How Big Can an ADU Be in Maryland? 75% Rule & County Size Limits

Maryland’s 75% ADU rule sounds simple: an accessory dwelling unit may be no greater than 75% of the size of the primary single-family detached dwelling. The difficult part is that Maryland law does not define exactly how that primary dwelling must be measured. Howard County uses the exterior footprint of the principal dwelling, Maryland Department of Planning recommends finished living area above grade, and several major counties are still transitioning their local 2026 rules.

Research baseline: September 9, 2026, with later jurisdiction-specific updates noted below. Maryland’s October 1, 2026 local-law deadline has passed. A passed deadline does not establish the rule or approval path for a particular property; check the linked local guide and reviewing office. This calculator is a planning tool, not a zoning or permit determination.

1. What Maryland’s 75% ADU rule actually says

Maryland Land Use §4-501 defines an accessory dwelling unit as a secondary dwelling on the same lot, parcel or tract as a primary single-family detached dwelling and not greater than 75% of the size of that primary dwelling.

The statute does not define size. Maryland Department of Planning therefore recommends using the square footage of the finished living area above grade when applying the 75% limit. That recommendation is state implementation guidance; it is not a universal statutory measurement formula written into §4-501.

2. Maryland ADU size comparison

3. How Maryland jurisdictions are measuring or limiting ADU size

The calculator separates a documented local measurement method from an unverified planning comparison. A numeric comparison does not establish the size your jurisdiction will approve.

4. Howard County: why exterior footprint can cut the number in half

Howard County’s current ADU FAQ states that maximum ADU size is calculated from the exterior footprint of the principal dwelling, with the ADU limited to 75% of that footprint.

That is not the same as total above-grade living area. A two-story house might contain 2,000 square feet of finished living space but sit on a 1,000-square-foot exterior footprint. Under the MDP planning recommendation, 75% of 2,000 is 1,500 square feet. Under Howard’s local footprint method, 75% of 1,000 is only 750 square feet.

Howard also requires the ADU application plot plan to show compliance with maximum size limitations and setbacks. Read the Howard County ADU guide.

5. Can a county keep an 800- or 1,200-square-foot maximum?

This is one of the most important unresolved implementation questions in Maryland. MDP’s HB 1466 FAQ says jurisdictions must authorize ADUs consistent with the state definition and therefore may not limit ADU size below the 75% limit.

That interpretation matters because several older local frameworks contain fixed caps. Montgomery County has since changed its rules: effective October 5, 2026, Ordinance No. 20-36 removed the attached 1,200-square-foot cap and basement/cellar alternative, and the detached least-of-50%-of-footprint, 10%-of-lot-area or 1,200-square-foot formula. Its general ADU definition now uses 75% of the primary single-family detached dwelling’s size. A detached structure legally constructed before May 31, 2012 retains setback and floor-area relief if it is not increased in size or height and no new windows are installed on the wall nearest an abutting property; confirm with DPS how that exception and the general size definition apply to your project. Anne Arundel’s current public guidance limits detached ADUs to the lesser of 800 square feet or 50% of the principal dwelling’s floor area.

Anne Arundel’s pending Bill 61-26 makes the issue especially visible: the introduced text retains the 800/50% detached cap but says all other ADUs may not exceed 75% of the principal dwelling’s floor area. The bill remains pending after the September 8, 2026 public hearing; no final passage or enacted ordinance is shown on the County Council’s current legislation page.

See the Anne Arundel transition guide and Montgomery County ADU rules guide.

6. Can you use SDAT square footage for the 75% calculation?

Use SDAT as a starting point, not as your final zoning measurement. Maryland’s Real Property Data Search can show a residential property’s Above Grade Living Area, which is conceptually close to MDP’s recommendation of finished living area above grade.

SDAT’s glossary says Above Grade Living Area generally includes residential living area above ground level and excludes finished attic or basement area. It also distinguishes that number from Above Grade Enclosed Area, which can include unfinished above-grade portions that living area excludes.

There are two major cautions. First, SDAT itself states that its Real Property Data Search is for informational purposes and is not to be used for legal reports or documents. Second, SDAT says that where more than one residential dwelling exists on an account, its living-area figure can reflect the combined area of all dwellings.

So if the property has additions, multiple dwellings, unusual assessment history or inconsistent records, compare SDAT with approved plans, permit records, a professional floor plan or the measurement source requested by the local zoning office.

Open Maryland SDAT Real Property Data Search →

7. How to find the right starting number before you calculate

1. Confirm the governing jurisdiction. A municipality may control zoning even when the property is inside a county. If you have not verified it yet, start with the Maryland ADU Jurisdiction Finder before entering a size.
2. Check the county’s current ADU status. Use the Maryland County ADU Rules Tracker before assuming the state-guidance method controls.
3. Open SDAT for a first-pass number. Note Above Grade Living Area, Finished Basement Area, structure details and property account information.
4. Compare SDAT against your records. Approved plans, a recent appraisal, survey/site plan or professional floor plan may show a different area.
5. Ask which measurement term the zoning office uses. Finished living area, floor area, gross floor area, building footprint and lot coverage are not interchangeable.
6. If you are in Howard County, establish exterior footprint. Multi-story living area is not the input Howard says to use.
7. Apply 75% only after identifying the basis. Multiply the verified starting measurement by 0.75.
8. Then test site feasibility. Setbacks, lot coverage, utilities, septic, stormwater, building code and other rules may reduce the size that can actually be approved.

8. Worked examples: the same house can produce very different ADU ceilings

The 75% number is still only a ceiling. A correct calculation does not prove the lot can physically or legally accommodate an ADU of that size. Run the Maryland ADU Property Feasibility Checker before paying for final design.

9. Maryland ADU size questions homeowners ask

Does Maryland allow every ADU to be 75% of the main house?

The state definition sets the maximum relative size at 75%, and MDP interprets HB 1466 to mean local jurisdictions may not impose a lower size cap. But local implementation, measurement definitions and unresolved transition rules still matter. A 75% calculation is not an automatic approval.

Does a basement count in the 75% calculation?

Under MDP’s recommended planning method, the starting point is finished living area above grade, so finished basement area would not be included in that recommended baseline. A local jurisdiction may use a different defined measurement method, so verify before designing.

Does an attached garage count?

Do not assume it does. Whether garage area is part of a local floor-area or footprint definition depends on the jurisdiction and the measurement term being used. Howard’s exterior-footprint method and MDP’s finished-living-area recommendation are fundamentally different concepts.

Can I use Zillow or a real-estate listing?

Use listing data only as a rough screening figure. It may combine above-grade and below-grade finished area or use a different methodology than the zoning office. SDAT, approved plans and permit records are stronger starting points, but the local measurement requirement still controls.

Can SDAT be the final proof of my house size?

Not automatically. SDAT describes its Real Property Data Search as informational and not for legal reports or documents. Ask the local zoning/permitting office what record or measurement it will accept.

Why does Howard County give me a smaller result?

Because Howard says to use the exterior footprint of the principal dwelling. A two-story house can have twice as much above-grade living area as ground-level footprint, so the input number—and therefore the 75% result—can be much smaller.

10. Bottom line

The hard part of Maryland’s 75% ADU rule is not multiplying by 0.75. It is identifying the correct number to multiply. State law does not define the measurement method, MDP recommends finished living area above grade, Howard County uses exterior footprint, and local implementation and measurement rules still need to be checked after the October 1, 2026 local-law deadline.

Use SDAT for a first-pass property record, then confirm the local measurement basis before buying plans. If the jurisdiction’s local text appears to impose a lower fixed cap, compare it with MDP’s current HB 1466 guidance and obtain a written local interpretation before relying on either number.

Related research: What happens if a county is not ready by October 1? · Maryland ADU parking rules · Garage-to-ADU conversions.

Primary sources used for this guide

• Maryland Land Use §4-501 — statewide ADU definition and 75% limit
• Maryland Department of Planning — HB 1466 FAQ, measurement recommendation and size-cap interpretation
• Howard County — ADU FAQ and exterior-footprint method
• Howard County — ADU Application
• Montgomery County DPS — October 2026 ADU guidance, effective October 5
• Montgomery County — signed Ordinance 20-36 (ZTA 26-06), adopted September 15 and effective October 5, 2026
• Anne Arundel County — current ADU Facts & Guidelines
• Anne Arundel County Council — Bill 61-26
• Prince George’s County Council — CB-074-2026 legislative record
• Maryland SDAT — Finding Your Property Information Online
• Maryland SDAT — Real Property Data Search field definitions

For the full Maryland statewide framework, see our Maryland ADU Guide 2026. Maryland ADU Guide is maintained by Gijo John as an independent research publication. We prioritize enacted statutes, official legislative records, local zoning materials and government guidance. This guide is informational and is not legal, architectural, appraisal or permitting advice.