Charles County now has an enacted 2026 ADU framework. Bill 2026-06 / ZTA 25-189 was adopted July 7, 2026 and became effective August 21, 2026. The law expands and modernizes ADU rules for attached, interior and detached units and replaces several older accessory-apartment restrictions.
Current status
🟢 ENACTED / ACTIVE — Bill 2026-06 is effective August 21, 2026.
Current baseline: one ADU per lot; attached/interior or detached; allowed where single-family detached housing is allowed; current single-family-detached ADU parking table says no additional off-street parking spaces are required; water/sewer or septic approval still applies.
1. Quick answer: can you build an ADU in Charles County?
Potentially, yes. Charles County’s new ADU law now permits an ADU with conditions on a lot, parcel or tract in a zoning district where a single-family detached residence is allowed, or where an existing single-family detached residence is a lawful nonconforming use.
| Issue | Current Charles County rule |
|---|---|
| Status | Bill 2026-06 enacted; effective August 21, 2026. |
| Number of ADUs | One ADU per lot, parcel or tract. |
| ADU types | Within or attached to the principal dwelling, or in a detached accessory building. |
| Lot size | ADU may be created on a lot of any size, subject to water/sewage requirements where public service is unavailable. |
| Maximum size under 3 acres | 1,000 sq ft or 75% of the principal dwelling floor area, whichever is smaller. |
| Maximum size at 3+ acres | 75% of the principal dwelling floor area. |
| Parking | Current table shows no additional off-street parking spaces required for the single-family-detached ADU use. |
| Water/sewage | Approval of the proposed water supply and sewage-disposal method is required before permit issuance. |
2. Where Charles County allows ADUs
The new 3.01.700 ADU use is written broadly around the single-family-detached use rather than a short list of legacy accessory-apartment zones. It applies on lots where single-family detached residences are allowed and can also apply where an existing single-family detached residence is a lawful nonconforming use.
Charles County’s July 7 meeting summary states that ADUs are permitted in any zone where single-family detached housing is permitted, with one ADU per lot. The County also states that the ADU use is not intended for the same lot as a duplex, triplex or multifamily apartment building.
3. Interior, attached and detached ADUs are now part of the County framework
The enacted ADU text allows the unit to be:
- wholly within the principal dwelling;
- attached to the principal dwelling; or
- in a detached accessory building on the same lot as the principal dwelling.
An ADU can be incorporated into an existing or new single-family detached dwelling. This is a major change from the older accessory-apartment framework, which contained more restrictive treatment of new detached ADUs.
For a detached project, however, do not assume that “detached allowed” means every backyard location works. Accessory-structure setbacks, zoning setbacks, lot coverage, easements, Critical Area/floodplain rules and private wastewater constraints can still control placement.
4. Charles County ADU size rules depend on lot size
The enacted 2026 framework uses a two-part size rule for single-family-detached ADUs:
- Property under 3 acres: the ADU may be no larger than 1,000 square feet or 75% of the principal dwelling floor area, whichever is smaller.
- Property 3 acres or larger: the ADU may be up to 75% of the principal dwelling floor area.
The law also limits the ADU bedroom count so it cannot exceed the number of bedrooms in the principal dwelling, and the detailed bill text imposes a separate maximum-bedroom standard.
For statewide context, see the Maryland 75% ADU Size Guide & Calculator.
5. Additional ADU parking was removed for the single-family-detached ADU use
Charles County’s adopted 2026 changes amended the parking table. The final bill materials show the old two-space requirement for the single-family-detached accessory-apartment use being replaced with no additional off-street parking spaces required for the ADU use.
This is one of the clearest homeowner benefits of the new law because it reduces the chance that an otherwise workable ADU project becomes infeasible purely because of an extra driveway or parking-pad requirement.
Townhouse ADUs use a different parking framework and should not be assumed to receive the same zero-additional-space treatment.
6. Water, sewer and septic approval still matter
Before issuance of a building permit and/or ADU permit, Charles County requires approval of the proposed method of water supply and sewage disposal for a new ADU, a conversion of existing space/accessory structure, or new detached construction.
For properties without public water and sewer, the County’s Health Department minimum-area and private-wastewater requirements can therefore still determine whether an ADU is feasible.
Use our Maryland ADU Septic & Well Guide.
7. Setbacks depend on whether the ADU is attached or detached
An ADU created wholly within the principal dwelling—or an attached ADU created by expanding that dwelling—must keep the principal dwelling compliant with the setbacks of the applicable zone.
Detached ADUs use separate setback provisions within the adopted text and also interact with the County’s accessory-structure regulations. Because the detailed requirement varies with property size and site context, this guide does not reduce the placement question to one universal number.
Before drawing a site plan, confirm the zoning district, front/side/rear setback rules, easements, lot coverage, environmental overlays and any private restrictions.
8. Charles County also created a limited townhouse-ADU path
Bill 2026-06 goes beyond Maryland’s minimum single-family-detached mandate by establishing a separate pathway for certain principal townhouse dwelling units with ADUs in specified zones and qualifying future developments.
The townhouse pathway has its own size, project-approval, design and parking rules. It should not be mixed with the single-family-detached ADU standards above.
For homeowners in an existing townhouse community, confirm that the development and unit actually qualify before relying on this provision.
9. Charles County ADU permit path
| Step | What to verify |
|---|---|
| 1. Confirm jurisdiction | Charles County zoning vs. incorporated municipality where applicable. |
| 2. Confirm principal use | Single-family detached dwelling allowed/existing under the applicable zoning path. |
| 3. Choose ADU form | Interior, attached or detached. |
| 4. Calculate size | Apply the under-3-acre or 3+-acre rule. |
| 5. Verify site constraints | Setbacks, lot coverage, easements, overlays and accessory-structure conditions. |
| 6. Obtain water/sewage approval | Especially critical for private well/septic properties. |
| 7. ADU / zoning approval | Zoning Administrator must determine compliance and issue the required ADU permit under the adopted framework. |
| 8. Building/trade permits | Complete building-code and inspection requirements before occupancy. |
10. What homeowners should verify before spending money
- Does Charles County or an incorporated municipality control zoning for this property?
- Is a single-family detached dwelling allowed on the lot, or does an existing home qualify as a lawful nonconforming use?
- Is the project interior, attached or detached?
- Is the lot below or above the 3-acre size threshold?
- What floor-area measurement is the County using for the principal dwelling?
- What detached-ADU setback applies to this specific parcel?
- Does the site have public water/sewer or private well/septic?
- Are Critical Area, floodplain, easement or HOA restrictions present?
- Which zoning/ADU permit and building permits are required for the chosen design?
11. Bottom line
Charles County is now one of Maryland’s clearest examples of an enacted 2026 ADU framework. Bill 2026-06 is effective, expands detached and attached options, removes the old extra parking requirement for the single-family-detached ADU use, and creates a property-size-based maximum that works alongside Maryland’s 75% standard.
The law still does not make every property automatically buildable. Zoning, setbacks, water/sewage approval, building code, environmental overlays and private restrictions remain property-specific checks.
Next step: use the Maryland ADU Jurisdiction Finder, then run the Property Feasibility Checker.
Primary sources used for this Charles County guide
- Charles County — enactment notice for Bill 2026-06 / ZTA 25-189
- Charles County — July 7, 2026 meeting highlights and enacted ADU changes
- Charles County Chapter 297 — signed Bill 2026-06 listed as a new law
- Charles County ADU public-engagement project page
- Maryland Department of Planning — HB 1466 ADU FAQ
Maryland ADU Guide is maintained by Gijo John as an independent research publication. This guide summarizes enacted local rules but does not establish parcel eligibility or replace County zoning, Health Department, building-code or professional review.