First Fix vs Second Fix: What You Need to Know Before Your Extension

Click to enlarge

Your electrician is two days into first fix. He stops and asks: "Where exactly do you want the sockets either side of the island?"

You don't know. The kitchen designer hasn't finished the layout yet.

That question costs money. Cables in the wrong place, back boxes repositioned through set plaster, work stopped while you chase a decision. First fix is a deadline, not just a build stage. And unlike most build jargon, it's a term with no legal definition, which means your builder, your electrician, and your plumber may all mean slightly different things when they use it.

What "First Fix" and "Second Fix" Actually Mean

First fix is everything that goes inside the walls, floors, and ceilings before they're covered over. Cables, pipe runs, structural timber, insulation, back boxes for sockets and switches. All hidden once the plasterer finishes.

Second fix is everything that connects to that infrastructure after plastering. Faceplates on sockets. Light fittings. Taps on supply pipes. Radiators hung on valves. The consumer unit energised and circuits tested.

Simple enough in principle. The complication is that these terms are not set down in any legal or regulatory standard. As one builder on PistonHeads put it: "there is no standard definition of first fix or second fix and everyone has their own definition." This matters because payment milestones in building contracts often reference "first fix complete" and "second fix complete." If your builder's definition of first fix doesn't match your electrician's, you end up in a dispute about what you've paid for.

The fix is straightforward: specify exactly what each stage includes in your contract, trade by trade. Don't rely on the phrase alone.

Three trades do first and second fix work on a kitchen extension: electrics, plumbing, and carpentry. Each has a different scope at each stage.

First Fix Electrics

First fix electrics covers all circuit cables from the consumer unit location out to every socket, switch, and light position. Back boxes get recessed into walls and ceilings. Earth bonding connections are made. Data cabling (Cat6 Ethernet) runs alongside the electrical cables if you've asked for it. The consumer unit carcass goes in, but nothing is connected or energised yet.

Your electrician runs cable through joists, chases it into blockwork, and positions every back box at the exact agreed location. Cables must run vertically or horizontally (never diagonally), and holes drilled through joists should be near the middle of the joist depth to avoid weakening the timber.

Nothing is live at this point. It's all there, waiting.

The decisions that drive first fix electrics are covered below, but the critical one is this: your kitchen layout must be finalised and a dimensioned drawing handed to your electrician before they start. Socket positions, appliance connections, under-cabinet lighting feeds, island power runs: all of these depend on knowing exactly where the cabinets go.

First Fix Plumbing

First fix plumbing covers hot and cold supply pipes, waste pipes, radiator pipe runs, boiler flue and gas connections, and underfloor heating pipe layout and manifold installation. Everything up to the point where you'd need a finished wall or floor to connect to. Taps, sanitaryware, radiators, and the boiler itself come later at second fix.

Two details that catch people out. First, if you're having an island with a sink, the waste and supply pipes to the island position must be routed through the floor slab or screed before it's poured. Missing this means breaking up the floor later at

. Second, if you're relocating the boiler (common on kitchen extensions where the boiler was on a wall that's now being knocked through), the new flue position must comply with Approved Document J: at least 300mm from opening windows and doors, 2,000mm below roof windows. Your plumber should confirm the flue position before running pipework to it.

Underfloor heating adds a hard dependency. The pipe loops go down at first fix, before screed is poured. The system must be filled and pressure-tested at 6 bar before the screed contractor arrives. A leak discovered under set screed requires breaking the floor up to find it. Do not let anyone pour screed over untested UFH pipework.

First Fix Carpentry

Carpentry first fix is often the first trade through the door after the building is weathertight. It covers studwork partitions, door linings (the frames that doors hang from, not the doors themselves), floor boarding in areas that need it, and noggins: short timber blocks fixed between studs to provide solid fixing points for TV brackets, heavy wall-mounted cabinets, and socket back boxes.

Insulation and plasterboarding may be done by the carpenter or by a separate trade. In many builds, a dryliner or boarder handles this phase: cutting insulation batts to fit between studs and rafters, fixing the vapour control layer, and screwing plasterboard into position. On larger projects, dryliners are often faster and more consistent on boarding than a general carpenter. Some contractors use a single carpenter throughout; others bring in a boarding gang specifically for the boarding phase. Confirm with your builder which trade covers insulation and boarding and check it is included in the price. Either way, insulation must be in place and visible for the building control pre-plaster inspection.

Carpentry first fix doesn't get the same attention as electrics and plumbing, but one decision here has lasting consequences: noggin positions for wall-mounted items. If you want a wall-mounted TV, a heavy mirror, or floating shelves, tell the carpenter exactly where before they board the walls. A noggin behind plasterboard gives you a solid fixing. Without one, you're relying on plasterboard anchors, which have weight limits and can pull out of the board.

Second Fix: What Connects to All of That

Second fix happens after plastering. It's everything visible in the finished room.

Second fix electrics: faceplates screwed onto back boxes, light fittings connected and hung, the consumer unit wired, circuits tested, and the installation certified. In a kitchen extension, this typically happens in two sub-visits. The electrician returns after plastering to fit socket and switch faceplates, before the kitchen goes in. Then they return during or after kitchen installation to connect built-in appliances: the oven, the hob, the extractor, the dishwasher isolator switch.

This two-visit structure confuses almost every homeowner who hasn't been through it before. "When does the electrician come back?" is one of the most common questions on r/DIYUK from people mid-build. If your electrician only mentions one return visit and you have built-in appliances, clarify this before they start. Get it confirmed in the quote.

Click to enlarge

Second fix plumbing: taps connected to supply pipes, sanitaryware installed (sinks, toilets), radiators hung on their first-fix valves, the boiler physically mounted and commissioned, and the hot water system tested. Your plumber issues a Gas Safe certificate for the boiler. Building control needs this at the final inspection.

Second fix carpentry: doors hung on their first-fix linings, skirting boards, architraves, and any built-in joinery. This is the finish carpentry that makes the room look complete.

The Sequence

The build sequence through first and second fix isn't arbitrary. Each step depends on the one before it.

Structure must be weathertight before any first fix trade starts. Roof on, windows in, external doors hung. You can't run cable through a structure that's open to rain.

Carpentry first fix goes in first: partitions, door linings, noggins. Then electrics and plumbing work simultaneously where possible. Electrician and plumber can work through the same space at the same time if they're not in each other's way. Coordinate their bookings. If one trade is booked two weeks after the other, you've added two weeks to your programme for no reason.

Building control inspects after first fix, before plastering. This is the pre-plaster inspection: all cabling, pipework, insulation, and fire stopping must be visible. Do not let the plasterer start before the inspector has been.

Plastering follows the inspection. In winter, allow two to three weeks for the plaster to dry properly. A dehumidifier running continuously speeds this up.

Screed (if you have UFH) follows plastering or runs alongside it, then needs its own curing time: roughly one day per millimetre of depth for sand-and-cement, faster for liquid screed but still four to six weeks before floor coverings go down.

Second fix trades return after plastering (and screed curing, if applicable). For the kitchen specifically, second fix electrics and plumbing complete in stages around the kitchen installation. Worktops must be fitted before the sink is connected. Units must be in position before the hob and oven connections are made. The sequence interlocks.

The Decisions You Must Lock In Before First Fix

Every one of these must be finalised before your electrician starts. Not roughly agreed. Confirmed, dimensioned, and signed off.

Kitchen layout, to the millimetre. Socket positions, switch positions, under-cupboard lighting feeds, island power, integrated appliance connections: all depend on knowing exactly where the cabinets go. Your kitchen designer should give you a dimensioned layout drawing. Hand it to your electrician. If they don't have it, they can't start.

Lighting zones. How many independently switchable circuits? A single switch for the whole extension, or separate zones for dining, cooking, and task areas? Dimmable circuits need different wiring. Decide before first fix. Changing zones later means rewiring.

Socket count. Plan more than you think you need. An island with four double sockets sounds excessive. It won't be. USB-C sockets in the kitchen, outdoor sockets for the garden, a socket at the consumer unit position for the broadband router: all cheaper at first fix than at any other time.

Data and ethernet points. Cat6 cable costs almost nothing to run at first fix (your electrician installs it alongside the electrical cabling). Running it after plastering means lifting floors or surface-mounted conduit. If you want wired connections for a home office, TV position, or ceiling-mounted WiFi access point, now is the time. A ceiling WiFi access point installed at first fix costs

. Retrofitting the same thing after plastering costs .

EV charger preparation. If you don't have an electric vehicle now but might in five years, a dedicated 32A circuit from the consumer unit to the driveway or garage costs very little at first fix. Installing it retrospectively means a new cable run all the way back to the consumer unit. A conduit buried at slab stage costs less than an hour of labour.

Consumer unit position. This affects every circuit run in the building. The consumer unit needs to be on a load-bearing wall, accessible (not inside a fitted cupboard), with switches at 1,350mm to 1,450mm above floor level (Part M of the building regulations), and critically: not within 3 metres of a hob. If your kitchen design puts the hob near a wall that would otherwise be the natural consumer unit location, you have a conflict that needs resolving before first fix starts. On my extension, the 3-metre rule pushed the consumer unit to a position behind the recycling bin area. Not ideal for aesthetics, but it met the regulation and kept every circuit run reasonably short.

UFH scope and zones. If you want underfloor heating, the pipes go in at first fix before the screed. This is not something you add later. Confirm UFH scope, choose a plumber who installs UFH (not all do), and agree the manifold position before first fix begins.

Boiler and heating changes. If the extension changes your house's heat load, the boiler may need upgrading or relocating. The pipework for any new radiators or zones is first fix work. Your plumber advises on sizing; you need to have their recommendation before the plumber starts running pipes.

Part P: Why Your Electrician Must Be Registered

New circuits in an extension are notifiable under Part P of the building regulations. All of them. A new circuit from the consumer unit to a socket, a new lighting circuit, the consumer unit itself if it's being replaced or relocated. All notifiable.

What that means in practice: the work must either be submitted to building control for separate inspection, or done by an electrician registered with a competent person scheme (NICEIC, NAPIT, or ELECSA). Registered electricians can self-certify their own work and issue an Electrical Installation Certificate directly, without a separate building control notification. This saves you the building control fee for electrical work and removes a scheduling dependency from your programme.

Ask every electrician you quote whether they're Part P registered before they start. If they're not, you need a separate building control inspection for the electrics, you pay for it, and you wait for it to be available.

Building Control at First Fix

Your building control officer inspects first fix before plastering. This is the pre-plaster inspection: cable routes and types, pipe runs, insulation in walls and roof, cavity closers around windows, fire stopping at the junction between old and new construction.

Have your insulation product datasheets on site. The BCO checks the product specification against Part L energy targets, not just whether something was installed. If the product is wrong (wrong thermal resistance, wrong thickness) or badly fitted (compressed, gapped), they'll flag it before it disappears behind plasterboard. Fixing insulation problems after boarding means stripping everything back.

Book this inspection as soon as first fix trades finish. Your plasterer is probably on a tight schedule. If building control can't come for a week and the plasterer has a gap in their diary, that gap becomes yours.

What This Costs

Real cost data from a ~55m2 kitchen extension, built 2022 in Oxfordshire.

First fix electrics on this project covered 29 spotlights across four independently controlled lighting zones, USB-C double sockets throughout, Cat6 Ethernet to a first-floor office, and all circuit runs back to the consumer unit. The quoted price was £3,196 for first and second fix combined. At 2026 rates, a similar spec runs

.

Total electrics for the project (first and second fix combined) came to approximately £2,785 invoiced. The quote was higher than the final invoice because scope was managed tightly. First fix is typically 50 to 60% of the total electrical cost because it's the more labour-intensive phase: running cables is harder than connecting faceplates.

Plumbing and UFH for 66m2 of heated floor area (the pipes covered both the extension and the existing kitchen area once walls came down, but not the zones under fixed kitchen units, where UFH is not installed), a new boiler, a water softener, and a boiling water tap was £10,400 all-in including VAT. That was a high-end spec. Basic extension plumbing without UFH or boiler replacement runs

.

An electrician's day rate runs

. Most extension electrics are quoted as a fixed price for the whole first-and-second-fix scope rather than day rate, which protects you from scope creep but means the electrician builds contingency into their price. Get three quotes and compare them line by line.

The Photograph Rule

When first fix is complete and before plastering starts, photograph everything. Walk every room and photograph every wall from floor to ceiling. Cable routes, back box positions, pipe runs, UFH manifold connections, insulation details. Take measurements from reference points (door frames, external walls) to each cable position.

These photographs serve two purposes. They give you a record of what's behind every finished wall, which matters the moment you want to hang a heavy picture, add a socket, or investigate a fault in twenty years. And they support any dispute about the first fix work: if something was missed or positioned wrong, the photos confirm the state before it was covered.

The photographs cost nothing. Not taking them is a mistake you can't undo.

Contracts and Payment Stages

Because first fix and second fix have no standard definition, payment disputes around these milestones are common. A builder who invoices for "first fix complete" has no universally agreed checklist to measure against.

Protect yourself by specifying in the contract exactly what each payment stage includes, broken down by trade. Not "first fix electrics complete" but "all circuit cables run to agreed positions per drawing revision 3, all back boxes installed, earth bonding complete, ready for building control pre-plaster inspection." That level of detail makes the payment trigger objective and measurable.

The same applies to second fix. Your plumber's definition of "second fix complete" might not include fitting the kitchen tap if the kitchen isn't installed yet. Spell it out. Retention (typically 2.5 to 5% of the contract value held back for 6 to 12 months) gives you the ability to get snags fixed after the trades have moved on to their next job.

The Summary

First fix and second fix are the framework your entire extension services programme runs on. The sequence: weathertight structure, carpentry first fix (partitions and noggins), electrics and plumbing first fix simultaneously, building control pre-plaster inspection, plastering, screed curing if applicable, second fix trades, kitchen installation, appliance connections.

Every decision about socket positions, lighting zones, UFH scope, kitchen layout, data points, and consumer unit location must be confirmed before first fix starts. Not after. Not during. Before.

Get these decisions locked in, and first fix is straightforward work your trades can complete efficiently. Leave them open, and you'll pay to change work that's already been covered up.