Clements

St. Mary's County (the southernmost county on Maryland's Western Shore, occupying the lower three-quarters of the peninsula between the Patuxent River on the north/east and the Potomac River on the south/west, terminating at Point Lookout where the Potomac meets the Chesapeake Bay; ~115,000 residents; county seat at Leonardtown — Maryland's only incorporated municipality within the county; home to Naval Air Station Patuxent River and the Webster Field Annex at St. Inigoes; the only county with NO 'county' in its self-name in Maryland — the official name is simply 'St. Mary's County' though it is properly a Maryland county) regulates land use through the St. Mary's County Comprehensive Zoning Ordinance (CZO) adopted under the county's charter authority pursuant to Md. Const. Art. XI-A and Md. Land Use Article. Maryland is a home-rule state; St. Mary's County is a charter county (adopted 1972; revised since) exercising zoning authority over all unincorporated parcels. The Town of Leonardtown, the county's only incorporated municipality, operates under its own town charter and zoning code. Accessory-dwelling-style uses in the unincorporated county fall under the CZO's 'accessory apartment,' 'guest house,' 'family member dwelling unit,' and 'farm tenant house' provisions, plus the per-district use schedules. Maryland enacted HB 538 (2024), the statewide ADU enabling act, requiring counties and Baltimore City to permit at least one ADU on most residentially-zoned single-family lots; St. Mary's County's pre-existing CZO accessory-apartment provisions are broadly aligned and the Department of Land Use and Growth Management has been updating administrative interpretations to align with HB 538.

St. Mary's County administers overlay regimes that bear materially on ADU projects. In approximate order of how often they bind on a typical project: (1) NAS Patuxent River AICUZ (Air Installations Compatible Use Zones) overlay — uniquely consequential in St. Mary's County because NAS Pax River is a major operational naval air station whose Accident Potential Zones (APZ-I, APZ-II, Clear Zone) and noise contours (65, 70, 75, 80, 85+ dB DNL) sweep across a substantial fraction of the Lexington Park / California / Great Mills / Patuxent River urban district, plus portions of Hollywood and Webster Field-adjacent St. Inigoes — the county's CZO incorporates the AICUZ recommendations and restricts new residential density in higher-impact zones; (2) Chesapeake Bay Critical Area — the 1,000-ft buffer landward of tidal waters and tidal wetlands established by Md. Natural Resources Code § 8-1801 et seq.; given the county's peninsular geography (bounded by Patuxent River, Potomac River, Chesapeake Bay, plus numerous tidal-creek systems), a very substantial portion of populated and shoreline parcels are within the Critical Area, subject to Resource Conservation Area (RCA), Limited Development Area (LDA), and Intensely Developed Area (IDA) sub-designations with associated impervious-surface and disturbance caps; (3) Floodplain — FEMA-mapped Special Flood Hazard Areas along Patuxent, Potomac, Bay frontages and tidal-creek systems; the county is an NFIP participant; (4) Historic Districts — Historic St. Mary's City buffer area, the Leonardtown Historic District (administered by the Town of Leonardtown for parcels inside town limits and by the county Historic Preservation Commission for parcels in the unincorporated vicinity), and several smaller rural historic districts; (5) the Sotterley Plantation National Historic Landmark and associated cultural-resource overlay; (6) Three Notch Trail and other planned-corridor overlays. St. Mary's County does NOT have a California-style coastal commission (Maryland has no coastal-commission analog; the Critical Area Act fills that role), does NOT have a CalFire-equivalent WUI overlay (Maryland has no statewide WUI overlay), and does NOT have a seismic-retrofit overlay. The AICUZ + Critical Area combination is the dominant county-specific regulatory feature.

• NAS Patuxent River AICUZ Overlay — Uniquely a St. Mary's County concern at this intensity — no other Maryland county hosts a primary operational naval air station of NAS Pax River's scale. The AICUZ overlay has been periodically updated as flight tracks and aircraft mix change; the most recent comprehensive AICUZ Study informs current overlay boundaries. The Joint Land Use Study (JLUS) process between the Navy, St. Mary's County, and the Maryland Department of Planning provides ongoing coordination.
• Chesapeake Bay Critical Area
• FEMA Special Flood Hazard Areas
• Historic District Overlays
• Federal Land and Adjacency

The Department of Land Use and Growth Management (LUGM) is the primary county permitting authority for unincorporated parcels in St. Mary's County. LUGM administers zoning, building, electrical, plumbing, mechanical, grading, sediment-control, and Critical Area permits, plus Floodplain Development Permits. The St. Mary's County Health Department (a local unit of the Maryland Department of Health) handles well and septic permits for parcels outside the MetCom service district — which is the majority of the county by parcel count, since MetCom service is concentrated in the Lexington Park / California / Great Mills / Leonardtown corridor and the Charlotte Hall / Wildewood areas, with the rural southern (Ridge, Saint Inigoes, Dameron, Piney Point) and northern (Mechanicsville, Loveville, Charlotte Hall fringe) county on private well and septic. A typical ADU permit bundle for an unincorporated parcel includes: (1) zoning permit and use confirmation by LUGM; (2) building permit with stamped residential plans; (3) trade permits (electrical, plumbing, mechanical) by Maryland-licensed contractors; (4) Health Department septic capacity evaluation (existing septic must accommodate the ADU load, or system must be upgraded — often a material cost on older systems); (5) Critical Area review where the parcel is within the 1,000-ft Chesapeake Bay Critical Area (which encompasses a substantial fraction of populated parcels given the county's extensive shoreline); (6) Floodplain Development Permit where mapped Special Flood Hazard Area applies (Patuxent River, Potomac River, Chesapeake Bay frontage, and numerous tidal-creek areas); (7) AICUZ (NAS Patuxent River) consistency review for parcels within the AICUZ overlay — Maryland Department of Natural Resources and Maryland Department of Planning coordination, plus Navy notification, may apply; (8) Historic Preservation Commission review where the parcel is in a designated historic district (Leonardtown vicinity, parts of Hollywood, the Historic St. Mary's City buffer area, and several small rural historic districts); (9) coordination with St. Mary's County Metropolitan Commission (MetCom) for parcels in the MetCom water/sewer service district. Inside the Town of Leonardtown, town permitting applies (the town has its own zoning and permitting; the county is not the issuing authority for parcels inside Leonardtown town limits).