Electrical Planning for a Guest Cabin or ADU on Your Property

October 9, 2026 · Family-owned electricians in North Idaho

Public domain, via Wikimedia Commons

If you're adding a guest cabin, ADU, or shop to your property in Bonner or Kootenai county, the electrical decisions you make before the slab is poured or the foundation is set will save you money and headaches later. Get the service size and subpanel location right at the planning stage, and the rest of the job goes smoothly. Get it wrong, and you're trenching twice or living with a panel that can't handle a future hot tub or heat pump.

What you need

• A load calculation for the new structure, done before you pick wire or breaker size
• Decision on service type: new subpanel fed from the main house, or a separate service with its own meter
• Feeder conductor sized for the distance and amperage (direct burial cable or conduit with individual conductors)
• A disconnect at the second structure, required by code even if it's fed from a subpanel
• Grounding electrode system at the second structure (ground rods or equivalent) if it has its own disconnect
• Permit from the county building department, since this is new electrical work on a second structure
• GFCI protection for all outdoor and bathroom receptacles, AFCI for bedroom circuits if the ADU has a sleeping area

Step by step

1. Do the load calculation first. Before anything else, figure out what the cabin or ADU will actually run. A simple guest cabin with lighting, a few receptacles, and a mini-split might need 60 to 100 amps. A full ADU with a kitchen, electric water heater, and laundry often needs 100 to 200 amps. Guessing low here is the single most common mistake, because upsizing later means redoing the feeder, the panel, and sometimes the trench.
2. Check what your main panel can support. If you're feeding the new structure as a subpanel off your house, your main panel needs spare capacity. A 100-amp main panel that's already running a well pump, electric range, and central air may not have room for another 60-amp subpanel without upgrading the service to 200 amps first. This is worth confirming before you commit to a design, since a service upgrade adds real cost and lead time.
3. Decide between a subpanel and a separate service. Most guest cabins and ADUs run as a subpanel fed from the main house panel, which is usually cheaper and simpler. A separate meter and service makes more sense if the structure is far from the house, if you want separate utility billing, if local code requires it for a rental ADU, or if the combined load is large enough that a single service can't reasonably support both buildings.
4. Plan the feeder route and size the conductor for distance. Voltage drop matters more than people expect once you're running 100, 150, or 200 feet from the main panel to a detached structure. A feeder sized correctly for a 60-amp subpanel at 50 feet may need to jump a wire gauge or two at 150 feet to avoid voltage drop under load. Direct burial cable needs to sit at the depth your local code requires, typically 24 inches, unless it's in conduit, which can sometimes go shallower. This is where a lot of DIY and unpermitted work gets undersized, because the math isn't obvious from looking at a wire gauge chart meant for short in-house runs.

Where this goes wrong

The most common issue is undersizing the subpanel for what the structure becomes over time. A guest cabin that starts with a window AC and some lights gets a mini-split split system added two years later, then an electric water heater, then a hot tub on the deck. A 60-amp subpanel that was fine at the start is now tripping breakers constantly, and the fix means trenching a new feeder because the existing conductor can't be reused at a higher amperage.

Another frequent problem is skipping the disconnect and grounding requirements at the second structure. Some owners assume that because the cabin is fed from the house panel, it doesn't need its own disconnect or ground rods. Code generally requires both at any structure with a subpanel, and an inspector will catch this during final inspection if the project is permitted. If it's not permitted, it becomes a problem later during a home sale or insurance claim, when unpermitted electrical work on a second structure can complicate both.

Voltage drop on long feeder runs is another one that shows up after the fact rather than during installation. Lights that dim when the well pump or an appliance kicks on, or a mini-split that struggles to reach full output, are classic signs the feeder conductor was undersized for the distance. This isn't usually a safety hazard right away, but it's inefficient and it's a sign the original calculation didn't account for the real-world distance involved.

Burying cable at the wrong depth, or without conduit where conduit is required, is a problem that's invisible until someone digs in the wrong spot with a shovel or post hole digger and hits it. North Idaho properties often have landscaping, fencing, or future outbuilding projects planned, and a feeder buried too shallow or without a locate-friendly marker becomes a hazard for whoever digs next, including the homeowner.

Finally, treating the new structure's panel like an afterthought in the overall site plan causes problems with septic, well, and other utility layouts. The feeder trench, the septic lines, and the water line all need to coexist, and if the electrical trenching isn't planned alongside the other site work, you end up with conflicts that require re-routing after the fact.

When to stop and call someone

The load calculation and the subpanel versus separate service decision are worth doing with a licensed electrician from the start, not after the cabin is framed. These decisions affect wire size, trench depth, panel location, and permit requirements, and they're much cheaper to get right on paper than to correct in the ground.

Running the feeder itself, whether direct burial or in conduit, and making the connections at both the main panel and the new subpanel, is licensed electrical work in Idaho. This isn't a place for DIY, both because of the safety risk of working in an energized main panel and because unpermitted feeder work on a second structure is one of the more commonly flagged issues during home inspections and appraisals.

If you're planning an ADU that will be rented out, or that needs to meet specific occupancy requirements for Bonner or Kootenai county, it's worth looping in both the building department and an electrician early, since occupancy classification can affect what the electrical code requires beyond a standard guest cabin.

Sageland Electric works with homeowners and builders across North Idaho on exactly this kind of planning, from load calculations to subpanel installs to full service upgrades, and getting a second opinion on the plan before trenching starts is usually the cheapest step in the whole project.