Maryland ADU lot coverage rules
An ADU can fit inside the setback lines and still be too large for the site. Local coverage limits, accessory-structure caps, drainage requirements and other property constraints can reduce the workable footprint. Treat size, placement and stormwater as separate checks before buying a detached ADU design.
This guide explains the calculations to prepare and the questions to ask. It does not establish a parcel’s permitted coverage or a universal Maryland impervious-surface allowance.
The difference between state guidance and local lot enforcement
The Maryland Department of Planning describes its Model Local ADU Ordinance as guidance to help local governments comply with the 2025 ADU legislation. The model document itself is not a building permit or permission to bypass local review. The underlying state law and the applicable local ordinance need to be assessed separately.
Start with the Jurisdiction Finder. A mailing city does not establish the zoning authority, and zoning and building-permit responsibilities may differ. Then use the local rules tracker to find the relevant research and official sources.
Lot coverage vs. impervious surface limits: what’s the difference?
- Lot or building coverage: a zoning measure whose exact definition comes from the applicable code. It may focus on building footprints, but do not assume roof area is the complete definition or that every projection, deck or pool receives the same treatment.
- Accessory-structure coverage: a separate limit may count several accessory structures together, sometimes as a square-foot cap rather than a percentage. The existing shed or detached garage may matter to this calculation.
- Impervious area: Maryland’s stormwater regulations describe surfaces that prevent runoff from infiltrating into the ground. Roofs and conventional paving are common examples. Ask how the reviewer treats each surface, including any proposed permeable system. See the state definitions.
- Land disturbance: the area affected by the construction operation is another calculation. Ask the designer to map excavation, grading, trenches and construction access rather than using only the finished building footprint.
Keep these totals in separate columns. A building footprint, an interior floor area, an impervious-area total and a limit-of-disturbance area are not interchangeable.
How new local text amendments factor in existing footprints
Before applying a newly adopted provision, confirm its effective date, the zoning district, the relevant definitions and what existing features it counts. Ask whether the proposal is a new structure, conversion, enlargement or replacement; these may raise different questions.
Howard County example: its ADU guidance discusses combined accessory-structure coverage and runoff obligations. For ADUs in newly constructed structures, it says the plot plan must demonstrate adequate water conveyance without impacts on neighboring properties or existing subdivision stormwater facilities. The map shows potential, not guaranteed eligibility. Check the Howard County guide for local details and source conflicts.
Inventory the existing house, sheds, garages, decks, pool and paving separately. Have the office or designer mark what counts under each applicable rule. Do not assume an older structure is excluded, or that removing paving creates extra building-coverage capacity.
“Grandfathered” needs a specific basis. Ask for the existing approval records and applicable nonconforming-structure provisions. The age of a building alone does not settle whether it may be converted, expanded or rebuilt.
Use the ADU Size and Measurement Guide and calculator to organize the dwelling-size check. Confirm the measurement method first. A 75% calculation does not resolve coverage, setbacks, height, utilities or stormwater, and it is not a final buildable allowance.
Worked example: one proposal, three different totals
Illustration only—not a Maryland or county allowance. Suppose a hypothetical 10,000-square-foot lot has a 2,000-square-foot house footprint and a 200-square-foot shed. A hypothetical rule permits 30% building coverage and counts those buildings. The arithmetic leaves 800 square feet: 3,000 − 2,000 − 200.
A proposed 600-square-foot ADU footprint would put that building-coverage total at 2,800 square feet, or 28%. If the lot also has 1,200 square feet of counted paving and the project adds a 150-square-foot paved path, its illustrative impervious total would be 4,150 square feet, or 41.5%. Passing the hypothetical building-coverage test therefore says nothing about whether the impervious total is acceptable.
Neither number establishes the land disturbed during construction. Nor does a two-story layout make its floor area identical to its footprint. Have the reviewer confirm the definitions and calculate each constraint separately.
When an ADU triggers stormwater management review
COMAR 26.17.02.05 requires an approved final stormwater management plan unless an activity is exempt under that regulation. It includes an exemption for development that does not disturb over 5,000 square feet. Its exemption for additions or modifications to existing single-family detached structures is also conditioned on that disturbance provision.
That is a land-disturbance threshold—not a 5,000-square-foot ADU allowance, and not a universal exemption from every drainage or permit requirement. Ask the approving agency how the rule applies to the entire project, what local provisions or other approvals remain relevant, and how an exemption is documented.
Howard County’s engineering FAQ also identifies the 5,000-square-foot disturbance threshold. Read that general guidance alongside its ADU-specific runoff requirements. Ask about any impervious-area trigger applicable to your parcel rather than assuming a single statewide net-new-area figure.
Why drainage design can change the budget
Stormwater work does not automatically mean a large engineered tank. Maryland’s stormwater measures regulation prioritizes environmental site design and identifies practices including runoff disconnection, dry wells and rain gardens, as well as alternative surfaces such as permeable pavement. Suitability and approval depend on the site and design criteria.
A rain garden or permeable-paver product is not an automatic permit solution. Ask the designer about soils, slopes, groundwater, nearby foundations, well/septic constraints, discharge routes and maintenance. Do not assume a material labeled “permeable” earns a particular regulatory credit without an accepted design.
Request separate scope and pricing for survey information, testing, drainage design, review fees, excavation, installation, inspection and maintenance responsibilities where applicable. Mark unresolved work clearly in the ADU budget and bid comparison tool. This guide supplies no dollar estimate because the required system and site work have not been established.
Your next step: assemble a site-constraint checklist
- Confirm the reviewing authorities and current local rule.
- Record the legal definitions and caps for each area calculation.
- Measure existing and proposed features separately.
- Map construction disturbance and ask about stormwater and drainage review.
- Bring coverage, setbacks, utility constraints and drainage together on the proposed site plan.
- Resolve the missing answers before committing to a final footprint.
Next: build your property feasibility checklist →
Use the setback and placement guide alongside these calculations. For a septic property, also check reserve-area records. Return to My ADU Plan whenever you need the overall sequence.
Sources reviewed September 15, 2026. Official sources are linked beside the relevant claims. This is planning guidance, not a property-specific determination.
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.