First Fix Electrics: What Happens and What to Watch For

If your electrician doesn't come back to issue the certificate, your building control sign-off is blocked. Not delayed. Blocked. The building control officer will not sign off your extension without the Electrical Installation Certificate. And if the electrician who did the work won't return (it happens more than you'd think), a replacement electrician may insist on opening up finished walls to verify what's behind them before they'll put their name to it.

That certificate dependency makes first-fix electrics one of the highest-stakes phases of your build. Not because the work itself is particularly complex for a qualified electrician, but because the consequences of getting it wrong, or of poor communication during the process, ripple through every phase that follows.

What First Fix Electrics Actually Involves

First fix is when your electrician runs every cable that will eventually serve every socket, light, switch, and dedicated appliance connection in your extension. All of this work happens before the plasterer arrives, because once plasterboard covers the walls and ceiling, these cables are buried.

The work breaks down into five activities:

Cable routing. Twin-and-earth cable runs from the consumer unit to each outlet position. 1.5mm cable for lighting circuits. 2.5mm for socket ring mains. 6mm for high-power circuits like the cooker. Cables run through drilled holes in timber studs or through chased channels in blockwork walls, always within what the regulations call "safe zones": horizontal or vertical runs only, at least 150mm from corners. No diagonal shortcuts. The name is misleading. These zones aren't safe to drill into (they're where cables are expected to be buried). Anyone drilling into a wall later should check these zones first, because that's where the wiring lives. Think of them as "cable zones" rather than "safe zones" and the logic makes more sense.

Back box installation. Metal back boxes (the rectangular boxes that sit behind every socket and switch) get recessed into the wall. The electrician chases a hole in the blockwork or cuts into the plasterboard, fits the box, and leaves cable tails of 150-200mm poking out for second fix connections later. Getting these flush matters. A back box that's too deep or at the wrong angle means the socket face plate won't sit flat against the finished wall.

Consumer unit work. If your extension requires new circuits (it will), your consumer unit needs spare ways or an upgrade. The consumer unit may also need relocating to comply with the 3-metre rule (more on this below). This is often a separate job that happens at the very start of first fix.

Specialist cable runs. Anything beyond standard sockets and lights: outdoor lighting, cat6 data cables, doorbell transformer wiring, extractor fan supply, island socket cables routed through the floor. These all need to go in now. Once the screed is poured and the plasterboard is up, adding a cable to the kitchen island or running an ethernet line to the living room becomes a full-day disruption involving cutting, patching, and redecorating finished walls.

Testing. Amendment 2 of BS 7671 now requires an insulation resistance test at first fix stage, before any equipment is connected. This is a two-stage process: test now while cables are accessible, then retest at second fix once everything is wired in. Ask your electrician explicitly to do this test and to give you written confirmation of the result. Many electricians still skip the first-fix test out of habit because it wasn't required before the 2022 amendment.

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The Consumer Unit and the 3-Metre Rule

Your consumer unit (the fuse board that distributes power to every circuit) is the starting point for all cabling. If it needs moving or upgrading, that happens before any other first-fix work.

Most extensions need new circuits that won't fit in the existing consumer unit. You'll likely need either spare ways in your current board or a new consumer unit altogether. Modern boards use individual RCBOs (one per circuit) rather than the older split-board design with shared RCDs. RCBOs are better because a fault on one circuit doesn't trip everything else in the house.

3m

Your consumer unit must be within 3m of the electricity meter. This is a DNO (Distribution Network Operator) requirement, not a BS 7671 rule. Some DNOs are stricter: UKPN in London and the South East allows a maximum of 2m.

That 3m limit is a hard constraint that catches people out. If your meter is on the front wall and your extension is at the rear, the consumer unit can't follow the extension. It stays near the meter. When the distance must be greater than 3m, your electrician has two options: install a switch fuse between the meter and the consumer unit, or run SWA (steel wire armoured) cable, which doesn't need the same fault protection because the steel armour acts as its own earth. Both add cost.

The rule produces some odd outcomes. The meter box sits outside, sometimes in an awkward spot behind the wheelie bins, and the consumer unit has to go inside within 3m of it. That can mean a hallway cupboard or garage wall you would never have chosen, and the electrician simply makes the best of a constrained situation. Know where your meter is, measure the 3 metres, and accept that the rule may force a compromise on consumer unit placement before your electrician even starts.

Surge Protection Devices (SPDs)

Amendment 2 of BS 7671 made surge protection devices a default requirement in all new consumer units. An SPD sits inside the consumer unit and absorbs voltage spikes before they reach your appliances. Lightning strikes, power grid faults, and even heavy machinery switching on nearby can send a surge through the mains. Without an SPD, that surge travels straight into your TV, fridge, oven controls, and anything else plugged in.

Your electrician must fit an SPD unless a documented risk assessment concludes the risk is acceptable without one. In practice, for an extension with expensive integrated appliances and smart home devices, there's no sensible argument against it. The device itself is one of the cheapest components in the entire consumer unit. Fitting it adds minutes to the installation because it clips onto the DIN rail alongside the RCBOs.

Here's where it gets awkward. The regulation allows the customer to decline surge protection in writing. Some electricians will ask you to sign a waiver to save themselves the cost and effort. Don't sign it. The SPD protects thousands of pounds of appliances for a negligible uplift on your overall electrical bill. If your electrician's quote doesn't include one, ask why.

If your existing consumer unit is in a reasonable location and just needs upgrading (not moving), expect £350 – £750.

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