First Fix vs Second Fix in a London House Extension
Short answer: on a London extension, first fix is the concealed preparation that goes in before walls, floors and ceilings close it in. Second fix is the visible fittings, final connections, finishes, and any testing and records that apply. Neither label is a statutory milestone, and neither one on its own proves that work is complete or compliant.
For a homeowner, each label marks a point where decisions become harder to change. Freeze layout and service positions in writing before anything is covered, then confirm actual products, compatibility and protection before visible fittings go in. Agree inspection timing with your appointed building control body before relevant work is covered.
What the labels do and do not mean
First fix and second fix are common construction shorthand rather than statutory milestones. Builders, electricians, plumbers and joiners all use the phrases, and each means something slightly different by them. The division between the two is project-specific: what sits in each stage depends on the current drawings, the written specification, the construction method, how trade packages are split, the contract, and the building control route in use.
That variation is normal, provided everyone works from the same written scope. Problems appear when a homeowner assumes first fix means one thing and the site team assumes another. A boarded ceiling is a poor place to discover that the pendant light was expected a metre to the left.
First fix in plain terms
First fix usually refers to concealed preparation carried out before finishes close it in: structural and service work, routes for pipes and cables, drainage connections, framing, noggins and fixing support. The covering and finishing layers that follow, such as insulation, boarding, plastering or screed, are programmed differently from project to project and are not automatically part of first fix. Once those layers are on, changing what sits behind them means opening it up again.
Second fix in plain terms
Second fix usually covers what you can see and touch: sanitaryware, taps, radiators, kitchen units and appliances, faceplates, light fittings and switches, together with final connections, finishing, and any testing and certification the work requires. Rooms start to look finished at this stage, which is why late changes here tend to be disruptive.
The decision freeze
The most useful habit is a written decision freeze agreed before hidden services are covered. Work through the points below with your builder and record the outcome in one place.
Decision freeze checklist
- Work from the current drawings, specification and written scope, and check that you and the site team hold the same revision.
- Decide how each room will be used, and where furniture and appliances will sit.
- Freeze the kitchen and utility information the services depend on, whether that is a confirmed product selection or confirmed dimensions and service requirements.
- Agree sanitaryware, heating, plumbing and ventilation positions and specifications, so far as they apply to your project.
- Agree socket, switch, lighting control, data and external service positions room by room.
- Identify wall-mounted items that need support before boards and finishes conceal the framing.
- Walk the rooms jointly, then issue one marked-up, versioned schedule that everyone works from.
- Confirm every variation in writing, including what it changes and who is responsible.
- Agree inspection timing and any notices with your appointed building control body before relevant work is covered.
- Keep dated progress photographs and approved schedule revisions as a practical project record.
Working through the freeze
Start with the drawings you hold
Drawings get revised. Before making positional decisions, confirm which revision the site team is building from and that it matches yours. Our house extension timeline guide sets out where these stages usually sit in the wider programme.
Decide the room before the sockets
Service positions follow furniture rather than the other way round. Sketch where furniture will go, then place sockets, switches and lighting around it. Positional changes are easier to resolve in the agreed drawing and schedule than after finishes have concealed the work.
Freeze the kitchen and utility information early
Kitchens carry much of the hidden service decision-making: waste and water positions, appliance supplies, extraction routes and the electrical demand of the appliances planned for the room. Those decisions rely on real product information, so check dimensions, clearances and service requirements against the actual selected product data and your project information rather than a showroom assumption. Not every appliance must be bought before first fix, but anything the services depend on needs to be confirmed and recorded, since it can affect work beyond the kitchen.
Plumbing, heating and ventilation positions
Where they apply to your scheme, agree sanitaryware layouts, radiator or underfloor positions, tap and shower selections, and ventilation routes and terminal positions. These remain design and installation matters for your appointed trades; your part is to confirm what you want and where, and to flag conflicts with how the room will be used.
Electrical, data and external positions
Go room by room and mark socket heights and locations, switch positions and which switch controls which light, any lighting control, television and data outlets, and external items such as garden sockets, outdoor lighting or a vehicle charging point. Leave circuits, cable routes and protection to a qualified electrician.
Support behind the finished surface
Wall-hung basins, wall-mounted televisions, heavy mirrors, shelving, handrails, kitchen wall units and blinds need something solid behind the finish. Once plasterboard is up, adding proper support means taking it back off. List these items during the freeze, mark them on the schedule, and photograph the framing before boarding.
One walk-through, one versioned schedule
Run the freeze as a joint walk-through rather than an email exchange. Stand in each room with the drawings, mark positions, and agree them on the spot. Then issue one marked-up schedule with a version number and a date, and treat it as the reference everyone works from. Later changes get a new version, confirmed in writing.
Before work is covered
Run through these points before plasterboard, screed or finishes conceal anything.
- Is the marked-up schedule current, dated, and held by everyone who needs it?
- Have late changes been confirmed in writing and carried into the latest revision?
- Have you agreed with your appointed building control body when it needs to inspect or be notified, and what must not be covered before then?
- Has support been provided for everything on the wall-mounted list?
- Have dated photographs been taken of the areas about to be closed in?
Leave the inspection of live electrical work and hidden safety-critical work to the competent trades and the appointed building control body. Checking that decisions and records are in order is your part; inspecting the work is not.
Notices, inspections and records
Do not assume building control attends automatically at first or second fix. Inspection and notification arrangements depend on the work and the route in use, and should be agreed with the appointed building control body before relevant work is covered.
Where local authority building control is used, the Planning Portal guidance on site inspections explains that notices are required at specified stages, including certain work before it is covered, that work should pause at relevant stages for inspection, and that opening up may be required if the relevant notice is not given.
GOV.UK describes another route. A registered installer in a competent person scheme may self-certify certain work, notify the authority where needed and provide the applicable compliance record. Which route applies depends on the work and the scheme.
For electrics, Electrical Safety First explains that fixed electrical installation work in homes must comply with Part P of the Building Regulations. Apart from some minor tasks, work is either reported to building control or completed through an applicable registered electrician route. Depending on the work and the route, records may include an Electrical Installation Certificate or Minor Work Certificate together with a Building Regulations Compliance Certificate.
Dated photographs help you and your builder remember what went where. They are a practical project record. They do not prove compliance and they do not replace inspection, certification or the appropriate paperwork.
Before second fix begins
The second decision point arrives before visible fittings are installed. Confirm the finishes and fittings now selected rather than those discussed earlier. Check that each item suits what is already installed behind it, including the positions frozen earlier. Agree who owns each lead time and who chases a delivery. Agree how finished surfaces will be protected while other trades work.
Lead times vary by product and supplier, so no universal ordering deadline is worth quoting. What matters is that every item has a named owner and a known status before it is needed on site.
After second fix
Once fittings are in, use ordinary controls only. Switch things on and off, run the taps, and report anything that does not behave as expected. Leave testing, commissioning and certification to the competent trades.
Anything still outstanding belongs in the next stage rather than in a mental list. Carry it into our house extension handover checklist, which covers snagging, certificates and the documents you should end up holding.
Where this sits
Our pre-start checklist covers household preparation before the site team arrives, and our house extensions page shows the range of work we take on in London. To talk through a project, get in touch.
This article is general homeowner guidance and not technical advice. Nor is it legal, electrical, gas, design or building control advice. Decisions about scope, specification, notification and certification belong with your appointed professionals and your building control body.
Photograph: a finished detail from BCS’s Albion Drive project in London.
