Maryland ADU Guide 2026: HB 1466 Rules, Size, Setbacks & Local Requirements

Maryland’s ADU rules changed statewide on October 1, 2025. House Bill 1466, now Chapter 197 of the 2025 Laws of Maryland, established a statewide policy supporting accessory dwelling units and requires counties and municipalities with planning and zoning authority to adopt local ADU laws by October 1, 2026.

The important point for homeowners is that Maryland now sets a statewide floor, but your exact approval path still depends on the local jurisdiction. This guide separates what the state law requires from what counties and municipalities may still control. For current local implementation, use our Maryland County ADU Rules Tracker.

Last reviewed: September 6, 2026. This page is informational and is not legal, zoning, architectural, engineering, or permitting advice.

Quick answer: what HB 1466 changed

Question	Maryland statewide rule	What still varies locally
Are ADUs allowed?	Local governments with zoning authority must adopt a law authorizing ADUs by October 1, 2026.	Approval procedure, safety standards and implementation details.
How large can an ADU be?	The state definition allows an ADU up to 75% of the size of the primary single-family detached dwelling.	Measurement methods and project-specific code limits may still matter.
Must approval be by right?	No.	A locality may use by-right approval, conditional use or special exception.
Can a locality require larger side or rear setbacks?	Not beyond its existing accessory-structure side and rear setbacks.	The underlying accessory-structure setback standard itself remains local.
Can extra parking be required?	Only after a parking study; any additional parking rule must include a waiver process.	The locality determines whether it adopts an additional parking requirement after the study.
Can an HOA ban an ADU?	Private restrictions may not impose an unreasonable limitation that effectively prevents development or rental of an ADU.	HOA rules may still regulate matters that do not cross that statutory line, and short-term-rental restrictions remain possible.

What counts as an ADU in Maryland?

Maryland defines an accessory dwelling unit as a secondary dwelling unit located on the same lot, parcel or tract as a primary single-family detached dwelling. The ADU may be attached to the house or located in a separate structure.

The Maryland Department of Planning also notes that jurisdictions may permit ADUs located entirely within the existing home, even though the statute does not separately define that configuration.

Official source: Maryland Department of Planning — HB 1466 ADU FAQ.

The 75% ADU size rule

Under HB 1466, an ADU may be up to 75% of the size of the primary single-family detached dwelling. Maryland Department of Planning guidance states that because local jurisdictions must authorize ADUs consistent with the state definition, a jurisdiction may not impose an ADU size cap below that 75% threshold.

The statute does not specify exactly how the primary dwelling should be measured for this calculation. Maryland Department of Planning recommends using the square footage of finished living area above grade. That recommendation is guidance rather than a substitute for the local ordinance or permit office’s measurement rules.

Use our Maryland 75% ADU size calculator and measurement guide to compare the statewide finished-living-area recommendation with Howard County’s exterior-footprint method.

Local approval can still vary

HB 1466 does not force every Maryland jurisdiction to approve ADUs by right. A county or municipality may authorize ADUs through a by-right process, conditional use, or special exception.

That distinction matters because a homeowner in one jurisdiction may have an administrative permit path while a homeowner elsewhere may face an additional review step. Before paying for design work, confirm the current local ordinance and the approval path that applies to your property.

Setbacks: a statewide limit on local restrictions

A local ADU law may not establish side or rear setback requirements that exceed the jurisdiction’s existing setback requirements for accessory structures.

This does not create one universal Maryland setback number. It means the relevant local accessory-structure standard becomes the ceiling for side and rear ADU setbacks under the state framework.

Parking: study first, waiver required

A jurisdiction cannot simply add extra off-street parking requirements for ADUs without first completing a parking study. Maryland’s guidance says that study must consider factors including the cost of constructing parking, available curb space, added impervious surface and differences in lot size.

If a local law does impose additional ADU parking requirements, it must also include a waiver process. A jurisdiction that does not require additional off-street parking for ADUs does not need to complete the study.

Utilities, septic, water and public-safety constraints still matter

HB 1466 does not erase building-code, public-health or infrastructure constraints. Local ADU laws must still provide for construction that meets relevant building codes and adequate public facilities requirements.

The statute also recognizes situations where new dwelling units are otherwise prohibited because of limitations involving safe drinking water, sewage disposal, fire, flood or landslide risk. For properties on private well or septic systems, capacity should be checked early.

Use our Maryland ADU Septic & Well Readiness Guide to check capacity, reserve-area, perc-test and Health Department review questions before paying for plans.

Can a Maryland HOA block an ADU?

HB 1466 reaches beyond zoning. It also amended Maryland real-property law so that deeds, declarations, contracts, HOA bylaws and similar private instruments may not impose an unreasonable limitation on a qualifying property owner’s ability to develop an ADU or offer it for rent.

Maryland Department of Planning explains that an unreasonable limitation includes a restriction that prohibits development either explicitly or by its effect. The chaptered law also makes clear that a restriction on the short-term rental of an ADU is not treated as an unreasonable limitation for this purpose.

The law separately authorizes an HOA governing body to treat an ADU as a separate lot for purposes of HOA voting and assessments. That does not mean the ADU becomes a legally subdivided parcel; it is an HOA governance and assessment rule.

We cover this issue in detail in our dedicated Maryland ADU HOA guide.

Historic properties are a special case

The real-property restriction provisions include an exception for certain historic properties. Maryland Department of Planning advises homeowners and local governments to treat historic-property questions separately and to consult the applicable historic-preservation requirements.

What homeowners should verify before spending money

Jurisdiction: Confirm whether your property is governed by the county, a municipality, or another zoning authority.
Current ADU ordinance: Check whether the jurisdiction has already adopted its HB 1466-compliant rules.
Approval process: Determine whether the ADU is by right, conditional use, or special exception.
Primary-home size: Establish the measurement the local authority will use for the 75% calculation.
Accessory-structure setbacks: These are critical because HB 1466 ties side and rear ADU limits to them.
Water and sewer: Verify public utility availability or private well/septic capacity.
Parking: Check whether the local ordinance imposes any additional requirement and, if so, how its waiver works.
HOA documents: Review declarations, covenants and bylaws, especially if the property is association-governed.
Historic status: Confirm whether the property is subject to historic-preservation review.

The October 1, 2026 deadline

The statewide policy has been effective since October 1, 2025, but local governments with planning and zoning authority have until October 1, 2026 to adopt compliant local laws authorizing ADU development.

That means Maryland is still in an implementation period. A county guide written six months ago may already be outdated, and a jurisdiction with an older ADU ordinance may still be revising it. Our local guides will therefore show a review date and link to the underlying local source rather than treating local rules as static.

Plan the budget and financing after feasibility

Once the state and local rule set is clear, build a realistic project budget with our Maryland ADU Cost Calculator & Contractor Bid Comparison. If you expect to borrow against the property or use a renovation mortgage, compare HELOCs, home equity loans, cash-out refinancing, FHA 203(k) and HomeStyle Renovation in our Maryland ADU Financing Guide.

Primary sources used for this guide

Maryland ADU Guide is maintained by Gijo John as an independent research publication. See our Editorial & Research Policy for how regulatory pages are sourced and updated, and our Disclaimer for important limitations.