Does an ADU Have Its Own Address or a Unit Number?

Yes, an ADU can have its own address, but one is not assigned automatically when the unit is built. Most jurisdictions give you two options: a completely separate street number next to the main home’s, or a secondary unit designation like “Unit B” or “123½” attached to the existing address. Which one you get depends on your local government’s addressing rules, the physical setup of the property, and in some cases whether the ADU has its own entrance or driveway. Either way, you have to ask for it.

Separate Street Number or Unit Designation

The difference matters more than it looks. A separate street number treats the ADU almost like a standalone home on the block. A unit designation keeps the primary home’s number and adds a secondary identifier such as “Unit B,” “Apt 2,” or a fractional number like “123½.” Both distinguish the ADU from the main house for mail, deliveries, and emergency response, but they signal different things to utility companies, insurers, and sometimes lenders.

USPS recognizes standard secondary unit designators including “APT,” “UNIT,” “FL,” and “STE,” and requires them to appear on the delivery address line of mail when they apply.¹ Whether your ADU gets a full new street number or one of these designators depends entirely on your municipality. Some default to unit designators for any secondary structure on an existing parcel. Others assign a fresh street number when the ADU is detached and has its own entrance from the street.

What Determines Which One You Get

Local zoning and municipal addressing standards control the decision, and they vary widely. A few factors show up repeatedly:

Separate entrance or driveway. An ADU with its own street-facing entrance or an independent driveway is more likely to qualify for a distinct street number. Some jurisdictions actually require a separate address in that situation.
Detached versus attached. A freestanding backyard cottage is treated differently from a converted basement or garage apartment. Detached units with clear physical separation from the main home are stronger candidates for their own number.
Utility metering. If the ADU has or will have its own electricity, gas, or water meters, the utility company often needs a distinct address for billing. In some places this works in reverse: getting a separate address may trigger a requirement to install separate meters.
Rental use. When the ADU will be rented, a distinct address simplifies leases, tenant screening, and the tenant’s ability to establish residency for voter registration and driver’s license updates.

None of these factors guarantee a separate address on their own. The local planning or public works department makes the final call under its addressing ordinance.

How to Request the Address

The process is bureaucratic but not complicated. Most homeowners run through the same steps.

Find the right department. Address assignments are usually handled by the local planning department, public works department, or a dedicated addressing division within county government. A call to your city or county’s main line will point you in the right direction.
Submit an application. You will typically need a site plan showing the ADU’s location on the property, proof of ownership, and either a building permit or a permit application number. Some jurisdictions fold the address request into the building permit process itself.
Pay the fee. Most local governments charge a one-time processing fee, typically in the range of $50 to $200, though this varies.
Wait for review. Staff verify that the ADU meets local requirements, confirm the address fits the numbering grid, and assign the number or unit designator. Turnaround runs from a few days in smaller towns to several weeks in busier metropolitan areas.

If your ADU is already built and permitted but was never given a distinct address, you can request one after the fact through the same department. The process is the same; it just happens on a different timeline than the original construction.

Getting Mail to the New Address

An address assigned by your local government still has to exist in the USPS system before carriers will deliver to it. Local governments are responsible for reporting new addresses to USPS for inclusion in delivery routes.² That handoff doesn’t always happen automatically or quickly. If the address has been assigned but mail still isn’t arriving, contact your local post office directly. They can confirm whether the address is active in their system and escalate the issue if it isn’t.

For unit designators, USPS requires the secondary designation on the delivery address line. The pound sign (#) should only be used when the correct designator, like “APT” or “UNIT,” is unknown.¹ If a tenant’s mail keeps getting misdelivered, check that they are using the exact designator in the USPS database rather than a shorthand.

911 and Emergency Dispatch

This is where a separate address earns its fee. When someone at the ADU calls 911, dispatchers use databases that match the caller’s address to a physical location. If the ADU has no distinct address, dispatchers see only the main home’s address. That ambiguity costs time, and for a fire or medical emergency, time is the thing you have least of.

Local governments synchronize new addresses with the Master Street Address Guide and other databases that 911 centers rely on. Discrepancies between those systems can delay a response. Getting a separate address assigned through the proper municipal channel is the way to make sure the ADU is correctly represented. Putting a number on the door yourself accomplishes nothing if that number doesn’t exist in the dispatch database.

Insurance and Property Tax

Some insurers treat an ADU with a separate address or its own utility meters as a standalone structure, which may require its own policy rather than an endorsement on the existing one. The more independent the ADU looks on paper, the more likely the insurer is to treat it as a separate risk. Call your carrier before the unit is occupied.

Building an ADU generally raises your property tax bill because you’ve added living space and value. That happens whether or not the ADU gets its own address. The assessment typically reflects the added value of the ADU without reassessing the main home, though the specifics depend on local rules. A separate address does not create a separate tax parcel; the ADU stays part of the same lot on a single tax bill. The building permit is what alerts the assessor, not the address assignment.

A Separate Address Is Not a Separate Property

This is the most common misunderstanding, and it can lead to expensive mistakes. An ADU with its own street number is still legally part of the same parcel as the primary home. You cannot sell it on its own, finance it as a standalone property, or subdivide it from the main lot just because it has a different number. Selling the ADU independently would require a formal lot subdivision, which is a different process governed by local subdivision ordinances and typically far more restrictive than ADU rules.

Lenders and appraisers pay attention to this distinction. Whether a property is classified as a single-family home with an ADU or as a multi-unit property can affect loan terms, and a separate address, separate meters, and rental history all feed into that classification. If you plan to refinance or sell after building the ADU, talk to your lender about the addressing and metering setup early, so nothing surprises anyone at appraisal.