St. Mary’s County ADU Rules 2026: Size, Owner Occupancy, Parking & Septic

St. Mary’s County allows accessory apartments today, but its published local rules are not yet cleanly aligned with Maryland’s 2026 ADU baseline. The County’s current FAQ and zoning ordinance still describe a legacy framework with a 300-square-foot minimum, a 40%-or-900-square-foot maximum, owner occupancy, and one additional independently accessible parking space. Maryland HB 1466, however, requires local ADU laws by October 1, 2026 and says jurisdictions may not cap ADUs below 75% of the primary dwelling’s size or impose extra parking without a parking study and waiver process.

The safest way to use this page is to separate what St. Mary’s County still publishes today from what Maryland now requires local governments to authorize. I did not find a final St. Mary’s County 2026 ADU replacement ordinance in the official County materials reviewed for this update, so the page treats the County as a transition / source-conflict jurisdiction rather than assuming the legacy numbers remain the final 2026 standard.

Last verified: September 13, 2026. This page relies on St. Mary’s County’s current zoning ordinance, its official accessory-apartment FAQ, St. Mary’s County Health Department guidance and Maryland Department of Planning’s HB 1466 FAQ. Maryland ADU Guide is an independent research publication and is not legal, zoning, architectural, engineering or permitting advice.

St. Mary’s County ADU status at a glance

What St. Mary’s County currently publishes

The County’s official FAQ #24 says an accessory apartment is permitted in all zoning districts except the CC, I and OBP districts. It may be located in the principal dwelling or in a detached accessory structure, and only one accessory apartment is permitted per legally created lot or parcel when the Section 51.3.108 criteria are met.

For an accessory apartment in the principal dwelling, the County’s published standards say it must share a common wall with the principal dwelling, have at least 300 square feet, and generally not exceed 40% of the gross floor area or 900 square feet, whichever is less. The FAQ says an attached garage is excluded from the gross-floor-area calculation, while a basement accessory apartment may occupy the entire basement.

For an apartment inside a detached accessory structure, the County’s published rule likewise says the apartment may not exceed 40% of that accessory structure’s gross floor area or 900 square feet, whichever is less.

The size conflict: why St. Mary’s is a transition county

Maryland’s Department of Planning says HB 1466 requires counties and municipalities with planning and zoning authority to adopt local ADU laws by October 1, 2026. The state defines an ADU as a secondary dwelling up to 75% of the size of the primary single-family detached dwelling.

MDP goes further and says local jurisdictions may not limit ADU size below that 75% threshold. That is materially different from St. Mary’s County’s still-published 40%-or-900-square-foot legacy formula.

Does St. Mary’s County require extra ADU parking?

The County’s published accessory-apartment standard requires one additional, independently accessible parking space.

Maryland HB 1466 changed the framework for local parking mandates. MDP says a jurisdiction that wants to impose additional ADU parking must first complete a parking study that considers factors such as construction cost, curb availability, impervious-surface impacts and lot-size differences. The local law must also provide a parking-waiver process.

I did not find a final St. Mary’s County parking-study-based 2026 ADU ordinance in the official materials reviewed for this page. That means the legacy one-space requirement and the County’s final HB 1466 parking implementation should be confirmed directly before design.

Owner occupancy

St. Mary’s County’s published rule says an owner of the lot must occupy at least one of the dwelling units on the premises, subject to bona fide temporary absences determined by the Planning Director. The older zoning text also requires an owner affidavit before issuance of the Certificate of Use and Occupancy.

HB 1466 does not create one identical statewide owner-occupancy rule for every county. Until St. Mary’s County publishes or enacts a replacement rule, homeowners should treat the County’s existing owner-occupancy condition as an active local requirement unless LUGM confirms otherwise.

Setbacks and detached accessory structures

Maryland’s HB 1466 says local side and rear ADU setback requirements may not exceed the setbacks otherwise imposed on accessory structures. St. Mary’s County’s current zoning ordinance includes district-specific development standards and accessory-structure rules rather than one universal detached-ADU setback number.

For example, the current zoning ordinance notes a 5-foot minimum accessory-structure side or rear setback in CMX, while other zoning districts use different development standards. The practical rule is therefore parcel-specific: identify the zoning district, then apply the accessory-structure setback framework and any overlay restrictions.

Septic and well review can decide whether the ADU works

For many St. Mary’s County properties, wastewater capacity is likely to be one of the first feasibility questions. The County’s own accessory-apartment FAQ directs homeowners to the St. Mary’s County Health Department for well and septic requirements.

The Health Department says properties not served by public sewer must undergo a perc test before beginning construction of any structure with plumbing. For additions to existing homes, the department reviews existing septic records and may require a perc test and system upgrade if the proposal increases sewage flow.

For new construction, the Health Department also requires a site plan by a Maryland-registered land surveyor as part of septic review. Do this early; a zoning-compliant ADU can still fail or become more expensive if the septic reserve area, system capacity or groundwater conditions do not support the additional dwelling.

Critical Area rules are stricter

St. Mary’s County’s published accessory-apartment rules include additional requirements in the Resource Conservation Area (RCA). For a detached accessory apartment, the perimeter of the accessory structure must be within 100 feet of the principal dwelling, and the accessory apartment must be served by the same sewage-disposal system as the primary dwelling.

The older zoning text also states that an RCA accessory apartment must be 900 square feet or less in total enclosed area and may not require a Critical Area variance for its footprint, appurtenances or woodland clearing. Because the 900-square-foot figure intersects the broader 2026 state-size conflict, confirm how LUGM is applying the RCA provision to new ADU proposals under HB 1466.

What St. Mary’s County homeowners should do now

1. Confirm the zoning district. The published County rule excludes CC, I and OBP.
2. Confirm the 2026 size standard in writing. Do not design solely around the legacy 40% / 900-sq-ft cap without asking LUGM how HB 1466 is being implemented.
3. Ask about parking. Confirm whether the legacy additional-space rule is still being applied and whether a parking study and waiver process now control.
4. Verify owner occupancy. The County’s published rule requires an owner to occupy one dwelling.
5. Check accessory-structure setbacks. Detached ADUs should be tested against the zoning district’s accessory-structure rules.
6. Check septic or sewer capacity before final design. If the property is not on public sewer, contact Environmental Health early.
7. Check Critical Area status. RCA properties have extra location and sewage-disposal conditions.
8. Re-check before paying deposits. St. Mary’s is still a transition jurisdiction in our tracker because the published local rules and Maryland’s 2026 baseline do not yet read cleanly together.

Primary sources used for this St. Mary’s County guide

• St. Mary’s County LUGM — FAQ #24: Second Home / Accessory Apartment
• St. Mary’s County Comprehensive Zoning Ordinance
• St. Mary’s County — Comprehensive Planning and ordinance resources
• St. Mary’s County Health Department — Perc Testing
• St. Mary’s County Health Department — On-Site Septic Systems
• Maryland Department of Planning — HB 1466 ADU FAQ

For statewide comparison, use the Maryland County ADU Rules Tracker, and for private well or septic planning use our Maryland ADU Septic & Well Guide. Maryland ADU Guide is maintained by Gijo John as an independent research publication.