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Gaithersburg, MD Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in Gaithersburg, MD, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

ADUs per lot: One per single-family lot
Detached size cap: 800 sq ft, or 1,200 sq ft with approval
Attached size cap: 49% of primary structure's floor area
Short-term rental: Prohibited in an ADU
Approval process: Minor amendment under § 24-12.6(J)
Historic district: Historic area work permit required

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Maryland under recent ADU streamlining laws. The Gaithersburg, MD rules below cover local size, height, and setback requirements.

Summary

Gaithersburg allows one accessory dwelling unit per single-family lot under section 24-5.2 of the zoning code. A detached ADU is capped at 800 square feet, or 1,200 square feet with planning commission site-plan approval, while an attached or integral ADU cannot exceed 49 percent of the primary structure's gross floor area. The unit cannot be rented short-term.

(C)Number restricted. Where accessory dwelling units are allowed, not more than one accessory dwelling unit shall be allowed on a lot, parcel, or tract of land associated with a single-family residential dwelling. ... (E)Applicable standards. ... (4)Accessory dwelling units that are integral or attached to the primary structure shall not exceed forty-nine (49) percent of the gross floor area of the primary structure;(5)Accessory dwelling units that are detached from the primary structure shall not exceed eight hundred (800) square feet in size or, with planning commission approval as a site development plan, no more than one thousand two hundred (1,200) square feet;...(13)The accessory dwelling unit shall not be used as a short-term rental as defined;

View official code

Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 52 Update 1).

Full Breakdown

13. The zoning code exempts ADUs from counting toward the maximum number of dwelling units approved on a conceptual, sketch, schematic, or site development plan. Only one ADU is allowed per lot associated with a single-family dwelling. Size limits differ by construction type: an ADU integral to or attached to the primary house cannot exceed 49 percent of the primary structure's gross floor area, while a detached ADU is capped at 800 square feet unless the planning commission approves up to 1,200 square feet as part of a site development plan.

4. 4(B)(2), cannot sit in the front yard, and cannot exceed the height of the primary structure except for a one-story bonus above a detached garage on a lot with a one-story principal house. 6. 5. 2(E)(13) bars using an ADU as a short-term rental, and a lot cannot be subdivided in a way that turns a detached ADU into the primary structure on its own lot unless all applicable subdivision and dimensional standards, including setbacks, are independently met.

Violations & Fines

Building or operating an ADU that exceeds the size caps, sits in the front yard, or is used as a short-term rental violates Chapter 24 and is enforced as a municipal infraction under section 24-15.1 and chapter 1, section 1-9. The city manager or designee can order an illegal ADU vacated or removed, and Planning and Code Administration investigates complaints. Fines run up to the council-set maximum, with an added penalty of up to $1,000 for nonpayment, and section 1-11 allows the city to lien the property for the cost of correcting the violation.

Frequently Asked Questions

How big can a detached accessory dwelling unit be in Gaithersburg?

Section 24-5.2(E)(5) caps a detached ADU at 800 square feet by right. The planning commission can approve up to 1,200 square feet if the ADU is reviewed and approved as part of a site development plan, but there is no path to anything larger.

Can I rent my Gaithersburg ADU on Airbnb?

No. Section 24-5.2(E)(13) specifically prohibits using an accessory dwelling unit as a short-term rental, so a homeowner cannot list the ADU on a nightly or weekly rental platform even though short-term rentals are separately permitted for the primary dwelling in several zones.

Do I need a permit to build an ADU?

Yes. New construction or an alteration to create an ADU requires a building permit under section 24-12.13, and the ADU itself is approved through a minor amendment application under section 24-12.6(J) rather than a full new site development plan.

Does a Gaithersburg ADU need extra parking?

Generally no, unless the property sits within a city parking permit program area. In that case section 24-5.2(E)(8)(a) requires one additional off-street space unless the board of appeals grants a waiver under section 24-7.6 after weighing construction cost, available curb space, and stormwater impact.

Sources & Official References

[1]Gaithersburg City Code, Chapter 24 - Zoning (Municode)

Other rules in Gaithersburg

Parking Rules: Overnight ParkingSome Restrictions Fence Regulations: Height LimitsSome Restrictions Shed & Carport RulesSome Restrictions Noise Ordinances: Barking DogsSome Restrictions Parking Rules: Commercial & Oversized VehiclesSome Restrictions Parking Rules: Snow Removal Parking RulesSome Restrictions Fence Regulations: Retaining WallsLight Restrictions

All Gaithersburg rules

How Gaithersburg compares: Easiest Cities to Build an ADU·Compare Gaithersburg to another location·View the Maryland accessory structures overview

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