The stages of building an extension.
Every extension runs through the same eight stages, in the same order. The Extension Build Stages Guide puts them on paper: an overview of all eight, then one printable page per stage.
- What happens, in the order it happens
- Who is involved, on site and off it
- Inspections and sign-offs, including every building control visit
- What to book or order before the stage starts
- The most common trap, and what it costs you
- A link to the full guidance for that stage
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The eight stages of building an extension.
Each stage hands over to the next, and the handover points are where programmes slip. Specification is the exception to strict order: it runs alongside structure, because the layout has to be fixed before first fix starts.
Stage 1: Planning and permissions
Settle the consent route, the drawings, the structural calculations and the budget. Nothing gets dug, but this stage decides what you are allowed to build.
Ends with permission or a confirmed permitted development route, drawings, structural calculations and a budget with contingency.
Every planning and permissions task, in build orderStage 2: Pre-construction
Decide who builds it, on what terms and in what order. Write one specification, get quotes against it, sign a contract with stage payments, and start the applications other people control.
Ends with a signed contract, payments tied to completed stages, and a programme of which trade is on site when.
Every pre-construction task, in build orderStage 3: Groundwork
Services traced, footprint set out, trenches dug, foundations and drainage in, floor built up to the damp proof course. Almost all of it is buried by the time the roof goes on.
Ends with foundations, drainage and the floor slab in, and the damp proof course at the correct level.
Every groundwork task, in build orderStage 4: Structure
Walls up, steels and lintels in, roof framed and covered, glazing fitted. The stage ends at weathertight, which releases every trade that follows.
Ends at weathertight: roof covered, glazing in, openings sealed.
Every structure task, in build orderStage 5: Specification
Fix the layout, fittings, appliances and suppliers, and place the orders. It runs alongside structure and has to be finished before first fix.
Ends with a layout drawing confirmed in writing and issued to every trade, and the orders placed.
Every specification task, in build orderStage 6: First fix
Cables, pipes, waste runs, ducting, heating and insulation, installed while the walls and ceilings are open and inspected before the plasterboard goes on.
Ends with everything tested and inspected, ready for boarding.
Every first fix task, in build orderStage 7: Second fix
Plaster, screed, sockets, switches, taps, floors, decoration and the kitchen. The order is set by what has to dry, not by who is free.
Ends with a finished, usable room, with each trade's work checked as they leave.
Every second fix task, in build orderStage 8: Completion
Final inspection, completion certificate, snagging and the paperwork file. Nothing here changes how the room looks. It changes what you can prove.
Ends with the completion certificate, a full paperwork file, and the snag list fixed before the final payment.
Every completion task, in build order
Common questions
Free, A4, nine pages. Budgeting as well? The extension budget spreadsheet is free too.
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