Scope: UK-wide planning guide. Practical considerations to adapt to your own home and circumstances.
Divide the budget into useful parts
List design and professional input, permissions where needed, construction, materials, fitting, disposal and finishing. Then add the cost of living through the work: storage, temporary facilities or accommodation if relevant. A budget focused only on the visible materials can leave out a large part of the project.
Mark the confidence level
Label each amount as a confirmed quote, a supplier price or a rough allowance. Date it and record what it includes. This makes the uncertain parts visible. A carefully formatted spreadsheet does not make an early guess more reliable.
Keep choices separate from necessities
Identify the work needed to produce a safe, usable result and the upgrades you would like if funds allow. Agree this distinction with anyone sharing the decisions. If costs rise, it is easier to defer a clearly optional finish than to debate the whole purpose of the project under pressure.
Create a change log
For every proposed change, record the description, extra or reduced cost, effect on time and approval. Update the forecast total before giving consent. Several small additions can materially change a budget even when none seems significant alone.
Review cash flow as well as the total
Map payment stages and ordering deadlines against the money available. Do not assume every supplier is paid after completion. Retain written records of payments and the work or goods they relate to. Ask questions about any payment request that does not match the agreed arrangement.
Finish with a realistic reserve
Set aside an amount for uncertainty based on the project and professional advice, rather than relying on a universal percentage. Keep it separate from the optional-upgrade list. A reserve has little value if it is already committed to a more expensive worktop before work begins.