Quick Answer
ADU electrical planning starts with capacity: an accessory dwelling unit adds a second dwelling’s worth of load, so a licensed electrician runs a load calculation to see whether your existing service can carry it or needs upgrading. From there, the plan covers how the ADU is fed — often a dedicated sub-panel — and meets California code, including Title 24 energy standards and the California Electrical Code. Getting capacity and code right early keeps the whole project on schedule.
Key Takeaways
- An ADU adds a whole second dwelling’s electrical demand, so the first question is always whether your existing service has the capacity to carry it.
- A load calculation — not a rule of thumb — determines whether your current panel serves the ADU or whether a service upgrade comes first.
- California code shapes ADU electrical from the start: the California Electrical Code governs the wiring, and Title 24 sets energy and efficiency standards.
- New California construction, including ADUs, is generally required to be built solar-ready and increasingly electric-ready, which affects the electrical plan.
- An ADU is usually fed by a dedicated sub-panel; whether it gets its own utility meter depends on your goals and local rules, and is worth deciding early.
ADU electrical planning is the process of determining how a new accessory dwelling unit will be powered — how much capacity it needs, whether your existing service can supply it, how it’s fed and metered, and how the whole design meets California code. It’s more consequential than it sounds, because an ADU isn’t a room addition; it’s a second dwelling with its own kitchen, heating, and increasingly its own EV charging. That’s a meaningful block of demand added to a property whose electrical service was sized for one home. Planning it right, early, is what keeps the electrical from becoming the bottleneck of the whole project.
What does ADU electrical planning involve?
A load calculation is the code-defined method of adding up a property’s total electrical demand — existing home plus new ADU — to determine whether the service is adequate or needs upgrading. It’s the foundation of every ADU electrical plan. Guessing here is expensive: assume the panel is fine and it isn’t, and you’ve built an ADU you can’t fully power; assume you need a 400-amp service when a load calc says 200 will do, and you’ve overspent. The calculation replaces both guesses with an engineered answer, and it’s the first thing a licensed electrician does when you’re planning a unit.
An accessory dwelling unit (ADU) is a secondary, self-contained dwelling on a residential lot; because it has its own kitchen, heating, and often EV charging, it adds a full second household’s worth of electrical demand.
Why Marin's older service sizes make capacity the first question
In Marin, ADUs have become a common way to add housing on existing lots — the California Department of Housing & Community Development tracks the statewide rules that have encouraged them — and the county’s older homes make the capacity question especially live. Many Marin homes still run service sized decades ago for a single, lighter-load household. Bolt a full second dwelling onto that, and the existing panel is often the constraint. That’s not a reason to hesitate — it’s a reason to plan. When the capacity and code questions are settled at the design stage, the ADU’s electrical becomes a clean, predictable part of the build rather than a mid-project surprise.
A load calculation is the California Electrical Code method of totaling a property’s electrical demand — existing home plus new ADU — to size the service and determine whether an upgrade is required before the unit is built.
How do capacity and California code shape an ADU electrical plan?
Capacity and code shape an ADU electrical plan from the first line: capacity determines whether your service can carry the added load, and California code determines how the ADU must be wired, powered, and made efficient. On the capacity side, a load calculation tells you whether the existing panel serves the unit or whether a service upgrade comes first. On the code side, the California Electrical Code governs the wiring itself, while the state’s Title 24 energy standards set requirements for efficiency, and, for new construction, provisions around solar-ready and electric-ready design.
What Title 24 and the CEC mean for your ADU
For an ADU, Title 24 and the California Electrical Code translate into real design decisions, not just paperwork. The California Energy Commission’s Building Energy Efficiency Standards (Title 24) set energy and efficiency requirements for new dwellings, and recent editions have moved toward solar-ready and electric-ready construction. The California Electrical Code — the state’s adoption of the National Electrical Code (NFPA 70) — governs the wiring, circuits, and protection. Together they mean an ADU should generally be planned with efficient design and future electrification in mind from the start, which is far cheaper than retrofitting later. Because code editions and local amendments change, a licensed electrician confirms the current requirements with your Marin jurisdiction rather than relying on memory. Our guide to making a home solar-ready covers the readiness side in more depth.
An ADU isn't a big room — it's a second home's worth of demand. The plan has to start with whether your service can carry it.— Towery Electric
How is an ADU fed — sub-panel, service upgrade, or separate meter?
An ADU is most often fed by a dedicated sub-panel connected to the main service, but the right approach depends on your capacity, your goals, and local rules. If a load calculation shows the existing service has room, a sub-panel serving the ADU is frequently the cleanest solution. If the service is already near its limit, a service upgrade comes first so both the home and the ADU have headroom. And if you want the ADU billed separately — for a rental, say — a separate utility meter may be worth pursuing, subject to your utility’s and jurisdiction’s requirements.
Each path has trade-offs in cost, complexity, and how the ADU is billed and controlled. There’s no single right answer; there’s the right answer for your property and plans, which the load calculation and your goals determine together. The table below lays out the common approaches so you can see how they compare before you’re deep into design. For a closer look at when a second panel is the answer, our sub-panel installation guide goes deeper.
| Approach | Fits when | Trade-off to weigh |
|---|---|---|
| Dedicated sub-panel off main service | Load calc shows the existing service has headroom | Shared meter; ADU billed with the main home |
| Service upgrade, then sub-panel | Existing service is near or over capacity | Higher upfront cost; gives both home and ADU room to grow |
| Separate utility meter | You want the ADU billed and metered on its own | More coordination with the utility; subject to local rules |
How does the ADU electrical planning process actually work?
The ADU electrical planning process works best when the electrician is involved at the design stage: it starts with a load calculation, moves through the feed-and-code decisions, then permitting and utility coordination, and ends with an install that matches the plan. Brought in early, the electrician settles the capacity and code questions before they can hold up the build. Left to the end, those same questions become expensive surprises. The sequence is deliberate — capacity first, then how it’s fed, then permits, then clean execution. Because ADU wiring often means a new sub-panel or service, our panel upgrade guide covers what that step involves.
- Load calculation. We total the existing home plus the planned ADU to determine whether your service carries it or needs upgrading.
- Feed & code decisions. We settle how the ADU is fed — sub-panel, upgrade, or separate meter — and confirm current Title 24 and CEC requirements.
- Permit & utility coordination. We pull the electrical permit and coordinate any needed PG&E service change with your Marin jurisdiction.
- Install to plan. We wire the ADU and its panel cleanly to the design, then have the work inspected before it’s energized for good.
When it runs this way, the ADU’s electrical is a predictable, on-schedule part of the project rather than the thing that stalls it. What you should expect from a good electrician is a real load calculation up front, a clear recommendation on how to feed the unit, and an honest answer about whether a service upgrade is genuinely needed or whether your existing panel has the room.
Planning an ADU and not sure your electrical service can carry it?
Request Your EstimateWhen should you plan the electrical for an ADU?
You should plan the ADU’s electrical at the very start of the project, during design — before framing, ideally before final drawings are set. The capacity answer influences almost everything downstream: whether you need a service upgrade, where the panel goes, how the unit is fed, and what the permit requires. Settling it early means the rest of the design can proceed on solid footing. Discovering a capacity problem after framing is the expensive version of the same conversation.
Why the capacity answer belongs at the front
The capacity answer belongs at the front because it’s the constraint everything else works within. If a load calculation shows your service needs upgrading, that’s far cheaper to plan into the project than to bolt on midway. If it shows you have room, you can design the feed with confidence. Either way, knowing early prevents the common ADU headache of a finished unit that can’t fully power its kitchen, heating, and an EV charger at once. If your existing service is the constraint, our guide to when a panel upgrade makes sense is a useful companion read.
What are the most common ADU electrical mistakes?
The most common mistake is assuming the existing service can carry the ADU without running a load calculation — and discovering otherwise after the walls are up. The second is treating code as an afterthought, then facing a failed inspection or a redesign because Title 24 or California Electrical Code requirements weren’t planned for. The California Energy Commission updates the state’s energy standards on a regular cycle, and local jurisdictions add their own amendments, so relying on outdated assumptions is a genuine risk.
How to avoid them
Avoid these by involving a licensed electrician at the design stage, insisting on a real load calculation before the electrical plan is set, and confirming current code with your Marin jurisdiction rather than assuming last year’s rules still apply. Plan for efficiency and future electrification from the start, since retrofitting is costlier than building it in. And decide the metering and feed strategy early, because changing it later means reworking the electrical. For the capacity foundation this all rests on, our guide to adding capacity with a sub-panel is the natural next step.
Key Terms
- Accessory dwelling unit (ADU)
- A secondary, self-contained dwelling on a residential lot, adding a full second household's electrical demand.
- Load calculation
- The California Electrical Code method of totaling a property's electrical demand to size the service and determine if an upgrade is needed.
- Title 24
- California's Building Energy Efficiency Standards, which set energy, efficiency, and readiness requirements for new dwellings including ADUs.
- Sub-panel
- A secondary electrical panel fed from the main service, commonly used to power an ADU or an added area of a property.
About the Author
Founder of Healthcare Marketing Group and author of The Ultimate Guide to Digital Marketing. Previously led a digital marketing company with 8,000+ clients. Writes on local-service SEO and E-E-A-T content strategy.
Reviewed By
Alex founded Towery Electric in Novato in 2014 and has planned and wired the electrical for accessory dwelling units across Marin’s older properties. Reviewed this article for technical accuracy on Oct 3, 2026, 2026.
“The single most useful thing on an ADU is the load calculation at the very beginning. Get that right, and the whole electrical scope becomes predictable. Skip it, and you’re solving capacity problems after the drywall’s up.”
— Alex Towery, Licensed Electrical Contractor
How We Ensure Accuracy
Sources
- California Energy Commission. “Building Energy Efficiency Standards (Title 24).” energy.ca.gov
- National Fire Protection Association. “NFPA 70 — National Electrical Code (basis of the California Electrical Code).” nfpa.org
- California Department of Housing & Community Development. “Accessory Dwelling Units.” hcd.ca.gov
- U.S. Department of Energy. “Home EV Charging — Alternative Fuels Data Center.” afdc.energy.gov
- California Contractors State License Board. “License Detail — Towery Electric, Inc. (#989290).” cslb.ca.gov
Frequently Asked Questions
Will my existing panel handle an ADU, or do I need an upgrade?
That’s exactly what a load calculation answers. An electrician totals your existing home’s demand plus the planned ADU and compares it to your service capacity. Some Marin homes have enough headroom to feed an ADU from the existing panel via a sub-panel; many older ones, sized decades ago, need a service upgrade first. There’s no reliable rule of thumb — the calculation gives you an engineered answer before you commit to a design.
Does an ADU need its own electrical meter?
Not necessarily. Many ADUs are fed from the main service through a dedicated sub-panel and share the home’s existing meter. A separate meter is an option — often chosen when the ADU is a rental you want billed independently — but it involves more utility coordination and is subject to local rules. Whether it’s worth it depends on your goals, and it’s best decided early, because changing it later means reworking the electrical.
What code applies to ADU electrical work in California?
Two frameworks matter most. The California Electrical Code — the state’s adoption of the National Electrical Code — governs the wiring, circuits, and protection. The state’s Title 24 Building Energy Efficiency Standards set energy, efficiency, and readiness requirements for new dwellings, including provisions around solar-ready and electric-ready construction. Because code editions and local Marin amendments change on a cycle, a licensed electrician confirms the current requirements with your jurisdiction rather than relying on prior assumptions.
Should an ADU be built EV- and electric-ready?
It’s worth planning for, and California code has increasingly moved in that direction for new construction. Building in capacity and conduit for future EV charging and electric appliances is far cheaper than retrofitting a finished unit. Even where a provision isn’t strictly required, designing the electrical with future electrification in mind protects your investment. A licensed electrician can build that readiness into the load calculation and panel design from the start.
When should I bring an electrician into my ADU project?
At the design stage, before framing and ideally before final drawings. The capacity answer influences almost everything — whether you need a service upgrade, where the panel goes, how the unit is fed, and what the permit requires. Settling it early lets the rest of the design proceed on solid footing and keeps the electrical from becoming the bottleneck. Discovering a capacity problem after framing is the expensive version of the same conversation.
Editor’s Summary
An ADU adds a second dwelling’s worth of electrical demand to a property that was likely sized for one home. That makes capacity the first question — answered by a load calculation, not a guess — and California code the framework, from the California Electrical Code that governs the wiring to Title 24’s energy and readiness standards. Settle capacity, feed, and code at the design stage, and the ADU’s electrical becomes a predictable part of the build instead of a mid-project surprise.