HomeAdviceWiring an extension: decide this before the slab

Wiring an extension: decide this before the slab.

The cheapest time to change electrical plans is before anything is built. The same socket that costs pennies to move on a drawing costs real money to move after plastering. Here is the list worth settling early.

Crest Electrical · AdviceExtensions~4 minute read
ExtensionsWiring an extension? Plan it first.

Check the board before the builder starts.

An extension adds circuits, and circuits need ways on the consumer unit. Plenty of extension budgets get a surprise when the existing board turns out to be full or dated. Getting the board assessed at design stage means any upgrade happens once, at the start, instead of as a mid-build variation.

Plan the rooms as you will live in them.

  • Kitchens. Appliance positions decide everything: each fixed appliance wants its own supply, and the island you might add later needs its feed in the slab now.
  • Sockets. Walk the plan and place them where furniture will actually sit. Corners of rooms and both sides of the bed beat a symmetrical grid nobody can reach.
  • Lighting. Decide the layers early: ceiling, task and accent, and which switch controls what from where. Two-way switching costs little at first fix and infuriates forever if missed.
  • Heating and cooling. Underfloor heating, air conditioning and heat pumps all want dedicated circuits planned in, not squeezed in.

Think past the back wall.

While the ground is open is the moment for outside power: garden sockets, lighting, the supply to a future garden office, and ducting for an EV charger even if the car is a few years away. An armoured cable in an open trench is a small job. The same cable under a finished patio is not.

The sequencing rule. The electrician should walk the job twice: once at design stage to agree the plan, once at first fix when the walls are open. If the first walk never happens, the second one becomes a negotiation.

Asked about this.

When should the electrician get involved in an extension?

At design stage, before the slab. The layout conversation costs nothing early and the board check prevents the classic mid-build surprise.

Does an extension need building control sign-off for the electrics?

The electrical work in an extension is notifiable, and a registered electrician self-certifies it as part of the job. You should receive the certificates with your completion paperwork.

Can you work alongside our builder?

That is most of what we do. First fix slots in when the structure is ready, second fix after plastering, and the programme gets agreed with the builder rather than around them.

Tell us about the job.

Send it through in under a minute, call, or WhatsApp us a photo of the job. You deal with the people doing the work, not a call centre.

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