Maryland ADU architect screening guide
Hire the designer after you understand the property.
Before you pay for plans, verify the five decisions that can force a Maryland ADU redesign: zoning authority, current rule set, size method, site constraints and permit scope.
This is not an architect directory. It is a mistake-prevention screen.
The question is not whether a designer can draw an attractive ADU. The question is whether the team has a repeatable process for verifying the rules and site assumptions that control those drawings. Use the questions below before a full permit set is commissioned.
Start with jurisdiction and zoning
Which government controls zoning?
Why it mattersCounty names alone do not always identify the final zoning authority.
Ask forThe parcel’s actual planning/zoning authority and the source used to verify it.
Which ADU rule set is in force today?
Why it mattersMaryland is still in the HB 1466 implementation period through October 1, 2026.
Ask forThe ordinance, bill or zoning section being used—and the date it was checked. Cross-check our tracker →
How will the maximum ADU size be calculated?
Why it mattersMaryland does not prescribe one universal measurement method for the 75% rule.
Ask forThe exact measurement method used locally. Use the 75% calculator →
Which setbacks, height and lot limits apply?
Why it mattersThe statewide setback guardrail does not create one Maryland setback number.
Ask forThe parcel’s zone, accessory-structure standards, easements, lot coverage and height limits.
Then test the site and approval path
What approval path applies?
Why it mattersMaryland allows local by-right, conditional-use and special-exception approaches.
Ask forThe path for this parcel and any extra review triggered by the concept.
What parking rule applies?
Why it mattersExtra ADU parking is constrained by HB 1466’s study and waiver requirements.
Ask forThe current local parking rule and whether a waiver process matters to the design.
What utility, well or septic issue comes first?
Why it mattersPublic-health and infrastructure constraints can change a layout before architecture begins.
Ask forWho verifies wastewater capacity, reserve area, well issues and utility routing. Open the readiness guide →
Is an existing garage or accessory structure legally usable?
Why it mattersA conversion is not automatically simpler just because the shell already exists.
Ask forPermit history, structural condition, setbacks, access, utilities and code implications.
Finally, protect the contract and permit scope
What site conditions could move or shrink the footprint?
Why it mattersA theoretical size ceiling is not the same as a buildable envelope.
Ask forEasements, stormwater, drainage, slopes, trees, access and utility corridors.
What assumptions are you making?
Why it mattersUnwritten assumptions become expensive when an agency reviewer changes the premise.
Ask forA written assumptions list and the cost treatment if one changes.
Can the work be staged?
Why it mattersUnresolved feasibility questions do not always justify a full construction set on day one.
Ask forSeparate feasibility, schematic-design and permit-drawing milestones.
Who owns permit coordination and resubmittals?
Why it mattersDrawing plans and managing agency review are different services.
Ask forWho submits, who answers comments, how many revision rounds are included and which consultants are extra.
Why “Maryland experience” is not specific enough
Two jurisdictions can sit under the same statewide law and still require different measurements or implementation checks.
Verified example
Howard County
Current county guidance uses 75% of the exterior footprint of the principal dwelling. A designer using total interior living area could start from the wrong ceiling.
2026 transition example
Montgomery County
The existing detached-ADU framework historically used a different local formula while ZTA 26-06 moves toward HB 1466 alignment. The operative rule should be rechecked when plans are prepared.
Contract screen
Before you pay a design deposit
- Zoning authority identified
- Current ADU rule dated
- Size method documented
- Setbacks and lot limits checked
- Approval path identified
- Parking rule checked
- Utilities or septic confirmed
- Site constraints assigned
- Assumptions written down
- Resubmittal responsibility defined
Use this page as the meeting worksheet
Print it or save it as a PDF and mark the 12 answers during the interview.
Walk-away signals
Red flags
- They give you a size from memory without naming the measurement method
- They assume the county answer is enough without checking zoning authority
- They quote “standard Maryland setbacks” without identifying the parcel’s zone
- They promise a permit date before confirming the approval path
- They defer septic, drainage or utility questions until after layout design
- The agreement is silent about agency-driven redesign or resubmittals
Best next step
Screen the property before you screen designers.
Build a clearer property brief first. Then take the flagged questions into the first architect or design-build meeting.
Primary sources
- Maryland Department of Planning — HB 1466 ADU FAQ
- Maryland Department of Planning — Model Local ADU Ordinance
- Howard County — Accessory Dwelling Units
- Montgomery County Council — Zoning Text Amendments
Last reviewed: September 7, 2026. Planning and consumer-screening information only; not legal, architectural, engineering, zoning or permitting advice.
Maryland ADU design-risk screen
Do not pay for detailed ADU drawings until these five things are verified.
A beautiful plan can still be the wrong plan if the zoning authority, current rule set, size method, site constraints or permit scope was assumed instead of checked.
Wrong zoning authority
The county name alone may not tell you which local zoning rules control the parcel.
Wrong rule set
Enacted, transition and legacy ADU rules can produce very different design assumptions.
Wrong size method
Maryland’s 75% rule does not use one universal measurement method in every jurisdiction.
Site surprise
Setbacks, septic, drainage, utilities or an existing structure can shrink or move the buildable envelope.
Unclear permit scope
Drawings, permit coordination, revisions and consultant work may be separate services unless the agreement says otherwise.
12 questions to ask before you sign a design agreement
Use these to test the firm’s verification process—not just its drawing ability.
01Which government actually controls zoning for this property?
Do not stop at the county name. A property may be inside a municipality or another local zoning framework. Ask the designer to identify the actual planning and zoning authority for the parcel and show you the source used.
02Which ADU rule set is in force today?
HB 1466 took effect October 1, 2025, while covered local governments have until October 1, 2026 to adopt compliant local laws. Some jurisdictions have enacted new rules; others remain in transition. Ask the designer to identify the current ordinance, bill or zoning section and the date it was checked.
03How will you calculate the maximum ADU size?
State law defines an ADU as no more than 75% of the size of the primary single-family detached dwelling, but Maryland does not prescribe one universal measurement method. Maryland Planning recommends finished living area above grade.
Howard County currently uses the exterior footprint of the principal dwelling for its 75% method. Montgomery County requires extra caution during its 2026 transition because its existing detached-ADU framework differs from the proposed alignment path.
04Which setbacks, lot-coverage limits and height rules apply?
HB 1466 says local ADU side and rear setbacks may not exceed existing accessory-structure setback requirements. That does not create one statewide setback number. The designer should identify the parcel’s zoning classification and document the applicable dimensional standards, easements and other constraints.
05What approval path applies?
Maryland does not require every ADU to be approved by right. A local government may use by-right approval, conditional use or special exception. Ask which path applies before detailed drawings begin and what additional review would be triggered if the concept needs relief from a zoning standard.
06What parking rule applies?
A jurisdiction cannot impose additional off-street ADU parking without first completing a parking study, and any additional parking rule must include a waiver process. The designer should verify the current local rule rather than automatically consuming buildable area for parking.
07What utility, well or septic constraints should be checked first?
HB 1466 does not override public-health, building-code or facility-capacity requirements. Ask who will verify wastewater capacity, reserve-area constraints, well issues, utility routing and any required health-authority review.
08If converting a garage or accessory structure, is it legally usable?
Ask the team to check permit history, setbacks, structural condition, access, utilities and code implications before assuming a conversion will be cheaper or simpler than new construction.
09What site conditions could force the footprint to move or shrink?
Ask about easements, drainage, stormwater, slopes, trees, access and utility corridors. A correct size calculation is only a ceiling; the actually buildable envelope can be smaller.
10What assumptions are you making—and what happens if one changes?
Ask the designer to list the assumptions behind the proposal: zoning authority, size method, setbacks, utilities, existing conditions and approval path. Then ask whether zoning-driven redesign or plan-review revisions are included in the agreement or billed separately.
11Can the work be separated into feasibility, schematic design and permit drawings?
For a property with unresolved questions, a staged scope can reduce the risk of paying for a complete construction set too early: confirm feasibility and the buildable envelope first, then develop the layout, then prepare permit/construction documents.
12Who handles permit coordination and resubmittals?
Preparing drawings and managing permit review are different services. Clarify who submits, who communicates with zoning/building/health reviewers, how many revision rounds are included and which third-party consultants may be needed.
Why local verification changes the design
Same statewide law. Different local implementation and measurement details.
Howard County
Current county guidance uses 75% of the exterior footprint of the principal dwelling. A designer using total interior living area could start from the wrong ceiling.
Montgomery County
The existing detached-ADU framework historically used a different local formula while ZTA 26-06 moves toward HB 1466 alignment. The operative rule should be rechecked when plans are prepared.
Before you pay a deposit
- Zoning authority identified
- Current ADU rule checked and dated
- Size measurement method documented
- Setbacks, lot coverage and height checked
- Approval path identified
- Parking rule checked
- Water/sewer or well/septic service confirmed
- Major site constraints assigned to the right professional
- Redesign and resubmittal responsibilities defined
Bring this to the first design meeting
Print this page and use the 12 questions as your screening worksheet.
Red flags
- They give you an ADU size from memory without identifying the measurement method
- They assume county rules apply without checking the actual zoning authority
- They quote “standard Maryland setbacks” without identifying the property zone
- They promise a permit date before confirming the approval path
- They postpone septic, drainage or utility questions until after layout design
- The agreement does not explain what happens if agency comments require changes
Best next step
Screen the property before you screen designers.
The strongest architect interview starts with a clearer property brief. Run the feasibility checker first, then bring the flagged questions into your first design meeting.
Primary sources
- Maryland Department of Planning — HB 1466 ADU FAQ
- Maryland Department of Planning — Model Local ADU Ordinance
- Howard County — Accessory Dwelling Units
- Montgomery County Council — Zoning Text Amendments
Last reviewed: September 7, 2026. Planning and consumer-screening information only; not legal, architectural, engineering, zoning or permitting advice.