First Fix Electrics Explained: What Builders Need to Know

First fix electrics is the wiring stage that happens before plastering: every cable, back box and chase that disappears behind the walls, floor and ceiling once the building closes them in. It runs from the incoming supply through to every point of use, and has to be finished before the plasterer arrives, well ahead of sockets, switches or lights going in. Second fix, the visible finish, follows once the walls are closed.

The most expensive mistake on a domestic build is rarely a wrong cable or an undersized breaker. It is the plasterer arriving on the Tuesday when the electrician was pencilled in for the Thursday. First fix electrics is the stage where everything gets buried, and once the boards are on and skimmed, a change that would have taken twenty minutes costs you three trades and a fortnight. There is plenty of work in the pipeline to get this wrong on: 5,440 dwellings across Bristol were under construction at 31 March 2025, according to Bristol City Council’s Residential Development Survey. This guide is for builders, site managers and homeowners running their own projects.

Table of Contents

• What First Fix Electrics Actually Covers
• First Fix vs Second Fix: The Split That Governs Your Programme
• Where the First Fix Wiring Stage Sits in the Build Sequence
• What the Builder Needs Ready Before First Fix Electrics Starts
• The Rules That Shape Every First Fix Decision
• What Goes Wrong When the Sequencing Slips
• Take a Typical Bedminster Kitchen Extension
• First Fix in Bristol’s Older Housing Stock
• First Fix on Small Commercial and Office Fit-Outs
• Booking First Fix Electrics in Bristol

TL;DR

• First fix is everything that gets covered up: cable runs, back boxes, chases, capping, earthing and bonding and the consumer unit position.
• The electrician needs to be finished before plasterboard goes up or walls are plastered. This is the one sequencing rule that costs real money when it is broken.
• Cables buried less than 50 mm deep must sit in the permitted zones and normally need 30 mA RCD protection, so cable routes are not a free choice.
• Vertical chases must not exceed one third of the wall or leaf thickness and horizontal chases one sixth, under Approved Document A.
• In new dwellings, switch and socket centre lines belong between 450 mm and 1200 mm above finished floor level, so screed depth has to be known before boxes are set.
• The builder’s job beforehand: studs and noggins in, kitchen layout frozen, floor build-up confirmed, and a plasterboard date everyone has seen.

What First Fix Electrics Actually Covers

Ask five people what is first fix electrical work and you get five answers, most ending at “the cables”. The cables are the visible part. The stage is better defined by one test: if it disappears behind plaster, plasterboard, screed or a floor deck, it belongs to first fix.

On a typical domestic job that means the full route from the incoming supply to every point of use. The meter position and tails route get settled first, because everything downstream hangs off where the consumer unit lands. Then the circuits: ring or radial socket circuits, lighting, dedicated supplies for the cooker, shower and boiler, and increasingly a run out to a car charging position whether or not the charger goes in this year.

The Physical Work on Site

Cable runs are pulled and dressed, back boxes fixed to studs or set into chased masonry, and every chase capped so the plasterer has a flat surface and the cable has some protection from the next person with a nail gun. Earthing and main protective bonding conductors go in at this stage, since those routes are inside the fabric. Smoke and heat alarm cabling is first fix too.

The quiet items catch builders out: draw wires for data and TV, oval conduit where a future cable might be wanted, a capped spare route to the loft. None of it takes long while the walls are open. All of it is a nightmare later.

What Is Not First Fix

Faceplates, light fittings, the consumer unit itself, alarm heads and final connections are second fix, as is the testing and certification. Plenty of jobs run into trouble because someone assumed the consumer unit goes in at first fix. It does not, though the position and the tails route absolutely do.

First Fix vs Second Fix: The Split That Governs Your Programme

The clean way to hold the two stages in your head is to think about who follows you. First fix is followed by the plasterer, second fix by the decorator and the client. Everything else falls out of that.

On a full house rewire the two stages sit either side of the plastering, and the first fix half is the noisy, dusty one. Our guide to how long a rewire takes puts the first fix stretch on a three bed house at 3 to 5 days.

Where the First Fix Wiring Stage Sits in the Build Sequence

The first fix wiring stage is not a fixed date. It is a window defined by two other trades. It opens when the structure is far enough along to fix to, and closes when the boarding starts.

On a stud-built extension or loft conversion, the sequence that causes the fewest arguments runs like this. Structure and stud work complete, including noggins where accessories and heavy fittings will land. Windows and external doors in, so the shell is secure. First fix plumbing next, because pipe routes are less forgiving than cable routes. Then first fix electrics, then insulation, boarding and plaster. We go deeper on this in our guide to second fix electrics.

On masonry the same logic applies, with chasing inserted before the plaster rather than the boards. Either way, the phrase to have in the programme is first fix before plastering, and it should be a dated line rather than an assumption.

What the Builder Needs Ready Before First Fix Electrics Starts

This is the section worth forwarding to whoever runs the site. Most delays we see on first fix electrics are not electrical problems. They are decisions nobody has made yet.

• Frozen layouts. Kitchen and utility drawings signed off, appliance positions included. A socket behind a fixed oven housing is a socket that no longer exists.
• Floor build-up confirmed. Screed and finish thickness sets finished floor level, which sets socket heights.
• Stud work and noggins complete. Boxes need something to fix to, and so do heavy pendants and extractors.
• Consumer unit position agreed with the client, not assumed. It has to stay accessible and cannot end up boxed inside a fitted cupboard.
• Incoming supply resolved. A new supply, moved meter or upgrade is a distribution network operator conversation with its own lead time.
• A lighting plan, even a rough one. Downlight positions have to reconcile with joists, and switching with which way the doors swing.
• The plasterboard date, in writing. Most sequencing failures are really communication failures.

Where we handle the building side of a project too, that coordination gets easier because it sits under one roof. Our building works and renovations service exists partly because splitting responsibility across trades is where programmes go to die.

The Rules That Shape Every First Fix Decision

First fix looks like a free hand. It is not. A handful of rules dictate where cables can run, how deep a chase can go and how high a box sits, and all are easier to satisfy before the walls close than after.

The current edition is BS 7671:2018+A4:2026, published on 15 April 2026. The previous version is withdrawn six months later, on 15 October 2026, so any first fix carried out now should already be designed to the new amendment rather than the one it replaces, according to NAPIT’s statement on Amendment 4.

Cable Zones and the 50 mm Rule

Cables buried in a wall or partition at a depth of less than 50 mm have to sit in a recognised zone, which the IET describes as a zone “within 150 mm from the top of the wall or partition or within 150 mm of an angle formed by two adjoining walls”, or running horizontally or vertically to the accessory they serve. Where such a cable has no mechanical protection or earthed covering, it needs additional protection from a 30 mA RCD, per the IET’s guidance on cables concealed in a wall or partition. A diagonal run across a wall is not a shortcut. It is a defect waiting for a picture hook.

Chase Depths Under Approved Document A

Chasing is structural work, whatever it feels like with a wall chaser in your hands. Approved Document A, paragraph 2C30, is explicit: “vertical chases should not be deeper than 1/3 of the wall thickness or, in cavity walls, 1/3 of the thickness of the leaf; horizontal chases should not be deeper than 1/6 of the thickness of the leaf of the wall”, as published in Approved Document A. On a single leaf that arithmetic gets tight quickly, and it is one reason a run goes the long way round.

Socket and Switch Heights Under Part M

In new dwellings, Approved Document M paragraph 1.18 requires that switches and sockets serving habitable rooms have “their centre line 450-1200mm above floor level”, according to Approved Document M Volume 1. Floor level means finished floor level, which loops straight back to knowing your screed depth before the boxes go on.

Part P and What Has to Be Notified

Approved Document P defines notifiable work in dwellings as the installation of a new circuit, the replacement of a consumer unit, or any addition or alteration to existing circuits in a special location, and requires the compliance certificate to reach the occupier and building control body within 30 days of completion, per Approved Document P. Most extensions and kitchen jobs trigger this on the new circuit alone. We cover the detail in our guide to the Part P Building Regulations.

Landlords extending or altering a let property have an extra reason to get this paperwork right: councils can issue a financial penalty of up to £40,000 for breaching the electrical safety duties in the private rented sector, according to gov.uk’s electrical safety standards guidance, quite apart from the cost of putting non-compliant first fix work right.

Part S and the Car Charging Route

Since 15 June 2022, the Building Regulations have required that “a new residential building with associated parking must have access to electric vehicle charge points”, with a minimum nominal rated output of 7 kW per chargepoint, according to Approved Document S. Even outside the requirement, a capped route to a parking position at first fix costs very little next to retro-fitting one, as our EV charger installation cost guide sets out.

If the property is let out once finished, landlords and renters installing a chargepoint can currently claim 75% of the cost, up to £500 per socket and up to 200 sockets a year, through the EV chargepoint grant for landlords, which is funded until 31 March 2027. Capping a route at first fix is what makes claiming it straightforward later.

Joist notching sits alongside these, though the permitted positions come from the timber design guidance and the engineer’s detail. Agree them before anyone cuts, particularly on engineered joists.

What Goes Wrong When the Sequencing Slips

Every one of these is something we have watched happen on somebody’s site. None of them are exotic.

1. Boarding starts before first fix is finished. Either the boarder works around half-run cables, or the run gets abandoned and re-routed the ugly way.
2. The kitchen layout changes after the cables are in. A fridge and a larder swap places, and there is now a socket behind a fixed panel and none where the coffee machine goes.
3. Socket heights measured off the slab. Then 75 mm of screed and tile goes down and every accessory sits low against the skirting.
4. Chases cut too deep. Usually horizontal, usually on a single leaf, usually discovered by someone else later.
5. A box fixed to plasterboard instead of a noggin. It holds until the first time somebody pulls a plug out with feeling.
6. No capping over chased cables. The plasterer’s trowel finds the cable, or the cable telegraphs through the skim as a line down the wall.

Take a Typical Bedminster Kitchen Extension

Picture a single storey rear extension on a Victorian terrace off North Street. Nine square metres, bifolds at the back, a new kitchen, and the rear wall opened up. The builder wants the boards on by the end of week four.

Weeks one and two are groundworks, blockwork and the steel. Week three is roof, windows and bifolds in, plus the stud partition to the utility with noggins at the heights the kitchen fitter specified. First fix plumbing runs on the Monday of week four, and the electrical first fix takes the Tuesday and Wednesday. Two new circuits come off the existing board, the hob and oven get their own supply, and a spare capped route runs to the rear wall for outside lighting the client has not committed to. Chases into the retained masonry stay vertical and shallow, drops come down within the permitted zones, and anything within 50 mm of the surface picks up RCD protection. Socket boxes are set from a finished floor level that assumes 100 mm of insulation and 75 mm of screed, because the builder confirmed the build-up rather than guessing.

Now change one thing. Suppose the client decides in week five, after the skim, that the island should have power. That is a lifted floor, a new run, a re-board, a re-skim and a repaint, where the same work on the Tuesday of week four would have been an extra hour. The cost of a change scales with how buried it already is.

First Fix in Bristol’s Older Housing Stock

Bristol makes first fix more interesting than a greenfield plot does. Much of the city’s work is extensions, loft conversions and knock-throughs on Victorian and Edwardian terraces in Bedminster, Southville, Totterdown and Brislington, where new fabric meets solid brick, lath and plaster ceilings, and whatever the last four owners did to the wiring.

Solid brick changes the chasing calculation, because there is no cavity leaf to give you depth. Lath and plaster punishes anyone fishing a cable through it in a hurry. And the join between a new stud extension and an old solid wall is where cable routes get improvised when the sequencing is tight.

There is also a lot of it about. Bristol recorded 1,703 net additional dwellings completed between 1 April 2024 and 31 March 2025, against a five year annual average of 1,777, and had 18,529 dwellings sitting with planning permission at 31 March 2025, according to Bristol City Council’s Residential Development Survey. Add the extension and conversion work that never appears in those figures and the volume of first fix happening across the city in any given week is considerable.

Across South Bristol specifically, Bedminster and Knowle terraces often still carry rubber or lead sheathed cable from before the 1960s tucked behind later plasterwork, which turns a first fix job on an extension into a conversation about a partial or full rewire of the rest of the house before anything new is connected to it.

We are a NAPIT approved contractor based in South Bristol, so most of this work is on our doorstep. Our new build and extension wiring service covers first and second fix on extensions, conversions and whole house projects, and our house rewiring page shows how the stages fit together on a full house rewire.

First Fix on Small Commercial and Office Fit-Outs

The same principles apply to office and small commercial fit-outs, with the containment scaled up. Instead of oval conduit and capping you are dealing with basket, trunking and dado, and instead of a rough lighting plan you have a lighting design with emergency lighting positions agreed before the ceiling grid goes in.

Data cabling is the item most often left too late: routes to desk positions, comms cabinet location and power for it. The sequencing question is identical to the domestic one. When does the ceiling close, when do the partitions get boarded, and is the electrician done before either.

Booking First Fix Electrics in Bristol

Most builders we work with do not want an electrician who turns up and asks where the sockets go. They want one who reads the drawings beforehand, flags the decisions that need making, and says honestly which week the first fix window needs to be. We work across Bristol on extensions, loft conversions, kitchens and full rewires, as well as small commercial fit-outs, and we self-certify notifiable work under Part P so the building control paperwork is handled rather than left hanging.

Costs vary by job, but to give a sense of scale: a full house rewire starts from £4,500 + VAT, with partial rewires priced from a survey (see our house rewiring price guide), a new consumer unit fitted alongside first fix starts from £680 + VAT, and a full EICR on the existing circuits you are extending from starts from £200 + VAT.

If you have a programme with a boarding date on it and a gap where the electrician should be, the useful conversation is the early one. Send us the drawings and the dates and we will tell you what we need from you and when. You can get in touch with our team to talk through a specific job.

First fix electrics is not complicated work. It is work with a short window and a long memory, and the jobs that run smoothly are the ones where somebody put the date in the programme early and told the electrician about it.