Many older Seattle homes have panels sized for a different time. Add a cottage with its own kitchen, heat and laundry, and the old service may not keep up. A DADU electrical upgrade comes into play when the property needs more capacity, a new panel or a new service to power both homes safely.

This guide covers load math, panel and service choices, trenches, heat and cooking loads and how to work with Seattle City Light. It also explains why this job belongs with licensed pros, not weekend projects.

Why a Cottage Changes the Electrical Picture

A cottage is a full home, not just an extra room. It needs its own power for daily life. According to SDCI’s Tip 116B, a DADU must meet the electrical code requirements that apply to single family dwellings. It also needs heating controls separate from the main house.

Those features add demand. Cooking, heat, hot water and laundry can run in both homes at once. So the team needs to know if the old service can carry the total load. If it cannot, the plan needs a fix before work starts.

Load Calculations Come First

A load calculation estimates how much power the lot will need. An electrician or engineer adds up each load, such as heat, cooling and appliances, using the code’s method. Then the result shows if the current service has room to spare. It is a math step, not a guess, and it guides every choice that follows.

City Light treats this as a core document. Its new or upgraded service page lists load calculations among the items most projects need, along with a legal site plan, an electrical site plan and a one line riser diagram. The same page also offers a plan set example for a single family home with a DADU.

Panel and Service Options for a DADU Electrical Upgrade

Results from the load calculation point toward a few broad paths:

• A feeder from the existing panel to a subpanel in the cottage, when capacity allows.
• A larger main service that replaces the current panel and meter equipment to power both homes.
• Separate service options that City Light may review based on its standards and the site.

Each path has tradeoffs in gear, trench work and timing. Tip 116B also notes that circuit breakers, fuses and the heat source may sit in the unit they serve or in common areas that all residents can reach. Therefore, panel location becomes part of the layout discussion.

Heating, Cooking and Other Big Loads

Electric heat and cooking often drive the load math. Heat pumps, ranges, water heaters and dryers all draw a lot of power. In addition, many owners plan for electric vehicle charging, which adds more demand.

City Light highlights this point for older buildings. It notes that understanding a property’s capacity and load is necessary for owners interested in EV charging or heat pumps, particularly in older buildings. For that reason, it helps to list every planned appliance before the load calculation starts.

Trenching to the Cottage

Power usually reaches a cottage through a buried line in a trench. The trench length and depth depend on where the cottage sits and where the power starts. That trench may share a route with water or sewer lines, depending on code clearances. As a result, utility routes need planning together rather than one at a time.

Before digging, buried lines need marking. Seattle Public Utilities recommends a free call before you dig, and it notes that owners must hire a private company to locate lines they own. Our guide to DADU water service covers routing for the water line.

Coordinating With City Light

Any change to the service involves the utility, so plan for its steps from the start. City Light asks for an electric service application when you change an existing service. You also need one for new permanent service and for temporary power on a job site.

Timing matters as well, since the utility works on its own schedule. City Light needs the electrical permit number and local approval before it approves equipment for permanent connection. It also describes two required inspections, first by the local permitting authority and then by City Light, and it requires payment before completing the connection.

Why a DADU Electrical Upgrade Needs Professionals

Service and panel work involves high energy gear and strict code rules. Mistakes can cause shocks or fires, which is why the city controls who may do it. SDCI’s electrical permit page states that professionals applying for permits must hold a Washington State electrical contractor license and a Seattle business license.

Owners may do their own work only when they own and live in the property. A rental or a house you do not occupy does not qualify. Even then, the work still needs permits and inspections. According to SDCI, most electrical permits require three inspections, covering cover, service and final stages, and a feeder to a subpanel adds feeder inspections.

Planning a DADU Electrical Upgrade Early

Power planning works best alongside design. It is far easier to change a drawing than a finished trench. Share the appliance list, heating choice and any EV plans with the electrician early. Then ask how the load calculation affects the panel, service and trench route.

Our guides to DADU utility expenses and DADU permit requirements explain how electrical work fits into the budget and permit process. Overall, a DADU electrical upgrade depends on accurate load data, clear service options and steady coordination with City Light.