If you’re mid-build or planning an extension, “first fix” and “second fix” are two phrases you’ll hear from every trade on site, not just electricians — but what they actually mean for the electrics specifically, and when each one needs to happen relative to the rest of the build, is worth understanding before your programme is set. This guide covers exactly that.
First fix: before the walls close up
First fix is the stage where cable gets run through the new structure — inside stud walls, under floors, through ceiling voids — and back boxes get fixed in place for every socket, switch and light fitting the design calls for. Nothing gets connected up or made live at this stage; it’s purely getting the infrastructure into the building before it’s sealed behind plasterboard.
This has to happen at a specific point in the build programme: after the structural work (walls, roof, floor) is in but before the plasterer arrives, because once plaster goes on, running a new cable means chasing into a finished wall rather than an open one. If you’re working with a builder, first fix electrics usually sits in the same window as first fix plumbing and any other services going into the walls — it’s worth confirming this timing explicitly with whoever’s running your programme, since a electrician arriving after plaster is one of the most common (and expensive) scheduling mistakes on a build.
Second fix: once the room is decoration-ready
Second fix happens much later, once plastering, flooring and decoration are essentially done. This is when the actual visible fittings go on: faceplates over the back boxes, switches, light fittings, and the consumer unit gets finished off if new circuits are being added. This is also when everything gets tested — insulation resistance, earth continuity, RCD operation — and, assuming it all passes, certified.
Second fix is typically one of the last trades on site, similar to kitchen fitting or final joinery, because fitting delicate faceplates and fittings before decorating is finished risks damage and paint splatter on the visible parts.
Why the sequencing matters for cost and disruption
Getting the order right isn’t just tidiness — it directly affects cost. If first fix is missed or under-specified and a socket or extra circuit is needed after plastering, that means chasing into a finished wall to add it, which costs meaningfully more than including it in the original first-fix design. This is the single biggest reason to have a proper design conversation before first fix starts, not after: it’s far cheaper to add “one more double socket in the corner” to the plan on paper than to retrofit it once the room’s finished.
What we need from you before first fix
A first-fix visit goes faster and costs less when the layout is settled beforehand. It helps to have: a plan of the extension showing roughly where you want sockets, switches and light fittings (even a hand-drawn sketch is enough to start from); any specific requirements — a home office circuit, outdoor sockets, pre-wiring for a future EV charger point — flagged early, since it’s far easier to run that cable now than add it later; and confirmation of whether your existing consumer unit has spare capacity, which we can check on the same visit if you’re not sure.
Will you need a new consumer unit?
Not automatically. If your existing board has spare ways and the right protection already, new circuits for an extension can often just be added into it. If the board is already full, or it’s an older type that doesn’t meet current regulations for the additional load, we’ll say so plainly at survey — this is one of the more common reasons an extension’s electrical quote comes in higher than expected, so it’s worth asking about directly rather than being surprised later. For more on what an older or undersized board means for cost more broadly, see our board upgrades information.
Working with your builder
Most extensions involve a main contractor or builder running the overall programme, and we slot into that rather than trying to impose our own schedule. In practice this means: a first-fix visit timed against the build’s structural completion, a gap while plastering and other trades finish, then a second-fix visit once the room is genuinely ready for fittings — not “nearly ready”, since half-finished decoration around fresh faceplates leads to damage and rework. If your build is being self-managed rather than through a main contractor, we’re happy to help advise on where electrics should sit in your overall sequence.
New build versus extension
Everything above applies equally to a full new-build house, just at a larger scale — more circuits, a full consumer unit specification from scratch, and usually a longer first-fix phase given the size of the job. New-build packages are quoted separately from extension work for exactly that reason: the scope and site logistics are different enough that a single “extension” price band doesn’t represent a new-build accurately.
Planning for what you’ll want later, not just now
One of the most common regrets we hear from extension owners a year or two on isn’t about what was done — it’s about what wasn’t planned for at first fix. Cable for a future EV charger point, an extra outdoor socket for garden lighting, or capacity for a home office that gets added later are all far cheaper to include at first-fix stage, even if you’re not ready to fit the end point yet, than to retrofit into a finished wall afterward. It costs very little to run one extra cable to a sensible location “just in case” during first fix; it costs considerably more to chase a wall open for it in eighteen months’ time. Worth a five-minute conversation before first fix, even for things you’re not sure you’ll ever use.
Building control, Part P, and notifiable work
Most electrical work on an extension counts as notifiable under Part P of the Building Regulations, which means it needs to be either carried out by someone registered with a competent person scheme, or separately signed off by building control. This isn’t extra paperwork we add on top — it’s a standard part of how extension electrics are done, and the certification you receive at the end (the EIC, plus Part P notification where applicable) is exactly what building control and your local authority expect to see at final sign-off for the whole extension. Keep it filed with the rest of your build documentation; it’s commonly requested again if you extend further or sell the house.
Common extension layouts and what they typically need
A single-storey rear extension — the most common type in Beaconsfield gardens — usually needs new circuits for sockets, lighting (including any feature or under-cabinet lighting if it’s a kitchen extension), and often an outdoor circuit if bi-fold or patio doors open onto the garden. A two-storey extension adds a second floor’s worth of circuits and sometimes triggers a consumer unit review sooner, simply because there’s more total load being added. A loft conversion has its own considerations again — usually a dedicated lighting and socket circuit plus, depending on use, requirements around smoke detection interlinked with the rest of the house. None of these are fixed rules; they’re just the patterns that come up most often, and your actual layout gets confirmed on survey.
Getting a fixed price
Once you can show us a floor plan (even a rough one) and tell us whether you know your existing board’s condition, we can arrange a site visit and get you a fixed quote covering first fix, second fix, testing and certification — before anything starts, and with any changes to scope quoted separately as they come up rather than added to the bill afterward. See our extension and new-build electrics page for full scope details and current pricing.
