Avoiding Extension Cost Overruns: 9 Practical Ways to Stay on Budget
Most extension projects dont go over budget because the builder is dishonest. They go over budget because the scope isnt clear, key decisions are made too late, or unexpected issues arent planned for.
Here are 9 practical ways to keep control of your spend before the first shovel goes into the ground.
The golden rule
Dont spend 100% of your budget on the build. Keep a contingency and treat it like it doesnt exist.
Start with a clear scope (whats included and excluded)
A good quote should be itemised and specific. The quickest way to overspend is to assume something is included when it isnt. Ask your builder to confirm inclusions and exclusions in writing especially for kitchens, flooring, decoration, and external works.
Decide your specification early
Late decisions cost money. If you choose tiles, lighting, doors, or kitchen layouts mid-build, it often means delays and rework. The more you can decide before you start, the smoother (and cheaper) the build will be.
Understand provisional sums
A provisional sum is an allowance not a fixed price. If your quote has lots of provisional sums, your final cost is likely to move. Ask what each provisional sum covers and what would cause it to increase.
Keep changes (variations) under control
Changes are normal but they should be agreed properly. Every variation should be priced and approved in writing before work continues. That way youre never surprised at the end.
Budget for the non-build costs
Your total budget should include drawings, structural engineer fees, planning (if needed), Building Regulations, party wall surveyors (if needed), and any items youre buying directly.
Assume there will be an unexpected
Older properties can hide surprises drainage routes, poor ground, hidden structural issues. A contingency of 1015% is sensible for most projects.
Dont chase the cheapest quote
If one quote is dramatically cheaper, its usually because something is missing. The cheapest quote often becomes the most expensive once the extras start.
Track spend as you go
Keep a simple running total. Stage payments, variations, and any direct purchases. If you track it weekly, you can make decisions early not when its too late.
Choose a builder who communicates clearly
Good communication prevents expensive mistakes. You want a builder who explains whats happening, flags issues early, and gives you clear options not one who disappears until theres a problem.
Want a ballpark figure first?
If youre early-stage and want a quick estimate before you commit to drawings, try our cost calculator then get in touch for a detailed, itemised quote.
Want to stay on budget and get it done properly?
We provide detailed, itemised quotations and clear variation pricing so youre never guessing what something will cost.