What did first fix electrics cost on your extension?

The electrician's revised quote for the wiring came in at £3,290.07, and it excluded the downlights and the switches. What I actually paid him was £6,624 across eight payments between July 2022 and December 2023, which covers the consumer unit move, materials the quote never carried, extras added once the room was real, second fix and the final certification.

Should first fix plumbing be push-fit or copper?

My plumber used push-fit nearly everywhere and joined it onto the copper that runs through the rest of the house, which is a legitimate choice and a faster one. The point is that it is a choice about your house, so agree it before the work happens rather than finding out from a photograph.

What does building control want to see before you board out?

Insulation and the gap above it. On my approval the relevant condition was a continuous 30mm air gap above the roof insulation board, and building control asked for thicker insulation part way through the build because of how much glass the extension carries. Once plasterboard is on, none of it can be checked without taking the ceiling down again.

Can you move a socket after first fix?

Yes, and you pay for it twice. First fix positions are chased into the wall, boarded and skimmed within a few weeks on most builds, so a change afterwards means cutting out finished plaster and making it good. Every socket on my build was set against a kitchen drawing months before the units existed.

First Fix: Wiring, Pipes and Insulation on a Real Extension

Nothing that happened between July and September 2022 shows up in a photograph of the finished room. Cables in a wall. Pipes under a floor. Boards between rafters. It is also the only phase where a decision made in an afternoon gets sealed behind plaster for the life of the building.

Working off a board that had not moved yet

First fix started nine months after the blockwork did, which tells you how far the programme had drifted by then. The thing that finally released it was the electricity supply. The network operator's hookup and the meter move both landed on 29 July 2022, and the electrician invoiced £690 for the consumer unit work that same day.

A fortnight later he was still branching the new circuits out of the old board. It sat on the inside of the garage wall, where the original meter had been, and every cable for the extension left from there and ran through the wall into a room that was still a shell. The board only reached its final position on the new external wall later on. If you are wondering why an electrician would wire a new room from an old board, it is because the alternative is a house with no power in it for a fortnight while you are living in it.

Interior brick wall with an exposed timber joist ceiling above, a thick bundle of black electrical cables and green and yellow earth wires descending from the ceiling and passing through a roughly cut rectangular opening in the brickwork. — 10 August 2022. New circuits branched out through the wall from the old consumer unit position, feeding the sockets and lights of a room that had no plaster and no floor.

The electrician's colleague did the bulk of the first fix between 15 and 23 August while the electrician himself was on holiday. We liaised by email, and he sent video of the progress. That worked better than it sounds, but only because the positions had already been agreed.

Socket heights were settled in that same window. Every one of them was set against the kitchen drawing rather than against the room, because the room had no units in it and would not have any until December. That is the whole dependency in one sentence, and the specification chapter is where the drawing itself gets argued over.

The lighting is the part of the design I would repeat exactly. In cable terms it came down to two rings at each end of the space, one carrying the downlights and one carrying the warmer fittings, the wall lights and the pendants. That split is what makes the finished room usable at seven in the morning and at ten at night without touching the same switch.

A wall with a vertical chase cut into it, two clipped cables running down inside the chase into three recessed metal back boxes at floor level containing wiring with orange connectors, and a metal pipe running vertically up the right hand side. — 7 September 2022. Back boxes at the old range cooker position, which became the wall behind the television. The pipe on the right is the old gas supply, capped by the plumber and taken out entirely when the kitchen floor was dug up.

One day of plumbing, and a wall nobody agreed

The plumbing scope went out on 9 August as a fifteen item list: a 30kW boiler, 66m² of underfloor heating, a water softener, the boiling water tap, an outside tap, and a gas feed run to the island and capped off rather than left out, so that a gas supply to that spot stays possible in future. The quote came back on 1 September at £10,400 including VAT, half of it up front.

First fix plumbing then took a single day. The plumber and a colleague arrived, worked, and left, and the whole of the new house's pipework was in.

Push-fit nearly everywhere. I am not going to pretend that is a fault, because it is fast and it is perfectly good, but it is a decision about the plumbing in your house that I discovered by looking at a photograph. Where the new work meets the original copper, which is what the rest of the house is built from, the two systems sit side by side in the same cavity.

They also chased pipework into the wrong wall.

Warning

First fix is fast and it is covered up quickly, so the review window is days rather than weeks. Mark up a drawing with which wall each run belongs in and walk it with the trade before they start, then photograph every wall and ceiling before plasterboard goes near them. Push-fit throughout is a legitimate choice, but it should be a choice you made rather than one you found out about.

An opened wall cavity with vertical copper pipes dropping from the ceiling into white plastic push-fit fittings and horizontal copper runs below, grey electrical cables running through the timber studwork alongside. — 7 September 2022. Copper dropping from the ceiling where the old boiler used to sit, meeting new push-fit below. The dark band along the left edge is the original steel in the house, which the new steelwork was eventually welded to.

The insulation building control actually cared about

The roof was the one part of first fix that building control took a direct interest in, and the reason is the glass. A room with that much glazing in it loses heat somewhere, so the roof was asked to make up the difference.

The original specification called for 195mm of PIR board between the rafters. The rafters that went in were shallower than the ones that had been specified, which is a story belonging to the structure phase, so the as-built arrangement ended up as 150mm of board in the rafter bay with a further 40mm layer beneath the rafters before the plasterboard. On 12 August the upgrade from 100mm to 150mm plus deeper battens was invoiced as an extra at £773, on top of the £1,460 already paid in June for the roof insulation itself.

Foil faced PIR insulation boards fitted between deep timber roof rafters, seen from inside an extension, with a painted steel beam below and a garden visible through the open wall beyond. — 15 September 2022. PIR board going in between the rafters. Building control wanted more insulation in the roof than the original detail carried, because of how much glass the extension has.

One condition on the approval was a continuous 30mm air gap above that insulation board, running through to the eaves. It is a ventilation requirement, and it is entirely invisible once the ceiling is boarded, which makes it exactly the sort of thing that quietly does not happen.

20mm

The gap left above 150mm of board sitting in a 170mm rafter, against an approval condition asking for a continuous 30mm. That arithmetic takes thirty seconds and it is worth doing yourself, on your own drawings, before anybody stands a sheet of plasterboard up against the ceiling.

What the two trades actually cost

The electrician's revised quote for the wiring came in at £3,290.07, and it explicitly excluded the downlights and the switches. I paid him £6,624, spread over eight payments between July 2022 and December 2023.

That gap is not an overrun, and it is not sharp practice either. It is the consumer unit move, the materials he bought that were never inside the quote, the second fix, the extras that only became obvious once the room was real, and a final invoice a full year after we had moved back in, covering the certification building control needed to sign the job off.

The plumber went the other way. His quote came in at £10,400 including VAT for the whole fifteen item list, and he was paid £9,694.75 across three invoices between September 2022 and March 2023. It landed under because the water softener came off his bill and I bought it separately. One line was disputed at the end, a pipe run into the garage for the fridge ice maker that I had assumed was inside the original scope, settled by paying £60 of materials against an invoice that had started at £140.

Tip

Ask any trade quoting first and second fix as one figure which materials are inside the number and which are not. On my build that single question would have explained most of the difference between the electrician's quote and his final total, a year before I worked it out from the invoices.

I took an unreasonable number of photographs that autumn, on the theory that if anything ever went wrong we would at least know what was running where. Three years on, that is still the best hour I spent on the entire build. Everything in this phase is invisible within six weeks and permanent after that.

Working through first fix on your own build? The first fix phase guides cover the same ground as a sequence of tasks rather than a story: electrical layout planning, first fix electrics, first fix plumbing and insulation, in the order the decisions have to be locked down.

First Fix

Working off a board that had not moved yet

One day of plumbing, and a wall nobody agreed

The insulation building control actually cared about

What the two trades actually cost

Frequently asked questions

Do this phase on your own build