Accessory Apartments in Great Mills: What’s Allowed

If you are exploring an in‑law suite or rental unit in Great Mills, you are not alone. Accessory apartments, often called ADUs, can help with multigenerational living, offset a mortgage with rent, or create a flexible suite for guests. The key is understanding what is allowed in St. Mary’s County and how to plan a smooth approval.

Why accessory apartments appeal in Great Mills

Families in Southern Maryland lean on accessory apartments for practical reasons.

• Multigenerational living: A private suite keeps loved ones close while giving everyone their own space.
• Rental flexibility: Long‑term rent can help with carrying costs, especially during PCS rotations.
• Aging in place: A first‑floor or detached suite can make single‑level living possible without leaving your neighborhood.
• Space without overbuilding: Many suburban lots can fit a modest ADU or an over‑garage conversion that blends with the main home.

With a clear plan and the right team, you can add the space you need while staying compliant with local rules.

What’s allowed locally: definitions and rules to check

St. Mary’s County treats an accessory apartment as a secondary residential use that is incidental to the main house on the same lot. It can be within the primary home or in a detached accessory structure on the same parcel per the Comprehensive Zoning Ordinance definition.

ADUs are different from duplexes. A duplex creates two primary dwellings, while an accessory apartment remains subordinate to the main home. ADUs are also different from short‑term rentals. Rental rules are separate and may require additional approvals.

Always verify the specifics for your address with the County’s zoning and permitting teams. Local regulations live in the Comprehensive Zoning Ordinance and are administered by the Department of Land Use & Growth Management (LUGM). Projects require permits and inspections, and some sites trigger Health Department review for septic and well capacity see LUGM and planning resources.

Zoning districts and eligibility

Where you can place an ADU depends on your property’s zoning. County materials indicate accessory apartments are allowed in most residential districts. Dwellings are generally not allowed in Community Commercial or Industrial districts, and are allowed in OBP only as part of a planned unit development. Your parcel’s zoning controls what is possible in Great Mills, so confirm your zoning in the County GIS and with LUGM before you design review the County briefing materials.

Size, height, and design limits

Typical local caps to research for your lot include:

• Interior ADUs: Common guidance requires at least 300 square feet and often limits the unit to no more than about 30 percent of the main home’s gross floor area. Confirm the exact formula in the ordinance for your layout as summarized by regional ADU references that cite the County’s FAQ and CZO.
• Detached ADUs: County guidance often limits detached ADUs to no more than two bedrooms and ties size to the accessory structure’s floor area, such as a cap near 50 percent of that structure. Exterior compatibility standards and height limits for accessory structures may apply. Verify the current limits for your zoning and structure type see County-referenced ADU summaries.
• Critical Area overlays: In the Resource Conservation Area, density and footprint rules are tighter. An ADU in the RCA may count as a principal dwelling for density, which changes what is allowed on the parcel. Check overlays early if you are near sensitive areas per County-cited ADU guidance.

Owner‑occupancy and number of units

Many places allow only one accessory apartment per lot. County guidance generally follows that pattern. Some jurisdictions require owner occupancy in one of the units. Because these details can change and may vary by zoning or overlay, confirm owner‑occupancy rules and the one‑unit limit with LUGM for your specific property see County FAQs referenced in local summaries.

Parking, access, and setbacks

Expect to meet setback rules for principal and accessory structures, along with off‑street parking requirements. Detached ADUs are often placed behind or beside the main house and may need similar exterior materials or roof pitch to blend with the neighborhood. Separate entries and safe access are common standards. Verify exact distances, parking ratios, and design notes in the CZO and with LUGM see County guidance overview.

Utilities, septic, and life‑safety

• Building permits and inspections are required for creation or conversion of an accessory apartment per LUGM’s permit process.
• If you are on public water or sewer, utility connections and potential tap fees may apply.
• If you are on well and septic, the Health Department must confirm capacity. A perc test and site evaluation may be required before plumbing can be approved. Perc testing has seasonal limits, so contact Environmental Health early to understand timing and feasibility Health Department perc testing.
• Fire separation, egress windows, smoke and CO alarms, and safe electrical and plumbing are required life‑safety elements that will be inspected.

Permits and approvals: steps from idea to keys

Feasibility and site review

Start with a quick feasibility screen:

• Pull parcel data in the County GIS to confirm zoning, floodplain, and any Critical Area overlays County GIS portal.
• Review HOA or recorded covenants for any private restrictions on additional units or detached structures.
• Walk the lot for grading, trees, and septic reserve areas that affect siting.
• For septic properties, contact Environmental Health to check existing records and whether a perc test is needed records request and guidance.

Then schedule a pre‑application conversation with LUGM Permit Services. Share a simple concept sketch and ask what reviews apply and whether your plan is by‑right or requires additional approvals LUGM permits page.

Plans, permits, and inspections

Most projects follow a similar path:

1. Concept and site plan: Prepare a to‑scale site plan showing property lines, the house, any accessory structure, well and septic, and the proposed ADU location. LUGM provides customer assistance guides for submittals permit services.
2. Permit applications: Submit building and zoning permits. LUGM routes reviews to Inspections, Environmental Health, DPW for driveway work if needed, and Critical Area reviewers when applicable permit routing overview.
3. Health approvals: On septic, obtain Environmental Health approval for capacity and design. If a new or expanded system is needed, plan for design and construction time perc and septic information.
4. Plan review and corrections: Respond to comments from plan reviewers.
5. Inspections: Expect inspections at key milestones, including framing, trades rough‑ins, insulation, and final occupancy Inspections office.
6. Fees and utility steps: Pay impact fees if applicable and complete any water or sewer taps before final occupancy see County FAQ overview.

Timeline, budget, and contractor selection

• Timeline: The schedule depends on plan completeness, review cycles, and septic testing windows. On septic sites, perc season and Health Department availability often drive the critical path perc testing guidance.
• Budget: Plan for design, permits and reviews, potential impact fees, trades labor, materials, and contingency for septic or utility upgrades.
• Contractors: Vet licensed builders and trades with ADU experience. Ask about code details for egress, fire separation, and sound control between units.

Converting existing space vs. building new

• Conversions: Basement, attic, or over‑garage conversions can be faster and less expensive, but you must meet egress, ceiling height, and fire separation. Interior ADUs are often capped by a percentage of the home’s floor area see County‑cited summaries.
• Detached builds: A detached garage or cottage provides more privacy and can improve long‑term flexibility. Expect site planning, setbacks for accessory structures, possible bedroom caps, and septic capacity checks.

Buying a home with an accessory apartment

Verifying legal status and permits

When you tour a property with a second kitchen or a finished apartment, do not assume it is legal. Ask for:

• Copies of building permits and approvals for the accessory apartment.
• Any final inspection documents or a certificate of occupancy for the unit.
• Septic permits showing the system and bedroom count if the home is not on public sewer. Cross‑check with Environmental Health records when needed records access.

If records are incomplete, your agent can request files from LUGM and the Health Department. You can also consult Inspections on what it would take to bring the unit to code Inspections office.

Financing, appraisal, and insurance

• Appraisal: Some loan programs and appraisers recognize legal ADUs differently from informal spaces. Clear documentation helps.
• Financing: Ask your lender how rental income from an ADU may be treated, and whether the unit must be legal, permitted, and separately metered.
• Insurance: Your insurer may rate a property with an ADU as a two‑unit risk or require proof of life‑safety features. Share permits and final approvals to avoid surprises.

Rental strategy and lease considerations

Long‑term rentals usually require a standard lease and may trigger local rental licensing or inspections. Short‑term rentals have different rules and are not guaranteed by an ADU approval. Confirm rental rules and any licensing or inspection steps with LUGM and Code Enforcement before you advertise a unit County inspections and code resources.

Value, rent, and resale: making the numbers work

Estimating rent and vacancy

Use comparable long‑term rentals of similar size and amenities in Great Mills to set a conservative rent. Assume some vacancy and set aside reserves for maintenance and turns. If you expect to house family, value the non‑cash benefits in your plan.

Impact on resale and buyer demand

Accessory apartments can broaden your future buyer pool: multigenerational households, military relocators seeking flexibility, and investors who appreciate a legal second unit. In some neighborhoods, detached cottages are niche. In others, a well‑designed over‑garage suite is a standout feature. Legality and documentation matter for value.

Maintenance, management, and compliance

Plan for routine repairs, annual safety checks for alarms and egress, and keeping permits and inspection records organized. Good records support appraisals and smooth resale.

Common mistakes and how to avoid them

• Building without permits: This can block financing and trigger costly fixes. File properly with LUGM permits portal.
• Ignoring septic limits: Bedroom count and system capacity control what you can add. Contact Environmental Health early perc and septic info.
• Overlooking setbacks and parking: Site constraints can change your design. Confirm setbacks and parking requirements with LUGM County guidance.
• Mispricing rent: Use conservative assumptions and factor utilities, wear and tear, and vacancy.

Next steps for planning an accessory apartment in Great Mills

• Confirm your parcel’s zoning and overlays in the County GIS GIS portal.
• Speak with LUGM Permit Services about your concept and required reviews permit services.
• If on septic, contact the Health Department to review records and perc testing timelines perc testing.
• Sketch your site plan and get bids from licensed contractors who have ADU experience.
• Keep a clean paper trail so your unit is legal, safe, and insurable when you are done.

If you want a second set of eyes on feasibility or you are looking for a home that already has a compliant in‑law suite, reach out. I can help you line up the right lots, review records, and coordinate the steps with County offices. Start the conversation with Laura Bernth - Hammer and Heels Realtor. Get local, construction‑savvy guidance from concept to keys.

FAQs

What is an accessory apartment in St. Mary’s County?

• It is a secondary residential unit on the same lot as a main home. It can be inside the house or in a detached accessory structure and remains incidental to the primary dwelling CZO definition.

Are ADUs allowed in my Great Mills neighborhood?

• Many residential zones allow accessory apartments, but your parcel’s zoning controls what is possible. Check zoning in the County GIS and confirm with LUGM zoning overview.

How big can an ADU be?

• Interior ADUs often must be at least 300 square feet and may be capped around 30 percent of the main home’s floor area. Detached ADUs are commonly limited to two bedrooms and a percentage of the accessory structure’s area. Verify current limits with LUGM and the CZO County‑cited summary.

Do I need permits and inspections?

• Yes. Building and zoning permits are required, with plan review and inspections. LUGM routes applications to the right agencies, including Environmental Health when septic is involved LUGM permits.

What if my property uses a septic system?

• The Health Department must confirm system capacity. You may need a perc test and site evaluation. Seasonal limits can affect timelines, so start early perc testing.

Can I rent the ADU short‑term?

• Short‑term rental rules are separate from ADU approvals. Check with LUGM and Code Enforcement about any licensing or limits before hosting inspections and code contacts.

How many ADUs can I have on one lot?

• County guidance typically allows one accessory apartment per parcel. Confirm with LUGM for your zoning and any overlays County FAQ reference.

Does the Critical Area change what is allowed?

• Yes. In the Resource Conservation Area overlay, density and footprint rules are tighter and an ADU may count as a principal dwelling for density. Verify overlays in the GIS and with LUGM ADU guidance reference.