How Long an Extension Takes, Stage by Stage

Ask how long an extension takes and you will usually be told how long the building takes. Those are different questions, and the gap between them is months.

The building half can be planned with real confidence once the ground is known. The half before it runs on statutory clocks that nobody involved can speed up, and it is the half that gets left off the mental calendar. This page is about the whole span, what governs each part of it, and which bits you can actually influence.

Before either half: the months that are entirely yours

The deciding stage has no clock on it at all, which is exactly why it expands.

Most people spend longer choosing than building. Working out what the house actually needs, getting two or three prices that are genuinely comparable, and then waiting for the person you picked to be free are all real time, and none of it appears on anybody’s programme.

Comparable quotes are slower to obtain than most expect, because a builder who is going to price properly wants to see the ground and the access first, and then wants the drawings. A price given over the phone can be had the same week, and it is worth precisely what it cost to produce.

Availability is the other piece. Anyone worth having is working on something already, so the question to ask early is not only what it costs but when the job could start. That answer, more than any statutory period, often sets the year the extension happens in.

The two halves, and the one nobody counts

Every extension has a paper phase and a building phase, and they are roughly comparable in length.

The paper phase runs from the first survey to the day work can lawfully start. Survey, measured drawings, an application of some kind, structural design, a Building Regulations submission, party wall notices, sometimes a water company agreement. Very little of it is visible and none of it feels like progress.

The building phase is the part everybody pictures, and it is the part per project type that has a reasonably reliable answer. A single storey rear extension on site is a different order of thing from a loft conversion or a garage converted inside its existing walls, and each of those has its own page with a realistic range on it.

The failure mode is almost always the same. Somebody decides in January that they would like the room finished for the summer, treats the build duration as the whole answer, and starts the paperwork in March. The paperwork, not the building, is what makes that impossible.

What is fixed in law, and cannot be bought

These periods are set by statute. No builder, designer or fee shortens them.

Two things in that table are worth pausing on. The planning clock starts at validation rather than at submission, so an application missing a document sits in a queue for a week or two before its eight weeks even begin, and what happens inside that period is worth knowing in its own right. And the party wall periods are the only ones on the list with a legitimate shortcut, because an adjoining owner who consents in writing removes the wait entirely. That is a conversation, not a process, and it is worth having early.

A refusal resets everything. Amending and resubmitting starts a fresh eight weeks; an appeal is measured in months and runs to the Planning Inspectorate’s timetable rather than the council’s.

What running things in parallel actually saves

The single biggest variable in the paper phase is not any individual duration. It is whether the items happen one after another or at the same time.

Drawings, structural design, the Building Regulations submission and the party wall notices do not depend on each other in the way people assume. Notices can go out while the planning application is being determined. Structural design can proceed once the shape is settled, which is well before a decision arrives. Steel can be ordered off approved calculations rather than after the trench is open.

Run sequentially, each waiting politely for the last, those items add a month or more to a project before anything is dug. Run together, they collapse into the eight weeks you were waiting anyway. This is the part of the programme genuinely within somebody’s control, and it is the main reason two identical jobs finish weeks apart.

There is one genuine dependency worth respecting. Commissioning structural calculations before the planning route is settled means paying for them twice when the scheme moves, so that particular piece of impatience costs money rather than saving time.

Where the clock stops without anybody noticing

Most lost weeks are not delays. They are waits nobody put on the chart.

• Validation. A missing ownership certificate or an unscaled drawing parks the application before it starts.
• Pre-commencement conditions. A permission granted with conditions attached is not a permission to start. Anything worded to be discharged before work begins has its own eight week application, and discovering that on the intended start date costs two months.
• The party wall award. Silence from a neighbour counts as dissent after fourteen days, which puts surveyors on the job and an award to be drawn up before the excavation can proceed. How the Act actually works matters here, because the wait is created by not serving early rather than by the neighbour.
• The water company. A public sewer under the footprint needs an agreement, and often a drain survey before the application can even be made.
• Made to measure items. A fabricated steel, a large glazed unit, a matched brick that has to be sourced. Ordered in week one they are invisible. Ordered when the opening is ready, they stop everything.

The build half, and why the middle looks slow

On site, the shape of the programme is consistent whatever the project. It starts with the stage of genuinely unknown duration, moves through a fast and visible middle, then slows again at the point where the outside stops changing.

Groundworks first, and that is where the range lives. Everything from the trench depth to whether the bucket finds a live drain is settled here, which is why the difference between a short programme and a long one is almost always underground. Then the shell goes up, which is the fortnight where a great deal appears to happen because it does. Roof on, openings closed, and the job becomes weathertight.

Then it goes quiet from the outside, and this is where people start to worry. First fix carpentry, electrics and plumbing all happen inside a building that looks finished from the garden. Every one of them has to be inspected before it is covered, and work that has not been seen cannot be plastered over.

Drying time is the item nobody builds into a mental programme. Screed and plaster both need to give up their water before anything goes on top of them, and that is physics rather than effort. In a cold damp house it takes longer, and laying a floor over a screed that has not finished drying causes an expensive problem months afterwards.

Second fix, tiling and decoration follow, and this is where a demanding specification shows up as days rather than weeks. What the weather does to all of this, and why the exposure is concentrated at the front of the job, is covered in detail on the single storey timings page rather than repeated here.

The tail nobody puts on the chart

Practical completion is not the end of the paperwork. A final inspection has to be arranged, and the completion certificate usually follows within eight weeks of the work being finished and found to comply. If you went through a registered building control approver rather than the council, the equivalent document is a final certificate. Either way it is the piece of paper a buyer’s solicitor asks for years later, so it is worth chasing rather than filing under done.

Snagging runs alongside that. A sensible list is made once, walked through together, and worked off in a single visit rather than dribbled out over months. Two weeks after handover is a realistic expectation for a properly kept list, and it is a fair test of whoever built it.

Choosing when to start

Because the exposure to weather sits almost entirely in the groundworks, the start date matters more than the season the rest of the job falls in. Breaking ground in February on heavy clay is a slower and muddier business than breaking ground in May, and the difference lands on the one stage that was already the least predictable.

That argues for using the winter for the paper phase and having permissions, calculations and notices all in hand by early spring. It is also the reason a job that starts in autumn is often finishing its wet work in the worst possible month. Which month to start in deserves its own answer, and it is not the obvious one.

The honest span, by project

Measured properly, from the first survey to the certificate, rather than from the first brick.

What separates these is mostly the front end rather than the building. A project that needs a full application waits eight weeks for a decision that a conversion inside an existing envelope never has to wait for at all, and that single difference is worth more weeks than any amount of speed on site.

Those are spans rather than promises, and every one of them assumes the paper phase was run in parallel. A garage conversion arranged one item at a time can take longer than a single storey extension arranged properly, which is not intuitive and is entirely down to sequencing.

What to ask for, and what a real answer looks like

Ask for a programme rather than a finish date. A programme names the stages, gives each a duration, marks which items are running in parallel, and identifies what is on the critical path so you can see what a week’s slip actually costs.

Then ask two questions of it. Which items are statutory periods that cannot move, and what is the assumption about foundation depth. Every honest answer to the second has a mechanism attached to it, because the depth genuinely cannot be known before something is dug.

A date offered without either of those is a hope. Living through the middle of it is a separate matter with its own answer, and what the house is actually like during the build is worth reading before you commit to staying put.

Statutory periods change from time to time, and the durations here are typical for the Colchester area at the review date below. They are ranges with reasons rather than promises.

Last reviewed August 2026. Planning and Building Regulations change, and your property may differ.