the Maryland Register. If your home may be historic, confirm with your local planning office.

Your HOA's governing body may treat an ADU as a separate lot for voting on HOA matters and for levying assessments. The Department of Planning suggests HOAs consult their legal counsel about whether their bylaws comply. [3][5][11][12]

Garage conversions

State law counts a separate structure as an ADU, and the Department of Planning's list of ADU types includes converted stand-alone accessory structures. One exception: a local law may ban converting an accessory structure into an ADU if the only vehicle access to it is from an alley. Side and rear setbacks for an ADU cannot be stricter than the existing accessory structure setbacks.

See our garage conversion guide for the general steps. [2][4][10]

Fire sprinklers and building codes

The Department of Planning says state law requires residential fire sprinklers in all new single- and two-family homes, which would include newly built ADUs. Local building codes or ADU laws may have their own sprinkler rules.

To learn how sprinkler rules apply to your project, contact the local office that runs the building code or the Office of the State Fire Marshal. [5]

Renting it out

HOA rules and deed covenants cannot unreasonably limit your ability to offer an ADU for rent. But they may limit short-term rentals of an ADU. The Department of Planning says HB 1466 "does not alter a local government's powers to regulate short-term rentals," so check your county or city rules too. [3][5]

Finding your local rules

The October 1, 2026 deadline for local ADU laws has now passed. Ask your county or city planning and zoning office for its current ADU law and process. The Department of Planning's ADU page links its HB 1466 FAQ, a model local ADU ordinance, and a 2024 inventory of local ADU ordinances.

A 2026 bill to require at least one internal and one external ADU on qualifying land, HB 1538, shows no action after its House hearing; its status reads "In the House - Hearing 3/13." Our ADU permit process guide covers the general steps. [2][4][5][13]

Maryland laws to know

• HB 1466 / SB 891 (2025), Chapters 197 and 196

  Cross-filed bills, both approved April 22, 2025 and effective October 1, 2025, that created the state ADU law and the HOA and covenant limits. [9][14][15]

• Land Use Article, Sections 4-501 to 4-504

  Defines an ADU and requires each local legislative body to adopt a local ADU law by October 1, 2026, with limits on density, setbacks and parking. [2][10]

• Real Property Article, Section 2-126

  Bars deeds, HOA rules and other covenants from unreasonably limiting your ability to build or rent an ADU. [3]

• Real Property Article, Sections 11B-111.11 and 11B-117

  Let an HOA's governing body treat an ADU as a separate lot for voting and for assessments. [11][12]

Questions people ask