An extension budget should tell you more than the amount you hoped to spend. Once work starts, you need to distinguish between the original budget, quotes received, money committed, payments made and the likely final cost.
Break the project into work packages
Instead of one construction allowance, split the budget into meaningful packages such as design and approvals, site setup, groundworks, drainage, steelwork, structure, roof, windows and doors, external finishes, first fix, plastering, second fix, kitchen, bathrooms, heating, electrical work, flooring, decoration and external works.
Keep the original budget visible
When a quote comes in above budget, do not simply overwrite the original number. Keeping both values shows where the project has moved and helps you decide where savings may be required elsewhere.
Track committed cost separately from paid cost
A project can look healthy if you only total invoices already paid. The more useful figure is committed spend: orders placed and contractors appointed even if the bill has not yet arrived.
Use a forecast final cost
Forecasting is the discipline of asking what each package is currently expected to finish at. Update it whenever a quote is selected, a variation is agreed or a risk becomes a likely cost.
Keep a contingency visible
Do not hide contingency inside other allowances. Keep it as its own line so you can see how much is still available and how much has effectively been consumed by changes.
Combine cost planning with programme planning
Budget and programme are connected. Delayed windows, steelwork or specialist items can affect labour sequencing and temporary works. Tracking major dates next to the cost plan makes it easier to see which decisions need to be made early.
The UK Extension Cost & Project Planner combines a structured cost plan and editable programme in one Excel workbook. For a simpler budget-only option, see the Home Renovation Budget Calculator.