Quick answer: Maryland does not set one statewide maximum height for every accessory dwelling unit. The 2025 state ADU law requires local jurisdictions to authorize qualifying ADUs by October 1, 2026, but local zoning rules still control many dimensional standards—including height—unless state law specifically limits that local authority.
That means a detached backyard cottage, garage apartment or two-story ADU may be allowed in one Maryland jurisdiction but need a variance, design change or lower roofline in another.
This guide explains what homeowners should check before assuming a detached ADU can be built to the same height as a house.
Does Maryland have a statewide ADU height limit?
No. Maryland Land Use § 4-504 establishes statewide requirements for local ADU laws, including rules dealing with zoning approval, setbacks, parking and other conditions, but it does not create a universal statewide maximum ADU height.
That matters because local governments may continue to apply their zoning rules to accessory structures and ADUs where those rules are not preempted by the state law.
Planning takeaway: do not use a generic “Maryland ADU height” number when sketching your project. Your actual limit may depend on your county or municipality, zoning district, structure type and whether the ADU is new construction or a conversion of an existing accessory building.
Why detached ADU height gets complicated
A detached ADU is often treated as both a dwelling unit and an accessory structure. That can pull several zoning rules into the same project.
- Maximum accessory-structure height
- Number of stories
- Roof form and how height is measured
- Side and rear setbacks
- Lot coverage
- Existing accessory-building rules
- Historic district or Critical Area requirements
- Variance procedures when the proposed design exceeds a dimensional limit
This is why two ADUs with the same square footage can have very different approval paths.
Garage apartments are where height issues show up most often
One of the most common height conflicts is a garage with an ADU above it. The homeowner may be able to fit the required living area only by adding a full second story, steeper roof or taller wall height.
Anne Arundel County records provide a useful real-world example. In a recent variance application for a garage with an ADU above, the proposed two-story structure was listed at 23.05 feet in height while the applicant sought setback relief. The example is useful because it shows how a garage-over-ADU design can quickly become a relatively tall accessory structure, even when height itself is not the variance being requested.
If you are considering this type of project, also read our Maryland garage conversion ADU guide.
Existing accessory buildings may be treated differently
Conversions deserve separate attention. Maryland’s new ADU framework limits how local governments can apply certain setback requirements when an existing accessory structure is converted without expanding its footprint. But that does not mean every existing shed, garage or outbuilding automatically qualifies as an ADU.
You still need to verify issues such as legal status of the structure, building code compliance, habitable space, utilities, egress, fire separation and any applicable dimensional rules.
In Montgomery County, for example, published ADU inspection guidance distinguishes certain older detached structures and addresses whether a building is increased in size or height. The county has also historically retained accessory-structure limits, including height, as part of its detached ADU framework.
Height and setbacks are separate checks
A common mistake is to assume that meeting the rear and side setbacks means the building envelope is fully compliant. It does not.
Your project can satisfy the ADU setback rules and still fail a height standard. The reverse can also happen: the proposed height may be allowed, but the structure may be too close to a property line.
For detached ADUs, check these as separate items:
- Is an ADU permitted on the property?
- What side and rear setbacks apply?
- What maximum height applies to the structure?
- Does the rule change based on distance from a property line?
- Does the zoning code treat a garage apartment differently?
- Will lot coverage or impervious-surface rules reduce the buildable footprint?
Height can affect more than appearance
A taller detached ADU can trigger practical design consequences even when zoning allows it.
- Privacy: second-story windows may overlook neighboring yards.
- Stairs: an upper-floor living space needs safe access and code-compliant geometry.
- Roof design: pitched roofs can increase measured building height faster than homeowners expect.
- Construction cost: a second story typically adds structural, stair and fire-safety complexity.
- Drainage: a larger or taller detached structure may still be constrained by lot coverage and stormwater requirements.
Howard County’s current ADU materials, for example, specifically require new detached ADU applications to demonstrate adequate water conveyance so runoff does not adversely affect neighboring properties or subdivision stormwater facilities.
How is building height measured?
This is another area where homeowners should not guess. “Height” may be measured to the roof peak, average roof height, midpoint of a sloped roof, or another point defined by the local zoning code.
Frederick County, for example, publishes separate zoning resources for building height and a worksheet for determining building height. That is a good illustration of why a drawing labeled “20 feet high” is not enough unless it uses the jurisdiction’s actual measurement method.
Before paying for final plans, ask the zoning office which code section defines height and exactly how it should be measured for your proposed ADU.
What Maryland’s 2026 ADU law changes—and what it does not
Maryland’s ADU law requires covered counties and municipalities to adopt local laws authorizing ADUs by October 1, 2026. It also limits certain local restrictions, including some setback, parking and lot-size requirements.
But the law does not replace every local dimensional rule with a single statewide development standard. Height therefore remains a local-code question in many jurisdictions.
This distinction is important during the 2026 transition period because counties and municipalities are updating their ordinances at different times. A rule shown in an older zoning table may also be amended as a jurisdiction brings its code into compliance.
Use our Maryland ADU Rules Tracker to find the current county or city guide for your property.
A simple height-check workflow
Before hiring an architect for a two-story detached ADU, work through this order:
- Confirm jurisdiction. Use the Maryland ADU Jurisdiction Finder. A municipal property may follow city rules rather than the surrounding county’s zoning code.
- Confirm the zoning district. Height standards can vary by district.
- Find the accessory-structure height rule. Do not rely only on the ADU section.
- Check how height is measured. Roof geometry can change the result.
- Check setbacks separately. Use our Maryland ADU setback guide.
- Check lot coverage. A smaller footprint can force a second story, but the taller design still has to comply with height rules. See our lot coverage guide.
- Ask whether a variance would be required. Do this before commissioning expensive final construction drawings.
Questions to ask your zoning office
- What is the maximum height for a detached ADU on this parcel?
- Is the ADU subject to the accessory-structure height rule?
- Can it be two stories?
- How is height measured for a pitched roof?
- Does height change based on setback from the rear or side property line?
- Is a garage with an ADU above treated differently?
- Can an existing taller accessory structure be converted without reducing its height?
- Would my proposed design require a variance?
Bottom line
There is no reliable statewide answer such as “Maryland ADUs can be 20 feet tall.” The state has created a broader right for qualifying homeowners to develop ADUs, but the permitted height of a detached ADU still depends heavily on local zoning and accessory-structure standards.
For homeowners, the safest sequence is jurisdiction → zoning district → height definition → setbacks → lot coverage → preliminary design. Doing those checks before final architectural drawings can prevent a costly redesign later.
Sources: Maryland General Assembly, Land Use § 4-504 and HB 1466 / Chapter 197; Howard County ADU application and CB3-2026 materials; Montgomery County ADU materials; Frederick County zoning information; Anne Arundel County 2026 variance records. This article is for general planning information and is not legal, architectural or zoning advice. Always verify current requirements with the agency that has zoning authority over your property.
What to do next
Start by finding the office that reviews your property. A mailing city or a county guide does not establish your parcel’s authority.
- Find Who Reviews My Property.
- Obtain your parcel identifier, survey, floor plans and utility records.
- Ask the reviewing office which current rules, supporting documents and approvals apply.
- Continue to your planning workspace.
Enable JavaScript to prepare a downloadable next-step brief. Planning guidance is not permit approval.