Getting the sequence right in a build programme sounds like the builder’s job, and it mostly is. But the electrician sits at two points in the programme where a late booking or a vague brief causes real knock-on damage. Miss the first-fix window and the plasterer waits. Miss the second-fix window and the kitchen fitter waits. Either delay costs money, usually yours.
This guide explains where the electrical work fits in a typical extension programme, what the lead times look like in Beaconsfield and the surrounding villages, and what to prepare before the slab is poured.
Why does timing matter so much for the electrics?
An extension has two distinct electrical stages, and a gap of several weeks sits between them. First fix is all the hidden work: cables pulled through studwork, blockwork chases or floor voids, back boxes set into the wall, and the consumer unit position established. None of it is visible once the plaster goes on, which is exactly the point. Second fix is everything you can see: sockets, switches, light fittings, the consumer unit itself wired up, and any dedicated circuits for appliances.
Between those two stages the plasterer works, the plaster dries, and often the screed is poured and dried too. That drying period can run to three or four weeks depending on the time of year and the heating available on site. If the electrician is not booked for first fix before the plasterer arrives, the plasterer either waits or goes in and leaves cables to be chased in afterwards. Chasing finished plasterwork is slow, dusty and adds cost that simply would not exist if the sequence had been right.
What happens at first fix?
First fix takes place once the external walls are weather-tight and the internal partition walls are up, but before any plasterboard is fixed or wet plaster is applied. The electrician runs cables from the consumer unit position out to every socket, switch, light, extractor fan and fixed appliance position on the drawing. Back boxes are set flush with the masonry or fixed to the studwork so they will sit correctly once the plaster skin is on.
This is also the stage where anything structural gets decided. Cables running across a joist need a notch or a hole in the right place. Cable routes through external walls need sealing against cold and damp. Under-floor heating thermostats, bathroom extractor timers and kitchen appliance circuits all need to be planned now, not discovered on second-fix day.
First fix on a modest single-storey extension in the Beaconsfield area typically takes one to two days. A larger double-storey extension with a kitchen and utility room at one end can run to three or four days. Either way, the time needed is predictable once the drawings are in hand.
What happens at second fix?
Second fix cannot start until the plaster is genuinely dry. Fitting a consumer unit while moisture is still in the walls causes problems with insulation resistance readings and can mean a failed inspection. A rough guide is to allow two to four weeks after the plasterer finishes before calling the electrician back, longer in winter when the building is cold.
At second fix the electrician fits all the accessories: sockets, switches, dimmers, USB outlets, shaver points, light fittings, extractor fans, the cooker connection unit, and anything else on the schedule. The consumer unit is fully wired, the circuits are tested, and the completion certificate is issued. That certificate is what building control need to close out the electrical element of your building regulations application.
Second fix on the same modest extension usually runs one to one and a half days. Larger extensions with more circuits take proportionally longer, but the work is predictable.
When should I book, and what are the lead times in South Bucks?
In the Beaconsfield, Amersham and Gerrards Cross area, a reliable electrician who works on extensions regularly is often booked four to eight weeks ahead. That is not a complaint, it is just the reality of a busy area with a lot of extension and renovation work running at any given time.
The practical rule is to make contact as soon as your architect has produced a layout you are reasonably happy with. You do not need planning permission granted yet. You do not need the building regulations drawings stamped. You just need enough of a floor plan to let the electrician understand the scale of the job and hold a date in the diary.
For a typical extension programme, that first contact should happen before groundworks begin. By the time the foundations are in and the blockwork is rising, the programme is already moving fast and the window for booking first fix can close quickly. Many homeowners in Penn, Knotty Green and Wooburn Green who have been through an extension project say they wish they had called the electrician at the same time they called the builder, not a few weeks later.
For new build and extension electrical work in Beaconsfield, the range at the top of this page gives a realistic guide to what the work costs across small, medium and large extensions.
What should I send the electrician before groundworks start?
The more information you share early, the less time everyone wastes making decisions under pressure on site. A useful pack to send before the slab is poured includes the following.
- A floor plan showing all rooms, even if dimensions are still approximate.
- The planned positions of the kitchen, utility room, bathrooms and any WCs.
- A list of appliances that will need their own circuit: electric oven, induction hob, washing machine, dishwasher, tumble dryer, EV charger, hot tub if relevant.
- Any underfloor heating zones, including whether they are electric or water-fed.
- The position of the existing consumer unit and whether there is spare capacity in it.
- Whether the extension will be lit with downlights, pendants or a mix, and whether any areas need dimming.
None of those questions need final answers at this stage. Approximate positions are fine. What matters is that the electrician can size the job, plan the cable routes and give you an accurate quote before the first brick is laid.
How does the electrician fit around other trades?
First fix is the stage where site coordination matters most. Plumbers run pipes through the same stud walls and floor voids that the electrician needs for cables. If both trades arrive on the same days without having spoken beforehand, they slow each other down and occasionally create problems that take time to resolve.
The straightforward fix is a short meeting at the start of the groundworks phase with the builder, the electrician and the plumber present. Fifteen minutes spent agreeing who runs where through which void saves hours on site. Most builders in the High Wycombe and Marlow area manage this as a matter of course, but if yours has not suggested it, ask.
Second fix has fewer clashes because it comes after the dirty trades have finished. The main coordination point is sequencing with the kitchen fitter. The electrician ideally visits just before the kitchen units go in to wire the appliance connections, then returns after the kitchen is fitted to commission and test. Some kitchen fitters prefer to have all the connections ready before they arrive; others are happy to work around a second visit. Agree this in advance and put it in the programme.
What about the connection back to the existing house?
An extension almost always draws its electrical supply from the existing consumer unit in the main house. The electrician will assess whether the existing unit has the capacity and the spare ways to accommodate the new circuits. If it does not, a consumer unit upgrade may be needed as part of the same project, which adds a small amount of time and cost but is far better discovered at the planning stage than on first-fix day.
The cable route between the existing house and the new extension also needs agreeing early. Running cables through a cavity wall, under a floor or across a ceiling void is straightforward when the building is open. Doing the same job after the extension is finished and decorated is a much harder task.
What do I do next?
If your extension project is at the drawing stage or about to move to groundworks, now is the right time to get in touch. Send the details of your project, including whatever floor plans you have and the rough programme your builder has outlined, and Jess will come back to you with a clear picture of when to book first fix, what the second-fix schedule should look like, and what the work is likely to cost. The contact form and phone number are at the top of this page.
