Maryland’s 75% ADU rule sounds simple, but the measurement method is not uniform. State law defines an ADU as a secondary dwelling up to 75% of the size of the primary single-family detached dwelling, yet the law itself does not say exactly how that primary dwelling must be measured.
Maryland Department of Planning recommends using finished living area above grade for the statewide 75% calculation. Howard County, however, expressly calculates the limit from the exterior footprint of the principal dwelling. Other counties are still transitioning their local rules, so a homeowner should not assume one method applies statewide.
Last verified: September 6, 2026. This tool provides a planning estimate, not a zoning or permit determination.
Quick answer: which number should you use?
| Jurisdiction / rule source | Measurement basis to use for planning | Can this tool give a clean 75% estimate? |
|---|---|---|
| Maryland statewide guidance | MDP recommends finished living area above grade because HB 1466 does not define how “size” is measured. | Yes — planning estimate only. |
| Howard County | Exterior footprint of the principal dwelling. | Yes — using the verified local method. |
| Montgomery County | The 2026 transition moves toward the state 75% standard, but we have not verified a final detailed local measurement method. | No final local calculator method yet. |
| Prince George’s County | CB-074-2026 and CB-092-2026 are still moving through the local process. | No final local calculator method yet. |
| Anne Arundel County | Current public guidance and the 2026 Article 18 update are not yet fully aligned on detached-ADU size treatment. | Do not rely on one calculator result yet. |
Maryland 75% ADU size planning calculator
Estimate your preliminary 75% ceiling
Select the jurisdiction first. The calculator only produces a numerical result where the measurement basis is sufficiently verified for planning use.
Drag the slider — your 75% planning estimate updates instantly.
Maryland statewide guidance: finished living area above grade
HB 1466 allows an ADU up to 75% of the size of the primary single-family detached dwelling, but the statute does not define the measurement method. Maryland Department of Planning says it recommends using the square footage of finished living area above grade.
That recommendation is important because “finished living area above grade” is not the same thing as the outside footprint of the house. A two-story home can have much more above-grade finished living area than ground-level footprint.
Example: if a home has 2,000 square feet of finished living area above grade, the statewide planning calculation is 2,000 × 0.75 = 1,500 square feet. That is a state-definition ceiling, not an automatic permit entitlement.
Howard County: exterior footprint changes the math
Howard County’s current ADU FAQ is more specific: the maximum allowable size is calculated from the exterior footprint of the principal dwelling, and the ADU may not exceed 75% of that footprint.
This can produce a much smaller number for a multi-story home. Imagine a two-story house with a 1,000-square-foot ground footprint and 2,000 square feet of above-grade finished living area. Under Howard County’s footprint method, the preliminary 75% ceiling is 750 square feet, not 1,500 square feet.
Why we do not use one Maryland calculator for every county
Maryland is still in the implementation period created by HB 1466. Local governments with planning and zoning authority have until October 1, 2026 to adopt compliant local laws. That means some counties have final rules, some are transitioning, and some official sources are not yet fully consistent.
For Montgomery County, the proposed 2026 transition moves toward the state 75% standard, but we have not verified a final detailed local measurement method. For Prince George’s County, CB-074-2026 and companion CB-092-2026 remain in process. Anne Arundel County’s current public page still publishes an older detached-ADU cap while its Article 18 update introduces 75%-of-floor-area language.
Our calculator therefore refuses to manufacture precision where the local rule is not yet sufficiently settled.
How to get the right measurement before you calculate
- Start with records you already have. Look for a survey, approved plans, appraisal, tax record, or recent professional floor plan.
- Identify the measurement the jurisdiction actually wants. “Living area,” “floor area,” and “exterior footprint” are not interchangeable terms.
- For statewide guidance, separate above-grade finished living area from basement area. Maryland Planning’s recommendation specifically refers to finished living area above grade.
- For Howard County, establish the building’s exterior ground footprint. A simple rectangular house may be estimated as length × width, but additions, bump-outs, porches, attached garages, and irregular shapes can complicate the permit-quality number.
- Use a tape measure only for an early planning estimate. Do not substitute a homeowner field measurement for a survey, site plan, or professional measurement when the permit office requires formal documentation.
- Then apply the 75% multiplier. Multiply the verified measurement by 0.75.
- Test the result against site constraints. Setbacks, lot coverage, stormwater, septic, utilities, building code, access, and other requirements can make the actually buildable ADU smaller.
Worked examples: same house, different planning result
| Scenario | Measurement used | 75% planning ceiling |
|---|---|---|
| Maryland statewide guidance: 1,600 sq ft finished living area above grade | 1,600 sq ft | 1,200 sq ft |
| Howard County: 1,000 sq ft exterior footprint | 1,000 sq ft | 750 sq ft |
| Two-story example: 2,000 sq ft above-grade living area but 1,000 sq ft footprint | State guidance: 2,000 Howard: 1,000 | 1,500 sq ft vs. 750 sq ft |
The 75% result is a ceiling, not a promise
A correct 75% calculation answers only one question: the preliminary size ceiling under the applicable ADU definition or local measurement rule. It does not prove that the full area can be built on your lot.
For example, Howard County still requires a plot plan showing compliance with maximum size and setbacks, and newly constructed ADUs must also address water conveyance. Other jurisdictions may have their own site, utility, septic, stormwater, building-code, parking, or approval requirements.
If the property uses private well or septic, run the Maryland ADU Septic & Well Readiness Checker before treating the 75% size ceiling as a buildable plan.
Primary sources used for this calculator
- Maryland Department of Planning — HB 1466 ADU FAQ
- Maryland Department of Planning — Model Local ADU Ordinance & Assessment Tool
- Howard County — Accessory Dwelling Units
- Howard County — ADU Application
- Anne Arundel County — ADU Facts & Guidelines
- Anne Arundel County — Article 18 Comprehensive Update
For the statewide legal baseline, read our Maryland ADU Guide 2026. For county-by-county implementation status, use the Maryland County ADU Rules Tracker. Once feasibility is clearer, compare delivery models in our Maryland ADU Builders & Prefab Options Guide. Maryland ADU Guide is maintained by Gijo John as an independent research publication.