Building permits, zoning and land rules in St. Mary's County, Maryland

Yes: unincorporated St. Mary's County requires a building permit for a single-family dwelling, whether stick-built, modular or a mobile home, as well as specified residential work such as additional living space, accessory apartments and townhouses¹. The county uses a Drop Off process for these residential applications; a project may also need review for septic or a private well, public utilities, floodplain or Chesapeake Bay Critical Area issues, and soil disturbance over 5,000 square feet¹. Permit Services routes the application to the agencies whose approvals are required under the building code, zoning ordinance and subdivision regulations². Expect a $20 application fee, a $0.16-per-square-foot new-dwelling fee and a $6,697 single-family excise tax, plus applicable agency charges⁷. St. Mary's County incorporates the 2021 International Residential Code alongside the 2021 building, energy, existing-building, and swimming-pool and spa codes, with Maryland amendments¹³. The ordinance adopted the 2018 fuel-gas and mechanical codes pending state updates; later editions take effect six months after the later state effective or publication date, so confirm the applicable editions with Permit Services¹³.

The Permit Services Division of the county's Department of Land Use & Growth Management processes unincorporated-area building, environmental, residential entrance, home occupation and sign permits, then routes permit information to other agencies for required review and approval². Reach Permit Services at (301) 475-4200, 41770 Baldridge Street, Leonardtown, MD 20650; applications are accepted Monday and Thursday from 8:30 a.m. to 2:00 p.m., and Tuesday, Wednesday and Friday from 8:30 a.m. to 4:30 p.m².. Inside Leonardtown, obtain the town's building permit before erecting a building or addition; septic applications still go to the St. Mary's County Health Department^4,6. Development Services handles zoning determinations and coordinates reviews of site plans, subdivisions, boundary-line adjustments and confirmatory plats at the same address and phone number¹⁴. Floodplain development permits for work in contact with a flood hazard area are also handled there¹⁵. A new home may be occupied only after required final sign-offs and issuance of the Certificate of Use and Occupancy¹⁶.

Yes: county zoning review applies in unincorporated St. Mary's County, and Development Services provides zoning determinations on request³. Inside Leonardtown, the town's own zoning ordinance applies; septic applications remain with the St. Mary's County Health Department^5,6. District-specific uses and dimensions must be read with the zoning map and the district assigned to the parcel¹⁷. A detached house in a qualifying residential cluster development has a 6,000-square-foot minimum lot size, but that cluster rule is not a general minimum for every parcel¹⁸. Minimum lot sizes and principal-structure setbacks are set by zoning district and, for setbacks, road classification in Schedule 32.1, and Critical Area rules can add requirements¹⁹. For example, a Commercial Marine parcel needs at least 1 acre, and a Critical Area RL-T parcel with the footnoted entry needs 2 acres¹⁸. The 720-square-foot minimum dwelling size applies only to mobile homes in the PUD-MHP mobile-home-park district, not to every dwelling²⁰. We could not confirm lot-coverage or building-height limits; ask Development Services for a written parcel-specific determination¹⁴.

The St. Mary's County Health Department's Environmental Health Division administers applications to construct or expand individual septic systems and reviews repair applications for failed systems⁶. A perc test is required for new construction or repair, and new construction requires a site plan from a Maryland-registered land surveyor; construction may begin after application approval, and the permit is valid for 2 years⁸. The Health Department must inspect the system before it is covered⁶. A property without public water and sewer needs a perc test before construction begins on any structure with plumbing²¹. Only 1% of St. Mary's County soils are rated without severe limitations for a conventional septic drain field, so make septic feasibility an early contract contingency and allow room in the budget for site-specific design and work¹². A new well is a separate permit: Maryland requires a well-construction permit for any well, and the St. Mary's County Health Department issues permits for new wells only to licensed well drillers¹¹.

An RV cannot be a permanent residence in unincorporated St. Mary's County¹⁰. One camping or recreational trailer may be occupied on a lot of at least 1 acre in an allowable zoning district only for intermittent private recreation, for no more than 7 consecutive nights; it must be screened from neighboring dwellings and have an authorized self-contained sewage holding tank¹⁰. We could not confirm a separate tiny-home occupancy rule. A small second unit may instead qualify as an accessory apartment, but it needs permit approval and is limited to one per lot or parcel; the owner must occupy one dwelling unit¹⁰. Ask Development Services for a written determination before buying or placing a tiny home¹⁴. A tiny home used as a dwelling remains subject to building-permit, inspection, septic and occupancy requirements^1,16.

1. Ask Development Services for the parcel's zoning district and a written zoning determination; inside Leonardtown, start with the town's planning and zoning authority^3,5.
2. Before dividing unincorporated land, obtain subdivision authorization or confirm an exemption; no lot may be sold or transferred and no building erected until the plat is approved or otherwise legally recorded^22,23.
3. If public water and sewer are unavailable, complete the Health Department's perc-test and septic process before building a structure with plumbing^6,21.
4. Apply through Permit Services using the residential Drop Off procedure; Permit Services will route the file to other agencies whose approval is required^1,2.
5. Obtain a floodplain development permit if the site is in contact with a flood hazard area¹⁵.
6. If soil disturbance exceeds 5,000 square feet, submit a Sediment & Erosion Control Plan prepared by a licensed surveyor or engineer¹.
7. Keep permitted work visible and accessible at each inspection point until it is approved, then obtain every final sign-off and the Certificate of Use and Occupancy before moving in^13,16.

Zoning and subdivision: obtain the parcel's zoning designation and a written determination; inside Leonardtown, confirm requirements with the town^5,17. A division of unincorporated land needs subdivision authorization or an exemption before a lot is sold, transferred or built on, and each new lot must have required water and wastewater service^22,23. Septic and soils: only 1% of county soils are rated without severe conventional drain-field limits, and land without public utilities needs a perc test before construction of a plumbed structure^12,21. A parcel in the MetCom service area may receive a waiver or deferral for interim private service but later be required to connect when public service is feasible²⁴. Check flood mapping before purchase because a floodplain permit may be required and new flood-prone building sites are prohibited in covered subdivisions¹⁵. Direct residential access to a major collector or arterial is allowed only when no feasible alternative exists, with 150 feet between entrances measured from centerline²³.