Timeline of a Home Extension

What to expect, week by week

One of the most common questions we're asked is: how long will it take? Every project is different, but a typical single-storey rear extension of around 25m² follows a recognisable pattern. Here's a week-by-week walkthrough so you know what to expect.

Note: this timeline covers construction only. The pre-construction phase (drawings, planning, structural engineer, building control) typically adds 8–16 weeks before this begins.

Weeks 1–2: Site setup and foundations

The first week is about preparation. We set up the site, protect your property, bring in plant and materials, and begin breaking out. By the end of week one, excavation for the foundations is typically underway.

Week two sees the foundations poured — concrete delivered, levelled and left to cure. A building control inspector visits to check depth and suitability before the concrete is poured.

Weeks 3–4: Substructure and drainage

With foundations set, we build up the substructure brickwork to damp-proof level, lay the damp-proof membrane, and install any drainage — soil pipes, manholes, connections. If your extension involves diverting existing drains, this is when it happens.

Another building control inspection checks the damp-proof course and drainage before we move on.

Weeks 4–5: Superstructure

Walls go up. The main leaf of brickwork or blockwork is built to plate height, with cavity insulation installed as we go. If your extension has structural steelwork — a beam over bi-fold doors, for example — it's installed now, bedded on padstones and bolted into place.

Weeks 6–7: Roof

The roof structure goes on — timber joists, insulation, and either a flat felt/fibreglass roof or tiles if it's a pitched design. The roof is a critical milestone: once it's watertight, work can continue regardless of weather, and the space starts to feel like a room.

Weeks 7–8: Windows, doors and first fix

Bi-fold or sliding doors, windows and external doors are fitted. Inside, the first fix begins — electrical cables pulled to sockets and lights, plumbing pipes run for radiators or underfloor heating, data cables if you've planned for them.

Weeks 9–10: Internal walls, plastering and screeding

Internal stud walls go up if your layout includes them. The floor is insulated and screeded (if underfloor heating is going in, the pipes are laid first). Then the plasterer arrives — plasterboarding walls and ceiling, skimming, and leaving it to dry.

Weeks 11–12: Second fix and finishing

With plaster dry, the second fix begins: sockets and switches fitted, lights installed, plumbing connected to radiators and sinks. Joinery — skirting, architrave — goes on. The kitchen is fitted, flooring goes down, and decoration begins.

Weeks 13–14: Snagging and completion

The final stretch. A detailed snagging list is worked through — minor touch-ups, adjustments, cleaning. A final building control inspection is arranged, and if everything passes, you receive your completion certificate.

What can delay the timeline?

• Weather — heavy rain can delay foundation and roof work
• Material lead times — especially for bespoke glazing or steel
• Unexpected ground conditions — poor soil, old foundations
• Changes to the scope — the most common cause of delays

A good builder communicates proactively about all of these. We provide a realistic timeline at the outset and keep you updated throughout.

Get in touch using the form on our contact page to discuss your project timeline.