Maryland ADU Setback Requirements 2026: Side & Rear Rules by County

How far does an ADU need to be from a property line in Maryland? There is no single statewide setback number. Under Land Use §4-504(d), a local government generally may not require a side or rear ADU setback greater than its existing accessory-structure setback. The practical distance can therefore be 2½ feet, 5 feet, 6 feet or another district-specific number. Start by identifying the property’s actual zoning authority with the Jurisdiction Finder; then use the local zoning district and accessory-structure rule to determine the setback that applies.

County-rule review: September 8, 2026. The October 1, 2026 local-law deadline has passed. The comparison below is dated research background; use the linked local guide and reviewing office to confirm current applicability. “Varies by district” means the rule is tied to the underlying zoning/accessory-structure table rather than one countywide distance. State setback provision rechecked October 6, 2026. Montgomery County’s effective ZTA 26-06 standards were verified separately on October 23, 2026; other county review dates are unchanged.

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

The article below is dated research background. Confirm current applicability with the reviewing authority.

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: accessory-structure standards now govern ADU setbacks

Ordinance No. 20-36 (ZTA 26-06) was adopted September 15, 2026 and became effective October 5, 2026. Under amended §59.3.3.3.A.2.c.v, an ADU must comply with the setback, height and building lot-coverage standards of an accessory structure in the underlying zone.

The ordinance removed the former detached-ADU rule tying the side setback to the principal dwelling and setting a 12-foot rear minimum. It also removed the separate ADU-specific increase for a length exceeding 24 feet along a side or rear lot line. Do not use those deleted provisions as current ADU requirements. The underlying zone’s accessory-structure standards still require a separate check; the amendment did not erase independent zone-specific rules.

Section 3.3.3.C.2.b retains the existing-structure exception: a structure legally constructed before May 31, 2012 that is not increased in size or building height and does not receive new windows on the wall nearest an abutting property may be used for a detached ADU without regard to setbacks or floor area. Ask DPS to confirm that the structure qualifies and how the exception applies to the proposed conversion.

See our Montgomery County ADU rules guide. For published R-60, R-90 and R-200 accessory-structure tables, independent height and length adjustments, and shed permits, use the Montgomery County setback requirements 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

CB-074-2026 was enacted September 29. Council lists November 16 as the effective date, with a commencement discrepancy requiring confirmation. CB-092-2026 remains pending. For setbacks, read the enacted ADU provisions together with the parcel’s zoning district and accessory-structure rules. Do not use a generic setback distance or assume the new bill is already effective.

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.

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

1. Confirm the jurisdiction. A municipal boundary can replace the county zoning rule entirely. Use the Maryland ADU Jurisdiction Finder first if you have not confirmed who controls zoning.
2. Find the zoning district. “Howard County” or “Anne Arundel County” alone may not produce the number.
3. Confirm detached vs. attached. Attached additions often use principal-building setbacks; detached ADUs commonly use accessory-structure standards.
4. Identify the actual property line. A fence, hedge or driveway edge is not proof of a legal boundary.
5. Check alleys and street-side yards separately. Baltimore County’s alley-centerline rule is a good example of a second measurement controlling the site.
6. Map easements and environmental constraints. Utility easements, septic reserve areas, wells, floodplains and buffers can create a larger practical setback than zoning.
7. For an existing garage, verify legal status and construction date. Existing-structure exceptions are highly date-specific.
8. 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 — signed Ordinance 20-36 (ZTA 26-06), effective October 5, 2026
• Montgomery County DPS — October 2026 ADU guidance
• 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.

Looking for shed setbacks rather than ADU setbacks?

An ordinary storage shed and a habitable accessory dwelling unit can be subject to different zoning, size and permit rules. For shed-specific county information, see Frederick County shed setbacks, Carroll County shed setbacks, and Harford County shed setbacks. Check whether your parcel is inside a municipality before relying on county rules.