Homeowners adding an ADU or rental unit typically choose between three common metering approaches: a separate utility service and meter for the ADU, a shared main meter with a dedicated subpanel feeding the ADU, or a master meter with submeters where local rules permit. Which option is allowed and practical depends on your utility and local jurisdiction, so contact both early and get written guidance and cost estimates.
Key safety and inspection items: neutrals must be isolated at subpanels (no neutral‑to‑ground bond at a subpanel), feeders to a subpanel must be four conductors (two hots, one neutral, one equipment grounding conductor), and detached structures or new services may trigger grounding electrode requirements under NEC Article 250—address these in your design and permit package.
Metering options: pros, cons, and where rules matter
1) Separate utility service and meter (independent service)
A separate utility service and meter gives clear electrical separation and independent billing for the ADU, but it typically requires the utility to approve a new service and may trigger a load assessment or service upgrade. Utilities vary in practice; some (for example SDG&E’s builder guidance) routinely expect separate meters or coordinated meter work for detached dwellings or ADUs, so contact the utility early for a formal assessment and written estimate.
Cited guidance: use the utility’s new-service process to determine whether a separate meter or service upgrade is needed and to get an on‑site estimate when required[1][2].
2) Shared main meter with a dedicated subpanel feeding the ADU
A shared main meter with a dedicated subpanel avoids the utility meter installation cost and keeps a single service, but it complicates tenant billing and may be prohibited for landlord billing in some jurisdictions. If you plan to bill tenants, verify state and local rules because some places require submeters or prohibit landlord billing from a master meter.
Confirm whether your jurisdiction allows landlord billing from a master meter before you rely on this approach, and get the required permitting/inspection guidance from the AHJ and the utility[6][7].
3) Master meter with submeters (where allowed)
Submetering lets one service serve the property while individual submeters record tenant usage for billing. Many jurisdictions that permit submeters require revenue‑grade meters meeting ANSI C12 or equivalent accuracy/registration standards; check local rules and NIST/ANSI guidance before planning to bill tenants using submeters.
Relevant references for metering accuracy and submetering rules: ANSI C12 and NIST guidance on electric metrology; check your state or local public utility commission for registration or accuracy requirements[8][9][6][7].
Grounding, bonding, and feeder rules you must plan for
Do not bond neutral to ground at a subpanel. The main bonding jumper that ties neutral to equipment grounding is located at the service disconnect/service equipment; subpanels must keep the neutral conductor isolated from equipment grounding conductors because bonding/grounding errors can create shock hazards and improper fault current paths, and inspectors routinely check for these separations.
Standard residential practice requires a four‑conductor feeder to a subpanel: two hot conductors, one neutral conductor, and one equipment grounding conductor so neutrals and grounds remain separate at the subpanel—this is commonly enforced at inspection.
Adding a detached structure or a separate service can trigger additional grounding electrode requirements (ground rods, UFER, etc.) and sizing rules governed by NEC Article 250; whether new electrodes are required depends on how the new service or subpanel is arranged, so verify with your inspector and utility early in the design.
Citations: bonding/grounding basics and inspection failure points are discussed in NFPA and industry practice summaries; subpanel separation and 4‑wire feeder practice are standard inspection checks described in code‑practice resources[3][4][5].
Common inspection failures to avoid
Bonding neutrals at a subpanel (neutrals must be isolated)[3][4]
Missing equipment grounding conductor in the feeder — a 4‑wire feeder is required for subpanels[4][5]
Missing or inadequate grounding electrode connections for detached structures or new services (NEC Article 250 issues)—confirm grounding electrode requirements with the inspector[3]
Permitting and coordination checklist (what building departments and utilities commonly require)
Typical required items you should prepare and expect to submit or coordinate:
Pre‑application contact with the utility to determine meter/service requirements and get a written determination and cost estimate for any service work[1][2]
Electrical permit application with a single‑line diagram and load calculation showing feeders, conductor sizes, overcurrent protection, and meter location (these diagrams and load docs are standard submission items)[1][12]
Building permit for the ADU where required by your jurisdiction (coordinate building and electrical permit timelines)[12]
Utility new‑service/meter application if a separate meter or service upgrade is needed; include any trenching/right‑of‑way applications if the meter or service is remote[1][2][13]
Plan for inspections (rough and final) and ensure the electrician’s scope addresses grounding/bonding, 4‑wire feeders to subpanels, and grounding electrode connections to avoid common failures[3][4]
Typical cost guidance (site dependent)
Installed cost ranges are highly site dependent and utilities differ. Consumer and industry summaries show installing a subpanel (materials, breakers, electrician labor, permit) commonly runs roughly $400–$2,200 for typical residential jobs, with simple small jobs at the low end and larger installs at the high end.
Installing a separate utility meter and any required service upgrade is more variable and often costs several thousand dollars; projects that need trenching, new service conductors, or full service replacement can run into the $5,000–$15,000 range or more. Obtain an on‑site utility and electrician estimate because costs are highly site‑dependent.
Sources for these typical ranges and the recommendation to get on‑site quotes: consumer cost guides and utility project‑resource guidance[10][11][1][2].
Practical step‑by‑step for homeowners before hiring
Contact the utility early and request a written determination about whether a separate meter or service upgrade is required for your ADU; ask for an on‑site load assessment and a written cost estimate when applicable[1][2].
Hire a licensed electrician for a site visit to produce a load calculation and single‑line diagram suitable for permit submission; this is often required by the electrical permit process[12][1]
If a separate meter or service upgrade is likely, obtain utility new‑service quotes and factor trenching/conduit and meter‑socket costs into the budget[1][13]
Ensure the electrician’s scope explicitly calls out four‑wire feeders to subpanels, neutral isolation at subpanels, and grounding electrode connections for detached structures if applicable—these items are common inspection failure points[3][4]
Submit the electrical permit with the load calculation and single‑line diagram, and schedule required inspections (rough and final) per the AHJ’s process[1][12]
When rules disagree or the answer isn’t clear
Utility guides and builder resources illustrate utility practice but are not universal: when sources disagree about whether a separate meter is required in a specific ADU scenario, the deciding authorities are the local utility and the AHJ (authority having jurisdiction). Always treat utility builder guides (for example SDG&E’s guide) as an example of one utility’s practice and confirm requirements for your jurisdiction in writing.
All work that affects service equipment, meter sockets, or service conductors generally requires coordination with the utility and an electrical permit—do not attempt to modify service/meter hardware without utility authorization and the required permits[1][2][12].
Final checklist to bring to your electrician and utility meeting
Property plan showing ADU location and route(s) for feeders or service conductors.
Preliminary load estimate for the ADU (appliances, HVAC, electric water heat if any) so the electrician can prepare a formal load calculation and single‑line diagram for permits[12].
Written utility guidance (if you already contacted them) or a note that a written utility determination is requested at the site visit[1][2].
Budget range expectation (subpanel vs separate meter/service upgrade) and authorization limits for quoting trenching or service changes[10][11]
Agreement that the electrician will provide the permit‑ready single‑line diagram and load calculation and will address 4‑wire feeders, neutral isolation at subpanels, and grounding electrode needs in the scope[1][3][4]
Bottom line
Decide your preferred metering strategy early, but verify it in writing with both the utility and the AHJ. For a subpanel, design for a 4‑wire feeder and isolated neutrals; for a new or detached service expect grounding electrode questions and possible service upgrades. Get a licensed electrician to produce a permit‑ready load calculation and single‑line diagram and obtain written utility cost estimates before you commit to the scope or a tenant‑billing plan.
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Typical required items you should prepare and expect to submit or coordinate:
The article does not provide specific guidance for when none of these permitting/coordination items apply.
Confirm requirements in writing with your utility and the authority having jurisdiction and hire a licensed electrician to produce permit‑ready documents.