Garage Conversion ADU in Maryland: Rules, Permits & Costs

Can you convert a garage to an ADU in Maryland? A garage conversion may qualify, but approval depends on your local zoning rules and whether the building can meet residential code. Attached and detached garages can follow different rules. Before paying for plans, confirm the jurisdiction, how the garage will be classified, the applicable size and setback rules, and whether the existing structure can meet residential building-code requirements. Start with the Maryland ADU Jurisdiction Finder, then carry the result into the Property Feasibility Checker.

Research dates: the county comparison began with a September 19, 2026 review. Prince George’s legislative status was checked October 10. Anne Arundel’s guidance and Bill 61-26 record, Howard’s ADU and application pages, and Montgomery’s linked DPS zoning handout were rechecked October 18. Baltimore County’s entry retains the earlier review. Follow each local guide for later changes; a proposed rule is not final law.

1. First question: what kind of garage are you converting?

The word garage conversion hides several different zoning situations. Before checking square footage or cost, identify which one you actually have.

The practical takeaway is simple: do not buy a generic “garage ADU plan” until you know how your jurisdiction classifies your exact structure. A plan that fits one county’s detached-ADU rules can fail another county’s size, setback or approval requirements.

What Maryland law says about converting an existing garage

Maryland Land Use §4-504 expressly anticipates conversion of existing accessory structures. A local legislative body may adopt ADU safety standards and may prohibit full or partial conversion of an accessory structure into an ADU if the only vehicular access to that structure is from an alley.

The same state law says local ADU side and rear setbacks may not exceed the jurisdiction’s existing accessory-structure setback requirements, while still preserving building-code, health, safety and adequate-public-facilities review. An existing garage shell therefore does not automatically become legal residential space without local approval.

2. Maryland garage-to-ADU rules by county

3. Anne Arundel County: garage conversion size and permits

Anne Arundel County provides one of Maryland’s clearest current examples. The 2023 law that created the county’s present ADU framework says an ADU may be located in an attached or detached garage or in another detached structure. It also states that conversion of all or part of an existing structure into an ADU must be constructed under the permits required by county code.

For a detached structure, the county’s current public rule still lists the lesser of 800 square feet or 50% of the floor area of the principal single-family detached dwelling. That size issue is currently part of a broader 2026 transition, so do not assume the current detached cap will remain unchanged after the Article 18 update is finalized.

Real Anne Arundel zoning cases show why garage projects are not just a matter of interior remodeling. In 2026, county hearing records included proposals to convert a detached garage into an ADU and to place an ADU above a garage where setback or height relief was requested. The structure may already exist, but its location can still create zoning problems once residential use is added.

Read our Anne Arundel County ADU transition guide before sizing a detached garage conversion.

City of Annapolis: garage conversions are expressly recognized

Annapolis is a useful reminder that a municipality can have its own ADU rules. The City’s current ADU FAQ expressly says an ADU may be created by a garage conversion, attached or detached, as well as within or attached to a single-family home or as a new detached structure. If your property is inside Annapolis, check the City rule rather than assuming Anne Arundel County’s zoning process controls.

4. Howard County: garage apartments are recognized, but the footprint math matters

Howard County’s current ADU guidance explicitly lists garage apartments among common ADU examples. CB3-2026 became effective April 8, 2026 and allows homeowners to construct attached and detached ADUs or renovate existing space for an attached ADU in qualifying residential districts.

The county’s maximum-size method is unusual and should be checked before buying plans: the ADU may not exceed 75% of the exterior footprint of the principal dwelling. Howard also requires the property owner to live in either the principal dwelling or the ADU, and the county prohibits ADUs from being used as short-term rentals.

A detached garage conversion must also be reviewed against the county’s accessory-structure lot-coverage and stormwater rules. Existing walls do not make those site constraints disappear.

Check the current Howard County ADU guide for the latest enacted-rule notes and source conflicts.

5. Montgomery County: apartments over garages are already part of the ADU framework

Montgomery County’s Department of Permitting Services describes ADUs as units that can be a stand-alone backyard structure, basement apartment, addition or apartment over a garage. Its homeowner permit page specifically points ADU applicants to indoor finishing permits and to detached-garage/accessory-structure permit resources.

Under the county’s currently published legacy framework, detached ADUs remain subject to a maximum gross-floor-area formula based on the smallest of three limits: 50% of the principal-dwelling footprint, 10% of the lot area or 1,200 square feet. ZTA 26-06 is intended to change several portions of the existing framework to align with Maryland’s 2025 state law, so homeowners should distinguish the rule being administered today from the proposed replacement.

Read the Montgomery County ZTA 26-06 transition guide before committing to a garage layout.

6. Baltimore County: garage apartment rules are still tied to the legacy framework

Baltimore County’s published zoning policy still uses the terms accessory apartment and in-law apartment. Under that framework, an apartment within an existing single-family detached dwelling can proceed through the county’s Use Permit process when the required conditions are met. A detached accessory building used as an in-law apartment has historically required a Special Hearing.

This matters for garage projects because converting an attached garage or building an addition is not necessarily treated the same as turning a detached garage into a second dwelling. Baltimore County’s own zoning policy manual distinguishes principal-dwelling accessory apartments from detached accessory-building cases.

Because this site has not yet verified a final 2026 HB 1466-aligned replacement ordinance for Baltimore County, do not assume the legacy pathway will be the final rule after October 1. See our Baltimore County transition guide for the latest status.

7. Prince George’s County: enacted zoning, commencement still matters

Prince George’s County status checked October 10, 2026: the County Council enacted CB-074-2026 on September 29. Its record lists November 16, 2026 as the effective date. Confirm commencement and the garage-conversion application path with DPIE before relying on the new standards. The separate CB-092-2026 record remains at Third Reading, with introduction on September 29 and no effective date shown. Its proposed rental, short-term-rental, impervious-surface and sewage-disposal requirements should not be treated as enacted merely because the zoning bill passed.

For an attached or detached garage, gather the original permits, a survey and current measurements. Ask DPIE which rules apply to your proposed conversion and submission date, including dimensional standards, building-code review and any separate municipal approvals. The zoning bill’s passage does not itself approve an existing garage for residential use. Track the Prince George’s County ADU transition here.

8. Garage-to-ADU permit checklist: what to verify before you design

Begin with the reviewing office. Send a survey or plat, existing garage permits, floor dimensions, photographs and a sketch of the intended entrance, kitchen and bathroom. Ask these questions before commissioning full drawings:

This is a preparation aid, not a county submission checklist. Organize the permit documents, then use the building-code guide for questions to take to your designer. Howard’s application instructions, for example, require a plot plan showing size and setbacks.

1. Confirm who controls zoning. Maryland counties do not always control incorporated municipalities. Check the exact county/city jurisdiction first.
2. Classify the garage. Attached, detached, over-garage and partially connected structures may follow different ADU standards.
3. Check the legal location of the existing garage. An old garage can be legally nonconforming for setback or height. Converting it to residential use can trigger a new zoning review.
4. Confirm the permitted ADU size. Do not assume the garage’s existing square footage automatically becomes allowable living area.
5. Get the building-permit path. Expect review of the change to habitable space and any new structural, insulation, electrical, plumbing, HVAC and energy-code work.
6. Check fire separation and egress. Sleeping rooms, attached garages and independent dwelling areas can create specific life-safety requirements. Use the Maryland ADU building-code guide to review sprinkler, separation and egress questions before final plans.
7. Check septic or well capacity early. Adding a bedroom or independent dwelling on a private system can trigger Health Department review. Use our Maryland ADU Septic & Well Guide.
8. Check rental licensing separately. Zoning approval to create an ADU does not automatically answer whether a rental license or short-term-rental license is required.
9. Verify before construction. Ask the local permitting office to identify the currently administered code section and permit path in writing.

Garage conversion building-code pitfalls: slabs, ceilings and separation

If the garage is in a flood hazard area, check its prior floodplain approval and any recorded nonconversion agreement before planning living space.

Considering the basement instead? Read the basement ADU conversion guide for finished-height, bedroom escape and dwelling-separation questions.

Added September 14, 2026. Before requesting a fixed conversion price, ask a qualified designer to document the existing structure and identify the applicable code path with the permitting office. If the garage already contains an old or unpermitted dwelling, first use the existing Maryland ADU legalization guide to determine whether the structure and dwelling use have the same legal status. The checks below are investigation questions, not construction specifications.

Slab and foundation: what is actually under the garage?

Ask for evidence of the existing foundation and slab construction, condition and suitability for the proposed loads and residential use. Have the designer address moisture, floor slope, insulation and proposed plumbing routes. A slab that looks sound does not establish the footing detail beneath it. Ask whether investigation or engineered alterations are needed before assuming the floor can simply be covered.

Anne Arundel publishes a 30-inch frost-line design criterion in its residential code guidance. That local criterion is not a statewide instruction to excavate every garage foundation to that depth, or proof that a particular frost-protected shallow foundation will be accepted. Obtain the project-specific design determination.

Ceilings: measure the finished space

Record height below beams and proposed ducts, and account for any new floor and ceiling build-up. Ask the designer to show finished clearances on the drawings. Anne Arundel’s guidance distinguishes habitable-room ceilings from other areas and obstructions; verify the applicable requirements and any existing-building provisions with your own authority.

Fire separation: identify what will remain a garage

A dwelling beside retained vehicle storage raises different details from a full conversion where no garage remains. Anne Arundel’s guidance separately addresses garage separation and separation between an ADU and an attached principal dwelling. Ask which assemblies, doors, penetrations and alarms your layout requires; “add drywall” is not a complete design scope.

Openings and setbacks: check both reviews

Ask the designer to map proposed doors and windows, sleeping-room escape routes and distances to lot lines. Zoning setbacks and building-code fire protection are separate checks. A location accepted under zoning does not, by itself, settle wall or opening details.

Put investigation, engineering, floor work, insulation, escape openings and separation work into the bid comparison tool as named scope items. If a contractor has not inspected or priced an item, mark it unresolved. Use the architect screening guide to choose someone who can explain these decisions.

9. Is converting a garage cheaper than building a detached ADU?

It can be, but the existing shell is only valuable if it is suitable for residential conversion. A garage may need slab/foundation work, insulation, windows, fire separation, plumbing trenching, electrical upgrades, heating and cooling, and structural changes before it functions as a code-compliant dwelling.

In our review of published Maryland-area builder pricing, garage conversions commonly appeared below fully detached new-build ADUs, but the ranges overlap heavily once utilities and structural work are added. Use the Maryland ADU Cost Calculator & Contractor Bid Comparison rather than comparing a garage-conversion headline quote with an all-in detached ADU budget.

10. Maryland garage ADU FAQs

Can I convert an attached garage into an ADU?

Potentially, yes. The local code determines whether the project is treated as an attached/interior ADU and what entrance, fire-separation and size requirements apply. Anne Arundel expressly allows ADUs in attached garages; other jurisdictions use different classifications.

Can I convert a detached garage into an ADU?

Potentially, but detached structures usually face more site-specific zoning constraints. Anne Arundel expressly allows ADUs in detached garages. Howard permits detached ADUs. Baltimore County’s legacy system has historically required a Special Hearing for an accessory apartment in a detached accessory building.

Can I build an apartment above my garage?

In some jurisdictions, yes. Montgomery County’s own ADU materials list an apartment over a garage as an ADU example. The exact classification depends on whether the garage is attached or detached and on local ADU height rules, setback, access and building-code rules.

Do I need a permit if I am not changing the outside of the garage?

Do not assume interior-only work is permit-free. Converting storage or vehicle space into independent habitable residential space can require zoning and building review even when the exterior barely changes. New plumbing, electrical, HVAC, sleeping rooms or a second dwelling unit can trigger additional permits and inspections.

Can I rent the converted garage?

Possibly, but rental rules vary. Some jurisdictions require owner occupancy or a rental license, and short-term rentals can be restricted separately. Confirm the current county/city rental rules after you confirm that the ADU itself is legal.

Does Maryland’s 75% rule mean my entire garage can become an ADU?

No. The state definition sets an ADU-size ceiling relative to the primary dwelling, but the measurement method and local implementation matter. Howard County, for example, uses 75% of the principal dwelling’s exterior footprint. Other jurisdictions are still finalizing 2026 rules. Use our Maryland 75% ADU Size Calculator & Measurement Guide before assuming a garage’s full area qualifies.

Before you commission conversion plans

A garage can be one of the most practical ways to create an ADU in Maryland, but the existing building does not bypass zoning or permit review. The most important questions are whether the garage is attached or detached, how the jurisdiction classifies it, whether the structure already complies with site rules, and which 2026 ADU framework is actually in force.

Start with your jurisdiction in the Maryland County ADU Rules Tracker, then use the Property Feasibility Checker before spending money on final garage-conversion plans.

Primary sources used for this guide

• Maryland Land Use §4-504 — accessory-structure conversion, setbacks and local ADU requirements
• City of Annapolis — ADU FAQ
• Anne Arundel County — Laws of 2023, Bill 6-23
• Anne Arundel County — Residential Code Requirements
• Anne Arundel County Council — Bill 61-26
• Howard County — Accessory Dwelling Units
• Montgomery County DPS — Homeowners Permits
• Montgomery County DPS — ADU Zoning Requirements
• Montgomery County DHCA — Class 3 ADU Information Sheet
• Baltimore County — Zoning Policy Manual, Accessory Apartments
• Prince George’s County — Accessory Dwelling Units Task Force
• Maryland Department of Planning — HB 1466 FAQ

Maryland ADU Guide is maintained by Gijo John as an independent research publication. We prioritize enacted laws, county code, official legislative records and planning/permitting guidance. Source-backed corrections can be sent to gijo.j@marylandaduguide.com.