How long does a house extension take in the UK?

A house extension usually takes 6 to 12 months from first idea to finished space. The build itself is the quickest part — a single-storey rear extension takes around 10 to 14 weeks on site — but the design, planning and approvals beforehand typically add three to six months.

Understanding the full timeline, not just the build, is the key to planning your project realistically. This guide walks through each stage and what causes delays.

If you're extending in London, our house extensions page explains how Beams manages the whole process.

On-site build times by type

These are typical durations once your builder starts work, assuming no major delays.

• Single-storey rear extension — around 10–14 weeks. The most common type and the quickest to build.
• Side return extension — around 10–14 weeks. The confined site can slow access and deliveries.
• Wraparound extension — around 12–16 weeks. More floor area and a more complex roof.
• Double-storey extension — around 14–20 weeks. Two floors, more structural work and scaffolding throughout.

A rough single-storey build runs in this order: groundworks and foundations (1–3 weeks), walls (2–3 weeks), roof (1–2 weeks), first-fix electrics and plumbing (1–2 weeks), plastering (a week, plus drying), second fix (1–2 weeks), then flooring, decorating and snagging. A double-storey roughly doubles the structural stages.

What happens before the build starts

The build is often the shortest stage. These come first:

Design and drawings — 2–6 weeks

A survey of your home, then design and the drawings needed for planning and building regs. Double-storey and wraparound schemes take longer to design than a simple rear extension.

Planning permission, if needed — 8–16 weeks

A householder planning application has a statutory eight-week target, but many councils take 10–12 weeks, and longer in conservation areas or where neighbours object. If your extension is permitted development, this stage drops out entirely and you go straight to building regs — see how to plan a house extension for whether yours qualifies.

Building Regulations — 5–8 weeks, often in parallel

Separate from planning, and almost always required. The Full Plans route gives you approval before work starts; it can run alongside the planning application to save time.

Party Wall process — 1–2 months

If your extension affects a shared wall or sits close to a boundary, notice must be served on neighbours, who have 14 days to respond. Starting this early protects your programme.

What causes extension delays

Most delays are avoidable with proper planning. The common culprits:

• Planning complications — objections, officer queries or a need to revise and resubmit drawings can each add weeks.
• Party Wall disputes — a dissenting neighbour and the surveyor process behind it is one of the most underestimated delays.
• Late design decisions — changing materials or layout mid-build is the single biggest cause of overruns. Lock choices in before work starts.
• Builder availability — the best builders are booked up; the start date is often the real bottleneck, not the build.
• Weather and ground conditions — frost, storms or unexpected obstructions in the ground can hold up groundwork and roofing.

Can you live at home during an extension

For most single-storey rear and side return extensions, yes — the work is contained to one part of the house, though there'll be disruption, dust and a period without a usable kitchen if you're extending one. For a double-storey or a large wraparound, the disruption is greater and some homeowners choose to move out for the most intensive phases. Your builder will talk you through the disruptive stages before work begins.

For what it all costs, see our guide to how much a house extension costs.