Short answer: No. Building an accessory dwelling unit in Maryland does not automatically give it a second street address, separate mailbox or independent 911 address. Maryland’s statewide ADU law does not establish one universal addressing rule, so this is a local verification item.
That distinction matters before you order signs, set up tenant mail, arrange insurance or utilities, or assume a detached backyard ADU will be treated as a second separately addressed house.
Maryland’s ADU law does not create a statewide second-address rule
Maryland Land Use Article §4-501 defines an ADU as a secondary dwelling unit on the same lot, parcel or tract as a primary single-family detached dwelling. The statewide framework promotes ADUs and requires local governments with planning and zoning authority to authorize them, but it preserves substantial local authority. It does not say that every ADU must receive—or is entitled to receive—a separate postal street address.
For homeowners, the practical rule is simple: do not assume that detached means separately addressed.
Anne Arundel County: same address, designated Unit B
Anne Arundel County provides a useful example of how specific a local rule can be. Its current ADU code requires an ADU to use the same street address as the principal single-family detached dwelling and be designated “Unit B.” The county’s ADU guidance repeats that requirement.
So a detached ADU in Anne Arundel County should not be assumed to receive an entirely new street number merely because it is a separate building.
Frederick County also illustrates why you must check locally
Frederick County’s addressing FAQ says accessory apartments use the same address as the principal dwelling. It also explains that official address changes must come through the county addressing process before owners update organizations such as utilities, the U.S. Postal Service or Maryland assessment records.
That is an important sequence: an owner should not simply invent “Unit B,” “Apt 2,” a half-number or another street number and begin using it as though it were an officially assigned address.
Havre de Grace: no separate address under the local ADU rule
Havre de Grace provides another clear example. Its ADU provisions state that no separate address will be assigned to the ADU. This reinforces the larger Maryland lesson: addressing can be governed by the county or municipality that has authority over the property, and the answer can be different from what a homeowner expects based only on the physical design of the ADU.
What about a separate mailbox?
A postal mailbox and a local-government address are related, but they are not the same decision. First establish the official address or unit designation with the appropriate local addressing authority. Then verify delivery setup with the U.S. Postal Service rather than assuming that adding another mailbox creates a recognized second delivery address.
If your jurisdiction assigns a unit designator, use that exact official format when setting up mail, leases and other records. A homeowner should avoid creating an unofficial unit number solely for convenience.
Is a 911 or emergency-services address different?
Potentially. Addressing systems are also used for public safety, permitting and geographic information systems. The agency that assigns or validates an address can therefore involve county or municipal GIS/addressing staff, planning, permitting or public-safety processes—not only the post office.
For a detached ADU, especially one with a separate entrance or a location that is not obvious from the street, ask the local addressing authority how the unit should be identified for emergency response. Do not assume that a mailing convention is automatically the same thing as an emergency-location record.
The four things Maryland homeowners should verify
- Official property address: Will the ADU share the main dwelling’s street number, receive a unit designation, or receive another officially assigned identifier?
- Mail delivery: After the official addressing decision, what format does USPS recognize for delivery to the ADU?
- Emergency identification: How should occupants identify the ADU to 911/fire/EMS, and does the local addressing or GIS office require anything additional?
- Consistency: Use the official designation consistently on permits, leases, insurance and utility records where applicable.
Who should you contact?
Start with the county or municipality that has land-use authority over your property. Depending on the jurisdiction, addressing may be handled by planning and zoning, GIS/addressing, permitting or another local office. After confirming the official designation, contact USPS about mail-delivery questions.
If you are not sure which county or municipality controls your property, use our Maryland ADU Jurisdiction Finder, then check the corresponding guide in our Maryland ADU Rules Tracker.
Address, utilities and legal status are separate questions
A separate address does not by itself make an ADU a separately owned parcel, and sharing an address does not mean the ADU cannot function as an independent dwelling unit. Likewise, address assignment should not be confused with whether the ADU receives a separate water, sewer or electric meter.
For the utility side of the project, see our Maryland ADU Utilities Guide.
Bottom line
A Maryland ADU does not automatically get its own street address or mailbox. Current local examples show that some jurisdictions explicitly require the ADU to share the principal dwelling’s address, sometimes with a unit designation such as Unit B. Confirm the official local addressing decision first, then verify postal delivery and emergency-addressing details with the appropriate agencies.
Maryland ADU Guide is an independent educational resource. This page is for general planning information and is not legal, postal, insurance or permitting advice. Local rules and administrative practices can change, so verify property-specific requirements with the responsible agencies.
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.
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