What's the biggest thing people underestimate when budgeting for a UK home renovation?

I'm interested in how people actually approach renovation budgets.

It seems easy to focus on the obvious costs — kitchen, bathroom, flooring, decorating — while forgetting the less visible expenses.

I'd personally separate the budget into:

• Essential repairs
• Plumbing/electrical work
• Heating
• Kitchen/bathroom
• Flooring/decorating
• Labour and materials
• Contingency

I'd also rank everything as must-do / should-do / nice-to-have.

That seems particularly useful when renovating an older property where unexpected problems can appear.

For anyone who's renovated a UK home, what cost or problem caught you off guard?

For reference, this HomeToWorld guide covers the different areas to consider when planning a UK renovation budget:
https://hometoworld.co.uk/home-renovation-costs-uk-complete-guide/