How long does a house extension take, realistically

People ask how long the build takes. It is the wrong question, or at least an incomplete one, because on most extensions the build is not where the time goes. The paperwork is.

A single storey rear extension might be twelve to sixteen weeks on site. Getting to the point where somebody can start can easily take longer than that.

Here is where the time actually goes.

Before anything is built

Drawings — two to six weeks. An architect or architectural technologist measures the house, produces existing and proposed drawings, and iterates with you. The variable is you, not them: the design conversation takes as long as it takes to decide what you want.

Planning, if you need it — eight weeks minimum. A householder planning application has a statutory determination period of eight weeks from validation, and validation itself can take one to two weeks if anything is missing from the submission. Assume ten to twelve weeks in practice, and more if the case officer asks for changes or the application goes to committee.

If your project falls under permitted development you skip this — but see the note on lawful development certificates below, because “skipping it” has its own timeline.

Building regulations — variable, and it depends which route. A full plans application is checked before you start and typically takes around five weeks for a decision. A building notice can be submitted around 48 hours before work begins, which is faster, but it means nothing is checked in advance and any problem is found on site with the work already built. On anything structural, full plans is usually the cheaper route in the end.

Structural engineer — one to three weeks. Needed wherever you are removing load-bearing structure or spanning an opening. Steel then has a lead time on top of the design.

Party wall notices — two months, and this one is a hard stop. Under the Party Wall etc. Act 1996, notice for work to a shared wall must be served two months before work starts. For excavation near a neighbour’s foundations it is one month. If your neighbour consents in writing you can proceed sooner; if they dissent, surveyors are appointed and an award is drawn up, which adds weeks. This is the single most common cause of a start date moving, and it is entirely avoidable by serving early.

Builder availability — four to sixteen weeks. Any builder worth waiting for is booked. This is not dead time if you use it to finish the drawings and serve the notices, but it is real.

On site

For a straightforward single storey rear extension, roughly:

Stage	Rough duration
Set up, break ground, dig and pour foundations	1–2 weeks
Drainage, oversite, slab	1 week
Brick and block to roof level	2–3 weeks
Structural openings, steel installed	Within the above
Roof structure and covering, watertight	1–2 weeks
Windows and external doors	1 week
First fix electrics and plumbing	1 week
Plastering and drying out	1–2 weeks
Second fix, kitchen or bathroom fit-out	2–3 weeks
Decoration, flooring, snagging	1–2 weeks

Call it twelve to sixteen weeks for a simple one. A double storey extension adds scaffolding, more structure, and work that has to happen inside the existing house — sixteen to twenty-four weeks is more realistic.

The five things that actually cause overruns

Ground conditions. The single biggest variable, and unknowable until you dig. Deeper foundations because of a nearby tree, made ground, a high water table, or a drain nobody knew about. This is why foundations often appear as a provisional sum.

Drainage. A manhole in the footprint is a day’s work. A shared sewer crossing the site needs an agreement with the water authority — that is a separate application on its own timescale, and it is much better discovered at drawing stage than at dig stage.

Decisions. Kitchens, tiles, sanitaryware and doors all have lead times, and the trades that fit them are booked around a date. A decision that slips by two weeks can move the finish by four, because the fitter has gone to another job.

Structural steel. Designed, fabricated, delivered, craned in. Order it late and the whole sequence waits for it.

Weather, at exactly one stage. Once the roof is on, rain is an inconvenience. Before that — during groundwork, before the slab, while the brickwork is going up — a sustained wet spell genuinely stops work. Concrete has a temperature floor too, so a hard frost pauses pours.

Note what is not on this list: the trades being slow. Overruns are far more often caused by something arriving late than by anyone working slowly.

How to actually make it faster

Almost all of the recoverable time is before the start, not during:

• Serve party wall notices as early as you possibly can. Two months is two months, and it can run in parallel with everything else.
• Choose the kitchen or bathroom before the build starts, not during. The unit schedule changes the first fix.
• Go full plans rather than building notice on anything structural. It front-loads a few weeks and removes the risk of a much longer problem later.
• Get a lawful development certificate if you are relying on permitted development. It takes around eight weeks, which feels like a delay, but it is the only document that proves the work was lawful — and its absence surfaces at the worst possible moment, when you sell.
• Book the builder before the drawings are finished, not after. Their lead time and your paperwork can run at the same time.

A realistic whole-project timeline

For a single storey rear extension needing planning permission, from first phone call to decorating:

• Drawings and design: 4 weeks
• Planning: 10 weeks
• Building regs and structural design, in parallel: 5 weeks
• Party wall notices, in parallel: 8 weeks
• Waiting for the builder: 8 weeks
• Build: 14 weeks

Sequenced well, with the parallel items genuinely run in parallel, that is around eight to nine months. Sequenced badly — waiting for planning before starting building regs, serving party wall notices after the builder is booked — the same job is comfortably a year.

If you want a view on where your project would actually sit, tell us about it. We would rather give you an honest programme at the start than an optimistic one you plan a kitchen delivery around.