First Fix and Second Fix Electrics Explained
Updated: Oct 9
First fix electrics is everything the electrician installs while the walls, floors and ceilings are still open: the cables, the back boxes for sockets and switches, and the routes to the consumer unit. Second fix is everything that happens after the plasterer has finished: fitting the sockets, switches and lights, connecting the consumer unit, then testing and certifying the work. On a rewire, an extension or a big refurbishment, the two stages sit either side of plastering, with other trades working in between.
What is first fix electrics?
First fix is the hidden part of the installation, the bit you'll never see again once the walls are finished. It is where most of the electrical decisions are locked in, and on a full house rewire it is also the longest and messiest stage. A typical first fix includes:
agreeing the final positions of every socket, switch, light, cooker point, spur and data point
chasing walls and fixing back boxes at those positions
lifting floorboards and drilling or notching joists to run cables
running every circuit's cable from each point back to where the consumer unit will be
running cables for things that come later, such as a cooker, an electric shower, extractor fans, outside lights, smoke alarms or an EV charger
leaving cables coiled and labelled at each point, ready for second fix
Nothing is live at this stage. The cable ends are left safe and the board is usually not connected yet.
The rules that matter at first fix
Where cables go is not a free choice. A cable buried less than 50 mm deep in a wall must run in a safe zone (straight up, down or across from an accessory, or within 150 mm of the top of the wall or a corner) and must also have 30 mA RCD protection unless it is mechanically protected, for example in earthed metal conduit (BS 7671 regulations 522.6.202 and 522.6.203). Approved Document P also points out that other parts of the Building Regulations apply: Part A controls the depth of chases in walls and the size of holes and notches in joists, and Part B covers fire resistance where cables pass through floors, walls and ceilings. Part M sets heights for sockets and switches in new dwellings. An electrician who drills every joist in the middle of its span, or chases deep horizontal runs across a wall, is creating problems that will be plastered over.
What is second fix electrics?
Second fix is the finishing and commissioning stage, once plastering has dried and ideally before final decoration. It usually includes:
Fitting faceplates for sockets, switches, fused spurs and cooker switches to the back boxes.
Installing light fittings, pendants, downlights and extractor fans.
Connecting fixed appliances such as a cooker, hob or electric shower.
Fitting and connecting the consumer unit (or making the final connections if it was mounted at first fix).
Inspecting and testing every circuit to BS 7671 before it is energised for use.
Issuing the certificates.
Testing is not a formality. It checks continuity, insulation resistance, polarity, earth fault loop impedance and RCD operation, and it is the evidence that the hidden first fix work was done properly.
First fix vs second fix at a glance
When: first fix before plastering; second fix after plastering, before or around decorating.
What you see: first fix is cables and empty boxes; second fix is the finished accessories and a working installation.
Who comes next: after first fix, the plasterer; after second fix, the decorator and you.
Hardest to change: first fix. Moving a socket after plastering means chasing and making good. Changing a faceplate at second fix is five minutes.
The order of work with other trades
On a rewire or extension, electrical work is threaded through the other trades. A typical sequence looks like this:
Strip-out and structural work. Old accessories and wiring are removed or made safe, and any builders' work is done.
Electrical first fix and plumbing first fix. Often in the same week. The two trades need to agree routes so pipes and cables don't clash in the same joist space.
Inspection before covering up. On notifiable work that isn't being self-certified, building control may want to see things before they're hidden.
Plasterboarding and plastering. Back boxes are set flush with the finished surface.
Kitchen and bathroom fitting. Units go in so second fix can connect to their final positions.
Electrical second fix and plumbing second fix.
Testing, certification and handover.
Decoration. Some electricians prefer to fit faceplates after the mist coat so they don't need removing for painting.
Second fix is usually quicker and much cleaner than first fix, which is why, in what a house rewire involves, most of the disruption sits in the first half of the job.
Where the consumer unit fits in
The consumer unit can go in at either stage. Some electricians mount the new board at first fix and connect it at second fix. On a rewire where you're living in the house, a temporary supply often keeps a few circuits running in between. Replacing the consumer unit is notifiable in its own right, and a new board brings modern RCD or RCBO protection for all the new circuits. If the board is the only thing that needs changing, a consumer unit upgrade is a separate, much smaller job.
What paperwork should you get at the end?
Nothing is certified at first fix, and our guide to electrical certificates explains each document you should end up with. At the end of second fix, a rewire or new circuits should leave you with an Electrical Installation Certificate, with the test results and a schedule of circuits. Because new circuits and a new consumer unit are notifiable under regulation 12(6A) of the Building Regulations 2010, you should also receive a Building Regulations compliance certificate within 30 days of completion when a registered electrician who can self-certify the work under Part P has done the job. If the work goes through building control instead, one of the other routes under Part P building regulations, the building control body issues a completion or final certificate once it is satisfied.
From our base in Shifnal we carry out first and second fix on rewires, extensions and refurbishments across Shropshire and the West Midlands, and we work to the other trades' programme. If you're planning a project and want the electrics sequenced properly from the start, get in touch.
First fix and second fix questions, answered
Can a different electrician do second fix?
It is possible, but it causes problems. The electrician who tests and certifies the installation is signing for hidden work they didn't see. Many won't do it, and those who do may need to open things up to be satisfied. Where possible, use one electrician for both stages.
How long between first fix and second fix?
As long as plastering, drying and any kitchen or bathroom fitting take. On a single room it may be a week or two. On a full rewire with a refurbishment, it can be much longer.
Can I change socket positions after first fix?
Before plastering, usually yes, at the cost of extra cable and a new back box. After plastering, it means chasing, a new box and making good. Walk round every room with your electrician before first fix and mark positions on the wall.
Do first fix cables need testing?
Good practice is to check them before they're covered. A damaged cable found before plastering is a small job; after decorating, it is a big one.
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