The short answer: An ADU’s address is assigned by the local addressing office, usually your city or county building department, and most often as part of the building permit. San José assigns an address to every ADU and JADU during the permit process and charges $454 with a two-hour minimum per addressing request; San Diego assigns addresses during plan check. The new number has to be visible from the street for emergency responders. Mail delivery is a separate step: in San Diego, the applicant must notify the post office.
Last verified: October 29, 2026 · Written by Marc
This guide summarizes the published addressing rules of two California cities, read on October 29, 2026. It is general information, not legal advice. Every city and county sets its own addressing rules, so ask your building department how it handles ADUs.
An address seems like a detail until it is missing. Without one, a tenant cannot get mail, a utility account may be hard to open in the tenant’s name and, more seriously, emergency responders may go to the wrong door. Owners often discover the issue late, after the unit is finished. The good news is that, in many cities, the address is already part of the permit process if you know to ask for it. This guide explains who assigns ADU addresses, the two common formats, what two California cities require and how to get mail and utilities working.
Who assigns an ADU address
Street addresses are assigned by a local addressing authority, not by the post office or the owner. In most cities that is the building or development services department; in unincorporated areas it is usually the county. The addressing office places the new number consistently with the street grid and tells other agencies about it.
San José describes the process clearly. New addresses are assigned consistent with existing addresses and the city-wide numbering grid. Once an address is assigned, city staff prepare an addressing notice and send it to the US Post Office, the Santa Clara County Assessor, utility companies, emergency services and city departments. The city also warns that failing to apply for an address assignment may delay the building permit.
Unit number or new street number
ADUs usually get one of two formats:
| Format | Example | Typical use |
|---|---|---|
| Unit number on the main address | 123 Main Street, Unit 2 | The most common approach for ADUs on the same lot |
| Separate street number | 125 Main Street | Sometimes used for detached units with their own frontage or access |
The choice is the addressing office’s, not the owner’s. Cities also set rules on the format. San José, for example, stopped allowing letters or hyphens in addresses in 1984, so formats such as “12A Elm Street,” “27-B Main Street” or “Unit A” are not used. Its ADU guidance says an ADU will be given a unit number.
How two California cities handle it
San José
San José publishes its ADU addressing rules in detail:
- Every ADU and JADU gets an address. The city’s ADU FAQ says an address will be assigned to all ADUs and JADUs as part of the building permit process.
- The request form. Owners who need an ADU address complete Form 302, the Address Assignment Request.
- The fee. A fee of $454.00 with a two-hour minimum is charged per request, and the minimum is collected when the application is submitted.
- Appointments. Submitting a request package requires an appointment, booked through the city’s addressing email.
- Visibility. Plans will not be approved unless the address numbers of both the main home and the ADU are visible and legible from the street, citing section 505 of the California Fire Code. The ADU’s unit number must also be placed next to the ADU’s entry.
Our guide to San José ADU requirements covers the rest of the city’s permit process.
San Diego
San Diego handles addresses through its Development Services Department:
- During plan check. If a building permit application has been submitted, an address will be assigned during the plan-check process.
- Fee. New addresses tied to a building plan check are charged a flat fee per address assigned, listed in the city’s Information Bulletin 501.
- Requests outside a permit. A separate street address request needs a letter with your contact information, the property’s legal description, the assessor’s parcel number, the current addresses, the reason for the request, any recent permit number and the owner’s or agent’s signature.
- Post office. The city says the applicant is responsible for notifying the US Post Office.
Our guide to San Diego ADU rules and fees explains the city’s other ADU fees.
Common mistake: waiting until the ADU is finished to ask for an address. In both cities above, the address is assigned during the permit or plan check process, and San José warns that skipping the request can delay the permit. Ask about the address when you submit plans, not at final inspection.
Why the address must be visible
The visible address is a safety rule, not a formality. San José’s ADU guidance ties it directly to emergency response: responders must be able to see both the main house’s number and the ADU’s unit number from the street, and the unit number must also appear at the ADU’s door.
For a detached ADU in a backyard, that often means two signs: one at the street showing both numbers, or an arrow toward the ADU, and one at the ADU entry. Your city’s fire department may publish its own guidelines for size and placement; San José refers applicants to its Fire Department’s guidelines.
Practical tip: Mark the path to a backyard ADU as well as the number. A small sign at the side gate, “Unit 2, rear,” helps delivery drivers and emergency crews find the door at night. Ask your fire department whether it has placement rules before you install anything permanent.
Mail, packages and utilities
Once the address exists, a few practical steps make it work:
- Mail delivery. Confirm with your local post office that the new address is in its system and where the mailbox for the ADU should go. In San José the city notifies the post office; in San Diego the applicant must.
- Mailbox. Many ADU owners add a second mailbox next to the main one, labeled with the unit number.
- Utilities. If the ADU has its own meters, the utility account needs the ADU’s address. If it shares meters, the address still helps for service calls. See our guide to ADU utility meters.
- Online maps and delivery apps. New addresses can take time to appear. Tenants may need to add delivery instructions in the meantime.
- Lease and records. Put the full ADU address in the lease, the insurance policy and your tax records. Our guide to ADU lease basics lists the other clauses to include.
A checklist for getting an ADU address
- Ask the building department how it assigns ADU addresses and whether a form is required.
- Submit the address request with, or during, the permit application.
- Pay the addressing fee, if any.
- Show both address numbers on the site plan, visible from the street.
- Install the numbers before final inspection.
- Confirm the address with the post office and set up a mailbox.
- Give the address to the utility companies, your insurer and the tenant.
Frequently asked questions
Does an ADU need its own address?
Often yes, and some cities require it. San José says an address will be assigned to all ADUs and JADUs as part of the building permit process. Even where it is not required, a separate address or unit number helps with mail, utilities and, most importantly, emergency response. Ask your building department how it handles ADUs.
Who assigns an address to an ADU?
Your local addressing authority, usually the city’s building or development services department, or the county in unincorporated areas. The post office does not assign street addresses. In San José, city staff assign the number based on the street grid and notify the post office, the county assessor, utility companies and emergency services.
How much does an ADU address cost?
It depends on the city. San José charges $454.00 with a two-hour minimum per addressing request, collected when the application is submitted. San Diego charges a flat fee per address assigned during plan check, listed in its Information Bulletin 501. Ask your city whether it charges a separate addressing fee.
Will my ADU get a unit number or a new street number?
That is decided by the addressing office, not the owner. Many ADUs get a unit number on the main address, such as “Unit 2,” while some detached units receive a separate street number. Cities also set format rules: San José does not allow letters or hyphens, so an ADU there gets a numbered unit, not “Unit A.”
How do I get mail delivered to my ADU?
After the address is assigned, confirm with your local post office that it is in the delivery system and where the ADU’s mailbox should go. In San José the city sends the addressing notice to the post office; in San Diego the applicant is responsible for notifying it. A labeled second mailbox avoids mixed-up mail.
The bottom line
To get an ADU address, ask your building department during the permit process, not after construction. In San José, every ADU and JADU gets an address with the building permit, the addressing request costs $454 with a two-hour minimum and both numbers must be visible from the street. In San Diego, addresses are assigned during plan check, and you must notify the post office yourself. Either way, the number protects your tenant as much as their mail.
Your next step is to ask your city how it assigns ADU addresses and add the request to your permit submittal. Our ADU permit application checklist shows where it fits among the other documents.
Sources
- City of San José: Address Assignment
- City of San José: ADU FAQs
- City of San Diego: Street Address Request or Change
General information, not legal advice. Local rules vary and change; confirm with your planning department.
Cover photo: “SD17 Gallery of Houses (36813107383)” by U.S. Department of Energy Solar Decathlon, public domain, via Wikimedia Commons.
Questions or a correction?
If something in this guide is unclear, out of date or does not match what your city told you, send me a message. Include the source if you have one.