There is no universal 4-foot ADU setback in Maryland. Maryland Land Use §4-504(d) instead limits how restrictive a local ADU law can be: a county or municipality may not require a side or rear ADU setback greater than the setback it already applies to accessory structures. That means the number can be 2½ feet in one jurisdiction, 6 feet in another, or vary by zoning district.
Quick answer
Your Maryland ADU setback depends on the local accessory-structure rule—not a single statewide number.
HB 1466 controls side and rear lot-line setbacks. Front-yard placement, height, lot coverage, alleys, easements, septic areas, utilities and existing-structure rules can still create separate constraints.
Last verified: September 8, 2026. Several Maryland jurisdictions are still changing their ADU codes before the October 1, 2026 deadline. “Varies by district” means the official rule is tied to the underlying zoning/accessory-structure table rather than one countywide distance.
1. Maryland’s setback rule: what HB 1466 actually says
Maryland Land Use §4-504(d) says a local ADU law may not establish setback requirements that exceed the existing accessory-structure setback requirements from the side and rear lot lines.
That wording matters. Maryland did not enact a statewide 4-foot, 5-foot or 6-foot ADU setback. Instead, the state tells each jurisdiction to look at the setback it already applies to an accessory structure and prevents the ADU rule from being more restrictive on the side or rear.
2. Maryland ADU setback lookup: current verified treatment
| Jurisdiction | Side / rear ADU treatment | Existing-structure relief? | 2026 status |
|---|---|---|---|
| Frederick County | 6 ft minimum from a lot line under the general accessory-structure location rule, subject to exceptions and other applicable provisions. | Property-specific/nonconforming rules can matter. | 🟠 Current ADU system + streamlining transition |
| Baltimore County | 2½ ft minimum from side or rear lot lines for accessory buildings in residence zones; also at least 15 ft from the center line of an abutting alley. | Separate hearing/legacy accessory-apartment rules may apply. | 🟡 Legacy framework |
| Howard County | Varies by zoning district. Detached structures use the accessory-structure setbacks in the underlying district; CB3-2026 says ADUs must comply with applicable underlying setbacks. | Front-yard placement relief exists for certain pre-March-31-2026 detached structures on parcels under 10 acres. | 🟢 Enacted CB3-2026 |
| Anne Arundel County | Varies by residential zoning district. County guidance says detached ADU setbacks are the setbacks specified for accessory structures in that district. | Side/rear setback variance generally unavailable except conversion of a structure that existed before May 27, 2023. | 🟠 Transition under Article 18 update |
| Montgomery County | Current legacy detached-ADU rules have stricter ADU-specific setbacks. ZTA 26-06 proposes that an ADU comply with the accessory-structure setback in the underlying zone. | Proposed ZTA retains special treatment for certain legally built pre-May-31-2012 structures used as detached ADUs. | 🟠 ZTA 26-06 pending |
| Baltimore City | Pending 26-0195 would allow an ADU to occupy required rear-yard area and list an ADU as a permitted rear-yard encroachment. Final side/rear administration is not yet enacted. | Existing carriage-house and nonconforming-structure pathways are separate from the pending ADU bill. | 🟠 Council Bill 26-0195 pending |
| Chesapeake Beach | 5 ft minimum from a side or rear property line under the Town’s current completely detached accessory-building rule. The 2026 ADU draft separately proposes 8 ft between a detached ADU and the principal house. | No separate existing-structure setback relief verified; draft adds an alley-access rule for conversions. | 🟠 Municipal transition |
| Prince George’s County | Final HB 1466-aligned ADU setback rule not yet verified from enacted text. | Do not rely on draft language as final. | 🟠 CB-074-2026 transition |
Why some rows do not show a number: many zoning codes set accessory-structure setbacks by zoning district, lot type, structure height, alley condition or other variables. Publishing one countywide number where the code does not create one would be misleading.
3. Frederick County: the cleanest 6-foot example
Frederick County Code §1-19-4.300.1 says no accessory structure may be erected within 6 feet of any lot line or within 6 feet of another building on the lot, except where another provision applies. Frederick’s ADU code requires an ADU located in or built as an accessory structure to comply with the County’s accessory-structure requirements.
So for a normal detached ADU planning screen in unincorporated Frederick County, 6 feet is a meaningful first-pass side/rear lot-line number—but it is not the entire feasibility test. Front-yard rules, accessory-structure footprint limits, zoning district rules, wells, septic reserve areas, easements and the pending 2026 ADU streamlining work can all change the usable envelope.
See our Frederick County ADU Rules deep dive.
4. Baltimore County: 2½ feet from the lot line, but watch the alley rule
Baltimore County’s general accessory-building rule is unusually easy to state. BCZR §400.1 says accessory buildings in residence zones generally may not be located less than 2½ feet from a side or rear lot line.
But §400.2 adds another urban/suburban constraint: accessory buildings, including parking pads, must be at least 15 feet from the center line of an alley abutting the lot. A structure can therefore satisfy the 2½-foot property-line rule and still fail the alley setback.
See the Baltimore County ADU transition guide.
5. Howard County: there is no single countywide ADU setback
Howard County’s adopted CB3-2026 requires ADUs to comply with applicable setbacks in the underlying zoning district. Howard’s resident guidance separately explains that a detached structure must meet the accessory-structure setbacks and maximum lot-coverage requirements of its zoning district, while a structure attached to the principal dwelling uses principal-structure setbacks.
This is why a blanket statement such as “Howard County ADUs need a 5-foot setback” would be unsafe. The correct number starts with the parcel’s zoning district.
CB3-2026 also created a front-yard nuance: on a parcel under 10 acres, a detached ADU may be in front of the principal dwelling only when it is within a detached structure that existed before March 31, 2026. That is a front-placement rule, separate from the state’s side/rear setback ceiling.
Use our Howard County ADU guide for the enacted framework.
6. Anne Arundel County: accessory-structure setback by zoning district
Anne Arundel County’s current ADU guidance directly states that the setbacks for a detached ADU are the setbacks specified for accessory structures in each zoning district. There is therefore no single Anne Arundel number that can safely replace the district lookup.
The County also has an important existing-structure rule: it says no variance may be granted to a side or rear lot-line setback to accommodate an ADU except for conversion of an existing structure that was in existence before May 27, 2023.
This can make an older garage fundamentally different from a brand-new detached ADU in the same neighborhood. See our Anne Arundel County transition guide.
7. Montgomery County: ZTA 26-06 is removing the ADU-specific setback penalty
Montgomery County is one of the clearest examples of why HB 1466’s setback provision matters. County Council staff identified existing ADU setback requirements as one of the local provisions inconsistent with State law.
The introduced ZTA 26-06 says that, unless an ADU-specific standard modifies the rule, an ADU must comply with the setback, height and lot-coverage standards of an accessory structure in the underlying zone. For a detached ADU built after May 30, 2012, the proposal changes the side and rear requirements to the same minimum setbacks as the accessory structure.
The same proposal preserves a valuable existing-structure exception: a structure legally constructed before May 31, 2012 that is not enlarged in size or height and does not add new windows on the wall nearest an abutting property may be used for a detached ADU without regard to setbacks or floor area.
See our Montgomery County ZTA 26-06 transition guide.
8. Baltimore City: pending bill proposes major rear-yard relief
Baltimore City Council Bill 26-0195 is still pending. Its first-reader text proposes that an ADU may occupy area otherwise required as rear yard and pervious surface, and it adds an ADU as a permitted rear-yard encroachment.
That could be a major urban-lot change if enacted, but it should not yet be converted into a numerical City ADU setback. The bill can still be amended before final action. Existing accessory-structure, carriage-house, building-code and CHAP rules also continue to matter.
Read the Baltimore City ADU Rules 2026 guide.
9. Prince George’s County: do not design to draft setbacks yet
Prince George’s County’s CB-074-2026 is the active zoning vehicle for ADU implementation, but the final enacted setback method has not yet been verified in our research. Until final text is adopted, homeowners should not design a detached ADU from an introduced or committee-stage setback number.
Use the Prince George’s County ADU transition guide and re-check the County Council record immediately before design.
10. Existing garages can have different setback treatment than new ADUs
A recurring Maryland pattern is that an existing legally built accessory structure may have a better path than demolishing it and constructing a new ADU in the same location.
| Jurisdiction | Existing-structure issue to check |
|---|---|
| Anne Arundel | Side/rear variance exception for conversion of certain structures existing before May 27, 2023. |
| Montgomery | Pending ZTA 26-06 preserves setback/floor-area relief for certain legally built structures predating May 31, 2012. |
| Howard | Certain existing detached structures predating March 31, 2026 can qualify for front-of-principal-dwelling placement on parcels under 10 acres. |
| Baltimore City | Qualifying carriage houses existing on June 5, 2017 have a separate conversion pathway under §15-504. |
If you are reusing a garage, read Can I Convert My Garage Into an ADU in Maryland? before assuming new-construction setback rules control.
11. How to measure an ADU setback before paying for plans
- Confirm the jurisdiction. A municipal boundary can replace the county zoning rule entirely.
- Find the zoning district. “Howard County” or “Anne Arundel County” alone may not produce the number.
- Confirm detached vs. attached. Attached additions often use principal-building setbacks; detached ADUs commonly use accessory-structure standards.
- Identify the actual property line. A fence, hedge or driveway edge is not proof of a legal boundary.
- Check alleys and street-side yards separately. Baltimore County’s alley-centerline rule is a good example of a second measurement controlling the site.
- Map easements and environmental constraints. Utility easements, septic reserve areas, wells, floodplains and buffers can create a larger practical setback than zoning.
- For an existing garage, verify legal status and construction date. Existing-structure exceptions are highly date-specific.
- Get the zoning interpretation in writing before final design. This is especially important in jurisdictions with pending 2026 legislation.
12. Bottom line
The Maryland ADU setback rule is not “4 feet.” The state-law protection is that a jurisdiction cannot make the ADU’s side or rear setback more restrictive than its existing accessory-structure setback. The homeowner still has to identify the local zoning district and then check every other site constraint that can control placement.
For a whole-property screen, use the Maryland ADU Property Feasibility Checker. For local-law status, use the Maryland County ADU Rules Tracker.
Primary sources used for this setback guide
- Maryland Land Use §4-504(d) — side/rear setback ceiling
- Howard County — resident bulk/setback guidance
- Howard County — final CB3-2026 legislative record
- Anne Arundel County — ADU Facts & Guidelines
- Montgomery County — ZTA 26-06 official packet
- Frederick County Code §1-19-4.300.1 — accessory-structure location
- Frederick County Code §1-19-8.321 — larger ADUs/accessory-structure compliance
- Baltimore County Zoning Regulations §§400.1–400.2
- Baltimore City Council Bill 26-0195
- Prince George’s County — CB-074-2026 legislative record
Maryland ADU Guide is maintained by Gijo John as an independent research publication. We prioritize enacted statutes, codified zoning rules, legislative records and official planning guidance. Source-backed corrections can be sent to gijo.j@marylandaduguide.com.