How Long Does a House Extension Take?

The full timeline, start to finish

The table below sets out a realistic programme for a typical single storey rear extension in the South East, with the two storey equivalent for comparison.

Typical extension programme, from first enquiry to completion. Stage | Single storey | Two storey. Designer appointed, drawings produced — 3 – 6 weeks — 4 – 8 weeks; Planning or lawful development certificate — 8 – 12 weeks — 8 – 14 weeks; Structural engineer and building control — 2 – 4 weeks (parallel) — 3 – 5 weeks (parallel); Party wall notices and award — 4 – 12 weeks (parallel) — 8 – 12 weeks (parallel); Tender, quotes and contractor booking — 4 – 8 weeks — 4 – 10 weeks; Construction on site — 10 – 16 weeks — 16 – 26 weeks; Snagging and completion — 1 – 2 weeks — 2 – 3 weeks; Realistic total — 7 – 12 months — 10 – 16 months Stages marked parallel run alongside others. Contractor availability is frequently the longest single wait.

The build, week by week

A single storey rear extension of around twenty-five square metres typically runs like this, assuming reasonable weather and no significant surprises in the ground.

Weeks 1–2: set up, protection, demolition, excavation and foundations · Weeks 3–4: oversite, drainage, and brickwork to damp proof course · Weeks 5–6: superstructure walls, steel installation, wall plate · Weeks 7–8: roof structure, covering, and made watertight · Week 9: windows and external doors installed · Weeks 10–11: first fix electrics and plumbing, insulation, plasterboard · Week 12: plastering, then drying time · Weeks 13–14: second fix, flooring, decoration · Weeks 15–16: knock-through to the existing house, making good, snagging

Why the knock-through comes last

A well-run project keeps the new extension sealed off from the house for as long as possible and opens the connecting wall near the end. This keeps dust, noise and weather out of your living space for three months of the build rather than four.

It also means the most disruptive fortnight, when the kitchen may be out of action and the house is genuinely open, is short and predictable. Ask any contractor when they plan the knock-through, because the answer tells you a great deal about how they run a site.

What actually causes delays

Very few extensions are delayed by lazy builders. They are delayed by decisions, discoveries and lead times, in that order.

Common causes of delay and how to avoid them. Cause | Typical delay | Prevention. Late client decisions on finishes — 1 – 4 weeks — Choose everything before the build starts; Window and bifold lead times — 6 – 12 weeks — Survey and order once the opening is formed; Unexpected ground conditions — 1 – 3 weeks — Trial pit before pricing; Party wall award not in place — 4 – 8 weeks — Serve notices at design stage; Variations and design changes mid-build — 1 – 6 weeks each — Freeze the design before starting; Prolonged wet or freezing weather — 1 – 3 weeks — Programme groundworks outside deep winter where possible; Building control inspection scheduling — 2 – 5 days — Book inspections in advance, not on the day

How to genuinely shorten a project

Start the planning application and the party wall notices as early as possible, because they run in the background and cost nothing in site time. Order long-lead items such as glazing, steel and specialist bricks the moment sizes are confirmed.

Then make every finish decision before the first day on site. Tiles, sanitaryware, sockets, paint colours, flooring, handles. A decision made in week nine stops a trade for a day; a decision made in advance costs nothing at all.

Living through it

Set up a temporary kitchen somewhere sealed off from the work, with a microwave, a kettle and a sink you can reach. Agree working hours in writing, typically eight to five on weekdays and mornings only on Saturdays. Decide where materials and the skip go before the first delivery.

Tell the neighbours yourself, before the scaffolding arrives, and give them a mobile number. It costs one conversation and prevents most of the complaints that stop sites.

Frequently asked questions

How long does a single storey extension take to build?

Ten to sixteen weeks on site for a typical rear extension, with the full project from first drawing to completion usually running seven to twelve months.

Can building work happen in winter?

Yes. Groundworks and masonry are affected by frost and prolonged rain, but once a roof is on, the internal programme is largely weather independent. Expect a week or two of weather allowance.

How long is the house unusable?

Usually only the final one to two weeks when the knock-through happens, provided the extension is built sealed off. A well-planned build keeps you in the house throughout.

What is the longest part of the process?

Usually planning and contractor availability rather than construction. Good builders in Kent are commonly booked two to six months ahead, so approach them at design stage rather than after approval.