Gaithersburg has its own ADU zoning rules, separate from Montgomery County, and its current code is not yet a clean match for Maryland’s 2026 ADU framework. The City allows attached, integral and detached accessory dwelling units, but the current zoning ordinance still uses size, parking and setback rules that should be read alongside HB 1466 before a homeowner pays for final plans.
The safest way to plan is to treat Gaithersburg as its own zoning jurisdiction. Montgomery County’s ZTA 26-06 does not automatically replace the City’s Chapter 24 zoning ordinance. Gaithersburg comprehensively revised Chapter 24 through its Retool Gaithersburg process, approved July 15, 2024, and the City continues to administer its own development review and zoning system.
2026 alignment check · Last verified September 13, 2026
🟠 Transition / Source Conflict — current City rules remain more restrictive than Maryland’s HB 1466 framework.
Gaithersburg’s current code still caps attached/integral ADUs at 49% of the primary structure, detached ADUs at 800 sq ft unless a site development plan allows up to 1,200 sq ft, requires one extra off-street parking space, and uses a 10-foot detached side/rear setback unless modified. Maryland’s HB 1466 guidance uses a 75% size standard and limits local parking and setback restrictions.
Last verified: September 13, 2026. We did not verify a newer enacted Gaithersburg ordinance that fully resolves these HB 1466 differences. This page therefore separates the current codified City rules from the statewide 2026 alignment question.
Gaithersburg ADU status at a glance
| Question | Verified status |
|---|---|
| Does Gaithersburg control its own zoning? | Yes. The City administers its own Chapter 24 zoning ordinance and development-review process. |
| Are ADUs allowed? | Yes. The current ordinance includes integral, attached and detached ADUs subject to Section 24-5.2. |
| Attached/integral size | Currently capped at 49% of the gross floor area of the primary structure. |
| Detached size | Generally 800 sq ft, or up to 1,200 sq ft with Planning Commission approval through a site development plan. |
| Parking | 1 additional off-street space is required for the ADU under the current ordinance. |
| Detached setback | Generally 10 feet from side and rear lot lines, subject to existing-garage and site-development-plan exceptions. |
| Short-term rental | Prohibited for the ADU under Section 24-5.2. |
| 2026 alignment | Not fully resolved. Current City size, parking and setback rules should be reconciled with Maryland HB 1466. |
The biggest issue: Gaithersburg’s current size limits vs. Maryland’s 75% rule
Gaithersburg’s current Section 24-5.2 says an integral or attached ADU may not exceed 49% of the gross floor area of the primary structure. A detached ADU may not exceed 800 square feet, although the Planning Commission may approve up to 1,200 square feet through a site development plan.
Maryland Department of Planning’s HB 1466 guidance says local jurisdictions required to authorize ADUs may not limit qualifying ADUs below the state’s 75% of the primary dwelling threshold. Because Gaithersburg’s current 49% attached limit and 800/1,200-sq-ft detached limits can fall below that threshold, homeowners should not assume the older numeric limits are the final 2026 compliance position without confirming with the City.
Does Gaithersburg require ADU parking?
Yes, under the current zoning ordinance. Section 24-5.2 requires one off-street parking space for the ADU in addition to the parking required for the primary structure, and Article 7 lists one space per accessory dwelling unit.
Maryland’s HB 1466 guidance says a jurisdiction that imposes additional ADU parking must have the required parking study and provide a waiver process. We did not verify a newer Gaithersburg ADU-specific parking waiver provision in the current materials reviewed, so this is another point to confirm before design.
What are the detached ADU setback rules?
The current City rule generally requires a detached ADU to be at least 10 feet from side and rear lot lines. There are exceptions: an ADU above a detached garage may use the garage’s applicable or approved existing setbacks, and the Planning Commission may approve a smaller setback through a site development plan.
Detached ADUs are generally not permitted in the front yard. On multi-frontage lots, the code provides a limited side-yard pathway where the yard is not coincident with the front of the building. Detached and attached ADUs also may not exceed the height of the primary structure.
Maryland HB 1466 separately says local side and rear ADU setbacks may not exceed the setbacks that otherwise apply to accessory structures. That makes the City’s 10-foot rule another 2026 verification point.
Can the ADU be used as a short-term rental?
No under the ADU-specific zoning rule. Section 24-5.2 expressly states that an accessory dwelling unit may not be used as a short-term rental.
This is important because Gaithersburg otherwise has a separate citywide short-term-rental licensing program. The general STR program does not override the ADU-specific prohibition in the zoning ordinance.
Permits, historic districts and site-plan review
Gaithersburg’s Planning Division administers development review and zoning. Depending on the project, a homeowner may need zoning review, building permits and trade permits, and a detached unit seeking more than 800 square feet or a reduced setback may require Planning Commission approval through a site development plan.
Properties in a local historic district can trigger additional review. Section 24-5.2 specifically requires historic-area-work-permit review in certain cases involving non-contributing properties and structures.
Why Montgomery County rules do not automatically control inside Gaithersburg
Gaithersburg is an incorporated municipality with its own zoning ordinance. That means a property with a Montgomery County mailing address may still be governed by Gaithersburg’s Chapter 24 rather than the County’s ADU zoning provisions.
Use our Montgomery County ADU guide only for properties actually governed by Montgomery County zoning. If you are unsure which authority controls your parcel, start with the Maryland ADU Jurisdiction Finder.
What Gaithersburg homeowners should verify before spending money
- Confirm that the property is inside Gaithersburg city limits.
- Ask Planning how the City is applying HB 1466 in 2026 to the current 49% attached and 800/1,200-sq-ft detached caps.
- Confirm parking compliance and whether an HB 1466-compliant waiver process applies to the property.
- Confirm the detached setback and whether the 10-foot rule or a site-development-plan modification controls.
- Check historic-district status before assuming ordinary permit review is sufficient.
- Do not plan Airbnb-style use. The ADU-specific zoning provision prohibits short-term rentals.
- Re-check the current City ordinance before final design. Gaithersburg notes that newly adopted ordinances may not immediately appear in the codified online code.
Primary sources used for this Gaithersburg guide
- City of Gaithersburg — Chapter 24 Zoning Ordinance
- City of Gaithersburg — Development Review & Zoning
- City of Gaithersburg — City Code & Regulations
- City of Gaithersburg — Short-Term Rentals
- Maryland Department of Planning — HB 1466 ADU FAQ
For statewide comparison, use the Maryland County & City ADU Rules Tracker. Maryland ADU Guide is maintained by Gijo John as an independent research publication. Verify property-specific requirements with the City before relying on this page for design or construction decisions.
What to do next
Start by finding the office that reviews your property. A mailing city or a county guide does not establish your parcel’s authority.
- Find Who Reviews My Property.
- Obtain your parcel identifier, survey, floor plans and utility records.
- Ask the reviewing office which current rules, supporting documents and approvals apply.
- Continue to your planning workspace.
Enable JavaScript to prepare a downloadable next-step brief. Planning guidance is not permit approval.