Extension cost overrun calculator

What the number actually means

The figure at the top is not a prediction of your project. It is the range that projects priced the way yours was priced, on a property like yours, tend to land in. Three quotes for the same extension routinely differ by forty per cent, and the final cost of the same job under two different builders can differ by more than that again.

What the tool is really doing is separating three very different kinds of money, because they behave differently and they are managed differently.

Scope that was never in the quote

Kitchens, flooring, decoration, landscaping, professional and statutory fees. This is not overrun in any meaningful sense — it is money you were always going to spend, sitting outside the number in your head. It does not go away when someone manages the project. It simply turns up in January instead of June, while you can still do something about it.

Variations from what nobody could see

Foundations that go deeper than the engineer assumed, a drain running where no drawing shows it, joists that will not take the load, a wall that is not carrying what everyone thought it was carrying. Some of this is genuinely unforeseeable. A good deal of it is foreseeable by anyone who has opened up enough Victorian terraces, and most of it is cheaper when it is priced before the work happens rather than after.

Changes you make once work has started

The most expensive pound on any renovation is the one spent changing something that has already been built. This bucket is almost entirely within your control, and it is the one that grows fastest when decisions are left open, which is why the calculator weights it against how much of the specification is still undecided.

Where SW London extension budgets actually go wrong

1. Comparing quotes that are not describing the same job. The cheapest of three quotes is usually the one that excludes the most. Line them up properly with thebuilder quote comparison checklistbefore you look at the totals.
2. Treating a provisional sum as a price. A glazing allowance is a placeholder until someone measures the opening and quotes against it. So is a kitchen allowance, a tiling rate and anything described as a PC sum.
3. Starting before the structural design exists. Steel sizes drive foundations, and foundations drive the groundworks price. Sizing steels after the quote is agreed means repricing the biggest early item on the job.
4. Paying ahead of the work. Stage payments made on request rather than against completed work remove the only leverage you have when something goes wrong.
5. Forgetting the programme costs money too. If you are renting, every week of slip has a price. A sixteen-week extension that takes twenty-two is six weeks of rent nobody budgeted for. Seehow long these jobs really take.

How I keep clients on budget, and where I cannot

Being honest about the split matters, because it is the difference between a service and a sales pitch. I cannot make the kitchen cheaper, and I will not pretend that finding the missing scope is a saving. What client-side management genuinely changes is the second and third buckets, and the way the first one is discovered.

• The gaps are found before you commit. The quote gets read line by line against the drawings, and the exclusions get priced while you still have three builders competing and the freedom to change the scope.
• Variations are priced before they are built. A written variation process, with each change costed and approved in advance, is the single biggest difference between a project that drifts and one that does not.
• Decisions are front-loaded. Kitchen, glazing, sanitaryware, electrics and finishes chosen before first fix, not during it, so the expensive kind of change never arises.
• Trade pricing and the right supplier. The same specification bought through trade accounts rather than retail — seewhere the real savings are.
• Someone on site every week who is not paid by the builder. Problems caught at first fix cost a fraction of the same problem caught at handover.

The honest caveat: a management fee comes out of the same budget, so on a small, straightforward job with a builder you already trust, the sums may not work. On a six-figure job with several trades and a structural element, they usually do. There is a worked example indoes a project manager’s fee pay for itself, and the fee models are set out onwhat a project manager costs.