Home addition electrical in San Antonio: planning power for an addition, garage conversion or casita

If you are planning an addition, a garage conversion or a casita, the home addition electrical plan decides whether the new space works on day one or whether the breaker trips the first hot afternoon the new AC runs. The stake is concrete. A service that was sized for the original house now has to carry more square footage, more cooling and often a second kitchen or bathroom. Planning that at the design stage costs a fraction of finding out at the rough-in inspection.

This guide covers how to plan power for new living space in San Antonio: the load calculation, panel capacity, when a subpanel makes sense, HVAC circuits, the permits and inspections, and how the electrician works alongside your builder. It also covers what sets the home addition electrical cost, so the number on the bid makes sense.

Start home addition electrical planning with a load calculation

The first question for any addition is whether the existing service can carry it. A load calculation answers it. It takes the square footage of the house and the addition, the cooking and laundry appliances, the water heater, the HVAC equipment old and new, and anything else you plan to add, then applies the demand factors the electrical code allows. The result says whether the service has room, how much, and what happens if you add an EV charger or a pool later.

Do this before the plans are final. If the calculation shows the service is short, the upgrade can be scheduled with the addition instead of after it, and the panel location can be planned so it does not end up inside a new closet or bathroom, where panels are not allowed.

A written load calculation for your house and the planned addition is listed in our flat-rate book as the Load calculation and capacity report task, from $709. It is signed by a Texas licensed master electrician, and you can hand it to your builder, your designer and the permit office. Starting price. Your tech confirms the final number on site before any work.

Call us with the square footage of the addition and what will go in it, and we will tell you over the phone whether the house is close enough to its limit that a calculation is worth doing first. Call or book a visit.

Panel capacity: subpanel or service upgrade

Once the calculation is done, the panel question has three possible answers. If the main panel has open spaces and the service has room, the new circuits run from the main panel. If the service has room but the panel is full or far from the addition, a subpanel in or near the new space is the cleaner answer. If the service does not have room, it needs an upgrade, and a subpanel may come with it.

A subpanel keeps the new circuits short, puts the breakers for the new space where the people using it can reach them, and avoids pulling a dozen home runs back across the house. What it does not do is add capacity. Every amp the subpanel delivers still comes through the existing service.

A 60-amp subpanel installed close to the main panel is listed in our book as the Subpanel installation, 60A task, from $1,859. Additions usually call for the higher tiers in the same task, which cover longer feeder runs across the house, and the estimate names the tier before you approve it. Starting price. Your tech confirms the final number on site before any work.

Our guide to subpanel installation cost covers feeders, trenching and grounding in detail. If the calculation says the service is short, our guide to the cost to upgrade an electrical panel to 200 amps walks through the upgrade step by step.

Room additions: what new living space requires

New living space is wired to the code San Antonio currently enforces, regardless of when the original house was built. In plain terms, that means:

• Wall receptacles spaced so that no point along a usable wall is more than 6 feet from a receptacle.
• AFCI protection on the circuits serving living areas and bedrooms.
• Tamper-resistant receptacles.
• A switched light or switched receptacle at each room entry.
• Smoke alarms in each new bedroom and the area outside it, interconnected with the existing alarms, and carbon monoxide alarms where the house has fuel-burning appliances or an attached garage.
• GFCI protection wherever the addition includes a bathroom, a laundry area or a wet bar.

Alarm placement is the item most often missed on additions, because the new alarms have to talk to the old ones. Our guide to smoke detector placement covers where they go, and our smoke and CO detector service covers the install.

Garage conversions have a few traps of their own

Converting a garage into a bedroom, office or game room is one of the most common projects in San Antonio, and it has a few electrical traps. The main panel is often on a garage wall, and once that wall is inside a bedroom closet the panel location is no longer allowed, so the panel either moves or the layout changes. The garage circuits are usually a single 20-amp GFCI circuit that also feeds the outdoor receptacles or the laundry, and that circuit has to be reworked when the space becomes living area. The water heater and any attic access in the garage need their own clearances planned in.

The conversion also takes away the garage receptacles that ran the freezer, the tools and possibly an EV charger. Plan where those loads go before the drywall goes up.

Casitas and detached living space

A casita or detached studio is fed like any detached building: a feeder from the main panel, a subpanel with a disconnect at the casita, a four-wire feeder with neutral and ground kept separate, and its own grounding electrode. The trench is often the largest single variable, and a long run may need a larger feeder to keep voltage drop in check. Inside, the casita follows the same living space rules as a room addition, plus the kitchen and bathroom rules where it has them.

If the casita includes a kitchenette, our guide to kitchen electrical code requirements covers the countertop circuits and appliance circuits it will need.

HVAC circuits for the addition

In San Antonio, cooling the new space is not optional, and it is usually the largest new load. There are two common approaches. Extending the existing ductwork works only if the existing system has spare capacity, which your HVAC contractor determines. A ductless mini-split is the more common choice for additions, conversions and casitas, because it serves the new space on its own.

Either way, the outdoor unit needs its own circuit sized to the nameplate on the equipment, and a disconnect within sight of the unit so a technician can shut it off while working on it. We coordinate with your HVAC contractor on the equipment model so the circuit is sized correctly the first time.

A dedicated circuit with the outdoor disconnect is listed in our book as the Mini-split or A/C circuit with disconnect task, from $919. The circuit size follows the equipment you choose, and the price is approved before work starts. Starting price. Your tech confirms the final number on site before any work.

Permits, inspections and coordination with the builder

An addition in San Antonio is permitted through the Development Services Department, and the electrical permit is pulled by a licensed electrical contractor. Wattsmith Electric is licensed by TDLR under TECL 41806. Outside the city, the suburban city or the county sets the process, and we handle that across our service areas. The electrical inspections follow the build:

• Underground or slab, where conduit runs in or under a new slab, inspected before the pour.
• Rough-in, with framing complete and walls open, before insulation and drywall.
• Final, with devices, fixtures, alarms and equipment connected.

Coordination with the builder matters most at two points. Conduit for an island, a floor receptacle or a feeder has to be in place before the slab is poured. Rough-in has to be inspected before the insulation crew arrives. We work from the builder's schedule and tell the builder when we need the walls open and when the inspection is booked, so the electrical never holds up the next trade.

For full new construction and larger additions, see our new construction wiring service, which covers how we work with builders from plan review to final.

Structural questions go to an engineer

Additions raise questions that are not electrical: whether a wall that is coming out is load-bearing, what beam carries the opening, and whether the new foundation ties properly into the old one. Wattsmith Electric is not an engineering firm. Our sister firm, 254 Engineering (Texas Board of Professional Engineers and Land Surveyors firm F-29811), handles foundation and structural evaluations. Your builder or designer may already have an engineer on the project, and either way, those answers belong to an engineer before the electrical plan is final.

What sets the home addition electrical cost

• Whether the service has room, or needs a subpanel or an upgrade.
• The number of new circuits, including HVAC, kitchen and bathroom circuits.
• The distance from the main panel, and for a casita, the trench.
• The lighting plan and the number of fixtures.
• Changes to the existing house, such as moving a panel out of a converted garage.

Each item is a line in our flat-rate book, and the estimate shows every line before you approve it. There is no hourly meter running while the walls are open.

To get that estimate quickly, send the floor plan for the addition with room uses marked, the HVAC equipment your contractor has proposed, a photo of the main panel with the door open and the label visible, and for a casita, the rough distance from the main panel to the new building. If the plans show a kitchenette, a bathroom or a laundry, list the appliances. With those in hand, the design visit confirms the details instead of starting from a blank page.

Compare the published rates on our pricing page, see what goes into panel upgrades and subpanels, and when the plans are close to final, call us to schedule the design visit. We handle the permit and the inspections, and we work to your builder's schedule.