Building an extension is one of the best opportunities you will ever have to sort out the electrics in your home, not just for today but for the next twenty years. The walls are open, the floors are up and access costs nothing extra. Miss that window and every improvement you want later means chasing plaster, lifting tiles and making good. This guide walks through the loads worth planning for, what each one needs from the wiring, and why the decisions you make at first-fix stage matter so much.
What loads actually change the picture for your consumer unit?
Most sockets and lighting circuits add modest demand. The loads that genuinely change the calculation are the ones that draw several kilowatts continuously or in long bursts. Electric underfloor heating is the main one in a typical extension, because a heated kitchen or living area can pull two to four kilowatts for hours at a time. An EV charger is another, typically drawing seven kilowatts whenever a car is plugged in. Add an induction hob if the extension includes a kitchen, and a heat pump if you are also improving the heating system, and you are potentially adding fifteen to twenty kilowatts of new demand to a house that may have been designed around a fraction of that.
None of this is a reason not to proceed. It is a reason to check the incoming supply, count the spare ways in the current consumer unit and have an honest conversation before any cables are ordered. Sometimes the existing board is fine. Sometimes it needs upgrading. Knowing early avoids an unpleasant surprise halfway through a build.
Does underfloor heating need its own dedicated circuit?
Yes, always. Electric underfloor heating is a fixed load that runs from a thermostat rather than a switch, so it cannot share a ring or a lighting circuit. The circuit conductor size depends on the floor area: a small bathroom mat needs much less cable than a full kitchen floor. The thermostat itself, which usually includes a floor-temperature sensor, is part of the wired installation and needs to be positioned where it will not be covered by furniture or rugs.
At first-fix stage, the heating element or mat goes under the floor finish, but the thermostat back box and wiring can be positioned at the same time as the rest of the wiring. Doing it in two separate visits adds cost and means opening the wall again after decoration. The tidy approach is to run the UFH circuit cable, position the thermostat back box and leave a tail ready for the element connection, all in the same first-fix visit.
If you are planning a large open-plan area with UFH across more than one zone, each zone may need its own circuit and thermostat, so the cable count rises accordingly.
How do you wire an extension to be EV-ready without fitting the charger yet?
The most cost-effective approach is to run the supply cable at first-fix stage and cap it off until you need it. A seven-kilowatt charger needs a dedicated circuit with a cable sized to suit that load, routed from the consumer unit to the point where the charger will eventually sit, usually on an outside wall of the garage or close to the driveway.
While the walls are open, that cable can be clipped neatly in the void or run in conduit, buried under the floor screed or dropped through the ceiling with no disruption. Once plaster and floor finishes are in place, the same run might mean cutting a channel from the board all the way through the house to the outside, which is a full day’s work and a significant amount of making good.
For a fuller picture of what the charger installation itself involves, see the guide to new build and extension electrical installations in Beaconsfield.
What is a spare way, and why does it matter?
A consumer unit has a fixed number of slots, called ways, for circuit breakers. A typical modern board has twelve to twenty ways. A well-designed house uses most of them, and an extension adds several more circuits. If the board is already full, new circuits cannot simply be connected: either a larger board is needed, or some circuits are combined in ways that reduce flexibility and can cause nuisance tripping.
The better solution is to plan ahead. If the current board has only two or three ways left and you are adding UFH, an EV feed, an outdoor socket circuit and extra lighting, the board needs upgrading as part of the extension project. Doing that upgrade at the same time as the extension wiring is straightforward. Doing it six months later, after plastering, tiling and decoration are finished, means revisiting the consumer unit area and potentially disturbing the finish around it.
Ask your electrician to count the ways before first fix and give you an honest view. It is a ten-minute check that prevents a much larger problem.
What about conduit for data, audio and future cable runs?
Structured cabling, home networks and entertainment systems all run on low-voltage cables, but those cables still need routes through walls and floors. Running a length of conduit from the consumer unit area to a media wall or a home office corner, while the studwork is exposed, costs very little. That conduit can carry ethernet, speaker cable or HDMI runs now, and something you have not thought of yet in five years.
In Beaconsfield and the surrounding villages, a lot of homes have extensions that were wired with just enough sockets and no thought given to data. A few years later the owner is drilling through walls or running cables under rugs because access was not planned for. It is one of those things that seems like a luxury at the planning stage and feels like an obvious oversight afterwards.
The same logic applies to outdoor sockets and lighting. Running a buried armoured cable to a garden socket or outbuilding connection point is simple work when groundwork is already happening. It becomes a landscaping project if you want it done later.
When does your incoming supply become the limiting factor?
Most domestic properties in the Chiltern and South Bucks area have a single-phase supply with a main fuse rated at either 60 or 100 amps. That ceiling is real, and adding several high-load circuits can push total demand close to it, especially in a household that already has an electric shower, a large cooker and modern appliances.
If there is a risk of exceeding the supply fuse rating, the electricity distributor needs to be involved, because upgrading the incoming fuse or supply is their work, not the electrician’s. This can add lead time to a project, sometimes several weeks. Again, knowing early is the difference between a minor scheduling adjustment and a project delay.
Your electrician can calculate the likely maximum demand based on the circuits you are adding and advise whether the current supply is adequate. In most extensions the supply is fine. But in a house that is also gaining an air-source heat pump and an EV charger, it is worth checking before the build starts rather than after.
What paperwork do you need when the work is complete?
Extension electrics are notifiable work. A registered electrician self-certifies the installation, notifies the local authority building control and issues an electrical installation certificate. That certificate is part of the building control sign-off for the extension as a whole, and you will need it when you come to sell the property.
The notification and certification process is the electrician’s responsibility, not yours, but it is worth confirming at the outset that your electrician will handle it and that a certificate will be issued. In Gerrards Cross and Amersham, as in most of South Buckinghamshire, building control expects to see the electrical certificate as a matter of course before closing out the extension application.
If you are also upgrading the consumer unit as part of the project, that upgrade is separately notifiable and will have its own certificate.
How do you make sure nothing is forgotten before the walls close?
The best approach is a simple walkthrough with your electrician before insulation and boarding start. Go room by room and confirm: socket positions and heights, lighting layout, thermostat positions for UFH, the EV feed route, any conduit for future cables, outdoor socket and lighting points, and the final position for any extractor fans or towel rails. Changes at this stage cost almost nothing. Changes after boarding and plastering cost significantly more, because the work is done twice.
If you are in the middle of planning an extension in Beaconsfield, Penn, Knotty Green or anywhere nearby and want to work through the electrical side before the build starts, send the details of your project using the form below. Jess can usually give an initial view on board capacity and first-fix scope within a day or two.
