Timeline Estimator

A kitchen extension in the UK runs across eight phases, from the first planning drawings to the completion certificate. On a typical 20 m² single-storey rear extension the schedule breaks down roughly as planning at 8 weeks, pre-construction at 4 weeks, groundwork at 2 weeks, structure at 6 weeks, kitchen specification at 6 weeks, first fix at 3 weeks, second fix at 4 weeks, and completion at 2 weeks. This estimator adds those phases up for your own size, type, and start month, and shows the total above.

Enter your floor area, pick the extension type and a start month, and decide whether a new kitchen is in scope. The result splits the schedule phase by phase, so you can hold your builder to a realistic construction programme instead of a vague "few months". Plotting those phases on a Gantt chart is the standard way builders and architects track an extension.

What changes the length

Three things stretch or compress the baseline schedule. The first is extension type. A single-storey rear extension is the baseline at 1.0×, a side return and a wrap-around run a little longer, and a double storey applies 1.5× to the on-site phases because it needs deeper foundations, a second-floor structural deck, and a staircase tied into the existing house.

The second is size. Above the 20 m² baseline the estimator adds 0.015 per m² for each extra square metre. That deliberately falls short of scaling in proportion, because setting out foundations, installing steels, and booking building control visits take the same time whether the footprint is small or large.

The third is when you start. A build scheduled to begin in November, December, January, or February applies 1.15× to every phase. Shorter days, saturated ground that will not take a foundation, frost that stops screed and external masonry, and thin trade availability over the Christmas break all pull against you. A spring start is the safest bet.

The lead times that set the pace

The phases above assume materials arrive on cue, and the two that most often derail a programme are glazing and the kitchen. Large glazing carries a lead time of 10-16 weeks, so the doors are ordered during design or early structure rather than when the opening is built. The kitchen units and appliances need ordering by the structure phase at the latest, and the layout has to be fixed before first fix electrics so sockets and switches land in the right places.

Two more items run on their own clock and need starting early. Utility meter relocation takes weeks to arrange and should begin the week your approval arrives. And the schedule only closes once building control has signed off the final inspection and issued the completion certificate.

A worked example

Take a 40 m² double-storey rear extension, kitchen included, starting in spring. Each on-site phase is scaled by the double-storey factor of 1.5× and the size uplift of 0.015 per m² applied to the twenty square metres above the baseline. Planning still sits at 8 weeks, but structure, first fix, and second fix all grow accordingly, and the estimator sums the result above.

Shift that same build to a December start and every phase picks up the winter factor of 1.15× on top, adding a couple of weeks to the total. Then remember these are optimistic numbers. One documented 55 m² Oxfordshire extension ran far longer than any estimate would suggest, held up by a supplier going insolvent and a steelwork saga that dragged the structure phase out by months. That is why the construction programme, a start-date buffer, and early orders for long-lead items matter more than shaving a week off any single phase.