If you're managing a building project — a kitchen extension, a loft conversion, or a full house renovation — understanding the build sequence is one of the most practically useful things you can do. Tradespeople often assume you know the terminology; when you don't, it's easy to miss signs that work is out of sequence, which can cascade into expensive delays. First fix and second fix are the two most fundamental phases of any building project, and getting them in the right order is essential.
What First Fix and Second Fix Mean
Electrical
First fix electrics involves everything that goes into the wall or ceiling before it's plastered over: running cable routes, installing back boxes for sockets and switches, running conduit where needed, and positioning consumer unit and distribution board cabling. None of the visible fittings are installed at this stage — just the infrastructure.
Second fix electrics is everything that happens after the plaster has dried and been decorated: fitting sockets, switches, luminaire ceiling roses, connecting the consumer unit, testing, and commissioning. For notifiable work under Part P of the Building Regulations, the second fix stage is when the final test certificates are issued.
Plumbing
First fix plumbing installs all pipework routes before walls are closed: hot and cold supply pipes, waste and soil pipes, central heating flow and return pipes, and boiler flue positioning. No sanitary ware, taps, radiators, or boiler is fitted at this stage.
Second fix plumbing connects everything visible: fitting the boiler, hanging radiators, connecting taps and waste traps, installing the bath, basin, WC, shower tray and enclosure. Pressure testing and commissioning happens at this stage.
Carpentry
First fix carpentry covers all structural and hidden timber work: stud partitions, door frames, window boards, floor joists (where applicable), and roof timbers (in a loft conversion). This work provides the structure that plasterboarders, plasterers, and insulation contractors work around.
Second fix carpentry is all the visible joinery: hanging doors, fitting skirting boards, architrave, window boards, built-in furniture carcasses, and staircases. It's the finishing work that happens after plastering and decoration.
Why the Sequence Matters
The sequence is not arbitrary — it reflects the physical constraints of construction:
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Plasterers cannot start until first fix is complete. The plasterer needs a clean, fixed surface to work on. If cables, pipes, or stud work are still being installed, plastering cannot begin.
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Second fix cannot start until plaster is dry. Fresh plaster contains significant moisture. Fitting timber skirting boards, door frames, or decorative joinery against wet plaster will cause the timber to swell, warp, or split as it absorbs moisture. Allow 4–6 weeks for new plaster to dry in a heated property (longer in winter or unheated spaces). Attempting to accelerate drying with excessive heat can cause surface cracking.
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Tiling must follow plastering and second fix plasterboard — you can't tile on skim plaster in a wet room without adequate substrate preparation (tile backer boards in shower areas).
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Flooring is almost always last — or at least protected throughout — because it's the most vulnerable surface to damage from other trades.
The Full Correct Build Sequence
For a typical extension or significant renovation:
- Structural works — foundations, walls, roof structure, structural opening works, steelwork
- First fix electrics, plumbing, and carpentry — all services into walls and floors, stud partitions, frames
- Insulation — within stud partitions, between joists, in roof structures
- Plasterboarding — closing the walls and ceilings
- Plastering — skim coat over boards (or sand and cement render on masonry)
- Drying out — 4–6 weeks minimum for new plaster
- Second fix electrics, plumbing, and carpentry — all visible connections and fittings
- Tiling — kitchens and bathrooms
- Flooring — hard flooring, carpets, vinyl
- Decorating — painting, wallpapering
- Snagging — final inspection and defects list
For a detailed guide to finding, vetting, and managing the tradespeople who work through this sequence, see our how to find a reliable builder UK guide and renovation project management tips.
How Delays Cascade
In a building project, trades are interdependent. If the plasterer can't start because the first fix plumber is delayed, the drying-out period is pushed back, which delays all second fix trades, which delays tiling, which delays flooring. A one-week delay at the plastering stage can result in a four-to-six week delay in project completion.
This is why project management — maintaining a programme, tracking dependencies, and resolving conflicts quickly — is not optional on larger projects. Your builder should be able to provide a basic Gantt chart or programme showing each trade and their dependencies. If they can't, that's a warning sign. For a full guide to managing a building project yourself, see our how to project manage a kitchen extension guide.
Building Regulations: Part P and Part L
Part P (Electrical Safety)
Part P of the Building Regulations applies to all domestic electrical installations. Not all work is notifiable — replacing like-for-like sockets and switches in dry rooms is permitted work that doesn't require notification. However, the following are notifiable (must be carried out by a registered competent person scheme member, or separately notified to building control):
- New circuits in any room
- Any work in kitchens, bathrooms, or outdoors
- Consumer unit replacement
- New circuits for electric vehicle chargers, storage heaters, or showers
Registered competent person scheme members for electrics include those on the NICEIC, NAPIT, or Electrical Competent Person registers. They self-certify their work and issue an Electrical Installation Certificate (EIC). If you use an unregistered electrician for notifiable work, you must submit a Building Notice to your local authority building control before work starts. Unregistered work without notification is a criminal offence under the Building Act 1984.
Part L (Conservation of Fuel and Power)
Part L affects renovation work whenever you're fitting a new boiler, adding an extension, or significantly upgrading the heating system. New boilers must be condensing (unless a specific exemption applies). Extensions must meet minimum U-value standards for walls, roofs, floors, windows, and doors. If your extension exceeds 25% of the existing floor area, you may also need to improve the energy efficiency of the existing building fabric.
For detail on building regulations approval routes, Full Plans vs Building Notice, and the Completion Certificate process, see our building regulations approval UK guide.
Project Timeline Table
| Project Type | Typical Duration | Key Milestones |
|---|---|---|
| Single-storey kitchen extension | 12–16 weeks | Structural works weeks 1–4 → first fix weeks 4–6 → plastering weeks 6–8 → dry-out weeks 8–12 → second fix/fit-out weeks 12–16 |
| Loft conversion (Velux/dormer) | 10–14 weeks | Structural works weeks 1–4 → first fix weeks 4–6 → boarding/plastering weeks 6–9 → dry-out weeks 9–11 → second fix/finish weeks 11–14 |
| Full house renovation (empty) | 6–18 months | 6 months for a smaller terrace with no structural issues; 12–18 months for a larger property with structural works, full rewire, replumb, and whole-house replastering |
| Kitchen renovation (no extension) | 3–5 weeks | Stripping/structural day 1–3 → first fix week 1 → plastering/dry-out weeks 2–4 → second fix/fit weeks 4–5 |
Snagging: What It Is and Why You Shouldn't Rush It
Snagging is the process of identifying defects, omissions, and items not completed to the required standard at the end of a building project. A properly produced snag list is a contractual tool — it records what the contractor must return to complete before you make the final payment.
What to include in a snag list:
- Incomplete work (missing door handles, unpainted surfaces, unfitted trims)
- Defective work (cracked plasterwork, leaking wastes, doors that don't close properly)
- Work not meeting the specification or building regulations
- Items not matching what was agreed in the contract
How to document snagging: photograph every defect, note the location, date the list, and deliver it in writing to the contractor. Use a formal snagging template if your contract requires it.
Retention: a typical construction contract allows you to withhold 3–5% of the contract value as retention until snagging is complete to your satisfaction. The JCT Minor Works Building Contract (the standard form for small domestic works, costing around £30–£50 as a template) provides for a defects liability period — typically 3–6 months after practical completion — during which the contractor must remedy any defects that emerge.
Why builders want you to sign off too early: practical completion triggers the release of any retention held, starts the defects liability period clock, and shifts risk for most damage to you. Don't sign off until you — or an independent professional — have properly inspected the work. You are not obligated to grant practical completion if there are outstanding defects or incomplete items. Paying the final account before snagging is resolved effectively waives your leverage.
Related Guides
- How to Find a Reliable Builder UK — FMB vs TrustMark, JCT Minor Works contract, vetting process
- Renovation Project Management Tips — sequencing, programme management, and how to handle delays
- How to Project Manage a Kitchen Extension UK — extension-specific build programme with trade dependencies
- Building Regulations Approval UK — Full Plans vs Building Notice, inspection stages, Completion Certificate
- How to Deal with a Bad Builder UK — when second fix quality is poor and how to enforce the contract
- UK Renovation Cost Guide 2025 — full cost benchmarks for all project types mentioned in this guide