Town of Chevy Chase MD ADU Rules 2026: Ordinance 26-03, 75% Size & New Setbacks

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Last researched: September 19, 2026. The Town of Chevy Chase has already completed its 2026 ADU update. Ordinance No. 26-03 was introduced May 19, adopted unanimously on July 8, 2026, and became effective August 7, 2026. The ordinance revises the Town’s accessory-building rules in response to Maryland’s statewide ADU law.

Homeowner takeaway: The biggest change is setbacks. Chevy Chase previously imposed much larger side and rear setbacks on detached ADUs than on ordinary garages and sheds. Ordinance 26-03 removes those ADU-specific setback provisions so detached ADUs now use the Town’s ordinary accessory-building setback system, including height- and length-based increases. The ordinance also expressly caps an ADU at 75% of the size of the main building.

Town of Chevy Chase ADU planning snapshot

Question Current rule verified
ADU ordinance Ordinance 26-03, adopted July 8, 2026; effective August 7, 2026.
ADU definition A dwelling unit in the main building or in an accessory building on the lot, subordinate to the dwelling unit in the main building.
Maximum ADU size 75% of the size of the main building.
Interior-lot accessory building under 12 ft high 5 ft from side and rear lot lines.
Interior-lot accessory building 12 ft or more to highest point 7.5 ft from side and rear lot lines before any height/length increases.
Corner-lot accessory building Generally 5 ft from the side lot line and 10 ft from the rear lot line, before height/length increases.
Rear-yard coverage Accessory building may not occupy more than 25% of the rear yard.
Front-lot-line setback Interior-lot accessory building must be at least 60 ft from the front lot line.
Height-based setback increase For qualifying taller accessory buildings, add 1.5 ft of setback for each foot above the ordinance threshold.
Length-based setback increase If a side/rear building dimension exceeds 24 ft, add 1.5 ft of setback for each excess foot.
Accessory-building height Limited by the ordinance’s 15-ft highest-point / 12-ft mean-roof-height framework.
Nonconforming accessory building A detached ADU cannot be created in a nonconforming structure unless the structure meets the current Chapter 4 requirements at conversion.

Ordinance 26-03 is final law, not a draft

The Town’s official ordinance list records Ordinance 26-03 as adopted 5-0 on July 8, 2026. The signed ordinance states that it took effect August 7, 2026, 30 days after adoption.

That gives Chevy Chase homeowners something many Maryland municipalities still do not have in September 2026: a final local ordinance specifically revised for the new state ADU law.

The ordinance now uses Maryland’s 75% size framework

Ordinance 26-03 adds a direct rule to § 4-5:

An accessory dwelling unit may not be greater than 75% of the size of the main building.

That matches Maryland’s current statutory definition of an ADU as a secondary dwelling unit that is no greater than 75% of the primary single-family detached dwelling.

The Town ordinance does not make the calculation question disappear. Before choosing a floor plan, ask the Town and Montgomery County how the “size of the main building” will be measured for the specific permit application.

Use our Maryland ADU size guide to understand the statewide calculation before applying it locally.

The major change: detached ADUs no longer have a separate large-setback rule

Before Ordinance 26-03, the Town Code treated detached ADUs differently from ordinary accessory buildings. The older rule required much larger setbacks, including a rear setback of at least 20 feet or 15% of lot depth in some situations.

Ordinance 26-03 strikes those separate detached-ADU setback provisions from § 4-5.

The practical result is that the Town’s ordinary accessory-building setback system now applies without a special ADU penalty. That change directly responds to Maryland law, which prohibits ADU side/rear setbacks that are more restrictive than the jurisdiction’s existing accessory-structure setbacks.

Interior lots: 5 ft or 7.5 ft before height/length increases

For an interior lot, an accessory building must be in the rear yard, occupy no more than 25% of the rear yard and sit at least 60 feet from the front lot line.

The base side/rear setbacks depend on height:

  • if the accessory building is less than 12 ft high when measured from average pre-development grade to the highest point, the minimum side and rear setback is 5 ft;
  • if it is 12 ft or higher by that measurement, the base minimum side and rear setback is 7.5 ft.

Those are base setbacks. A taller or longer structure can require more distance under the additional formulas below.

Corner lots: 5-ft side / 10-ft rear base setbacks

For a corner lot, the Town defines the rear yard using both front building lines and the rear building lines of the main house.

The ordinance keeps a base accessory-building setback of:

  • 5 ft from the side lot line; and
  • 10 ft from the rear lot line.

Again, height and length can increase those minimums.

Tall accessory buildings require progressively larger setbacks

Garrett Park is not the only Montgomery County municipality using more nuanced accessory-building geometry. Chevy Chase’s ordinance increases setbacks when an accessory building gets taller.

For an accessory building that reaches the ordinance’s height trigger:

  • 12 ft or more for a flat roof; or
  • 15 ft or more to the ridge for other roof types;

the required side and rear setbacks increase by 1.5 ft for each foot of height above the applicable threshold.

The increased setback applies to the entire accessory building.

Long buildings also require more setback

If an accessory building has a linear dimension greater than 24 ft along a side or rear lot line, the setback increases by 1.5 ft for every foot that the building dimension exceeds 24 ft.

The height-based and length-based setback increases apply cumulatively.

This matters for ADU design. A long 800- or 1,000-square-foot rectangular footprint can lose more buildable width than a homeowner would expect from looking only at the 5-ft base setback.

Accessory-building height is tightly controlled

The ordinance says an accessory building may not exceed the greater of:

  • 15 ft measured from average pre-development grade in front of the structure to the highest point of the roof surface; or
  • 12 ft measured to the mean height between the eaves and ridge for a gable, hip, mansard or gambrel roof.

Because setback requirements also increase with height, roof design can materially change both the legal envelope and usable interior space of a detached ADU.

Only 25% of the rear yard can be occupied by the accessory building

Both interior- and corner-lot provisions retain a 25% rear-yard occupancy limit for an accessory building.

That means ADU feasibility cannot be determined from the 75% dwelling-size cap alone. A property can legally support a larger ADU by the state size calculation but still have a much smaller practical detached footprint because of rear-yard coverage, setbacks, building dimensions and existing accessory structures.

Nonconforming garages do not automatically become ADUs

Ordinance 26-03 also changes the Town’s developmental-nonconformity provision.

A nonconforming accessory structure can generally continue to be maintained, altered and repaired subject to the existing rules. But the ordinance says a detached ADU may not be constructed within a nonconforming structure unless the structure complies with Chapter 4.

The building containing the detached ADU must meet the required setbacks and other Chapter 4 standards at the time the ADU is created.

That is a major issue for an older detached garage sitting too close to a side or rear lot line. Legal existence as a garage does not automatically mean it is eligible for ADU conversion.

Chevy Chase requires its own Town permit in addition to County permits

The Town’s Building & Permitting page warns that the Town has its own setback requirements for detached ADUs and tells applicants to contact the Town Office.

The Town permitting process generally requires Town building-review materials in addition to Montgomery County permitting. Depending on the project, this can include:

  • Town building-permit application;
  • Montgomery County permit documentation;
  • certified house-location survey or plat;
  • construction drawings;
  • site/construction-management plans;
  • tree-protection materials;
  • water-drainage review; and
  • other Town-specific approvals.

Do not assume a Montgomery County ADU approval alone satisfies the Town’s Chapter 4 requirements.

Montgomery County still controls the underlying ADU zoning use

The Town of Chevy Chase is within the Maryland-Washington Regional District. Montgomery County zoning governs the underlying ADU land-use rules, while the Town exercises additional authority over residential building location, setbacks, dimensions, lot coverage, parking and similar local matters.

A homeowner therefore needs to reconcile both systems:

  1. Montgomery County ADU zoning/use and County permits; and
  2. Town of Chevy Chase Chapter 4 building/setback requirements.

What to ask the Town before paying for ADU plans

  • How will the Town calculate the 75% “size of the main building” limit?
  • Is my lot treated as an interior lot or corner lot?
  • What base side/rear setbacks apply to the proposed height?
  • Will the height-based setback increase apply?
  • Will the 24-ft length rule increase my setback?
  • How much of my rear yard is available under the 25% coverage rule?
  • Is my existing garage conforming under current Chapter 4 standards?
  • Can that existing structure legally be converted to a detached ADU?
  • What Town building permit is required?
  • What Montgomery County ADU approval and building permits are required?
  • What tree, drainage or construction-management approvals apply?

Town of Chevy Chase homeowner checklist

  1. Confirm the property is inside the Town of Chevy Chase, not another Chevy Chase municipality or unincorporated area.
  2. Confirm Montgomery County’s current ADU zoning/use standards.
  3. Measure the main building using the method accepted for the 75% rule.
  4. Determine whether the lot is interior or corner.
  5. Measure the proposed accessory-building height and side/rear dimensions.
  6. Calculate the base setback plus any height and length increases.
  7. Check the 25% rear-yard occupancy limit.
  8. If converting an existing garage, verify that the structure conforms to current Chapter 4 setbacks.
  9. Prepare the Town and County permit packages.
  10. Check tree-protection and drainage requirements.
  11. Do not order a prefab unit until the Town confirms the legal building envelope.

Town of Chevy Chase ADU FAQs

Has the Town of Chevy Chase adopted its 2026 ADU ordinance?

Yes. Ordinance 26-03 was adopted unanimously on July 8, 2026 and became effective August 7, 2026.

What is the maximum ADU size in the Town of Chevy Chase?

Ordinance 26-03 says an ADU may not be greater than 75% of the size of the main building. Confirm the accepted measurement method with the Town and Montgomery County before design.

What is the setback for a detached ADU?

There is no single number for every project. On an interior lot, the base side/rear setback is 5 ft for an accessory building under 12 ft high and 7.5 ft for one 12 ft or higher by the ordinance’s highest-point measurement. Height and building length can increase those setbacks. Corner lots use a 5-ft side and 10-ft rear base setback, again subject to increases.

Can I convert a nonconforming detached garage into an ADU?

Not automatically. Ordinance 26-03 says a detached ADU cannot be constructed within a nonconforming structure unless the structure meets current Chapter 4 setback and other requirements when the ADU is created.

Does Montgomery County or the Town issue the permit?

Both systems can apply. Montgomery County controls the underlying ADU zoning/use and County building permits, while the Town of Chevy Chase imposes additional local building, setback, lot-coverage, drainage, tree and related requirements.

Who should I contact about a Town of Chevy Chase ADU?

Start with the Town Office at 4301 Willow Lane, Chevy Chase, Maryland 20815. The Town lists 301-654-7144 and townoffice@townofchevychase.org.

Primary sources checked

Maryland ADU Guide is an independent research publication, not the Town of Chevy Chase, Montgomery County, M-NCPPC, a law firm, architect or contractor. This page is based on the final adopted Ordinance 26-03 and current Town permit guidance reviewed September 19, 2026. Confirm parcel-specific Montgomery County zoning and Town building requirements before spending money on design or construction.

YOUR NEXT PRACTICAL STEP

What to do next

Calculate the real building envelope—not just the 75% size cap—before choosing a detached ADU plan.

  1. Confirm the property is inside the Town of Chevy Chase.
  2. Confirm Montgomery County’s current ADU zoning/use standards.
  3. Use the Town setback rules to test height, length and rear-yard coverage.
  4. Continue to the Maryland ADU planning tools.

Planning guidance is not permit approval.