Not treating the tax bill as a real cost from day one. Ask a room full of business owners this question and that one comes up more than almost anything else. The pattern is the same whether it's a sole trader in Cardiff or a small Ltd in Belfast: money lands in the account, it feels like income, it gets spent on stock, a van, or just living costs. Then the bill arrives and there's a scramble.
The specifics of how it bites depend on the setup:
- Sole traders get caught by the first Self Assessment bill, which can be a nasty surprise because HMRC also asks for payments on account towards the following year at the same time. In effect you can be paying around 150% of a year's tax in one go the first January.
- Ltd companies have Corporation Tax due nine months and one day after the year end, plus PAYE if you're on payroll, plus your own personal tax on dividends. Three separate pots, three separate deadlines.
- VAT catches growing businesses that sail past the £90,000 threshold without noticing, as the check is on a rolling twelve months, not the tax year.
The fix is boring but it works: open a separate business bank account before the first invoice goes out, and move a fixed percentage of every payment received into a savings pot the same day. Somewhere between 25% and 30% is a sensible starting figure for most sole traders; a Ltd owner should ring-fence 19% to 25% of profit for Corporation Tax and think about dividend tax separately. Never touch it. If it turns out to be too much, that's a pleasant problem to have at year end.
A decent bookkeeper for a couple of hours a month is worth it long before you think you can afford one. Waiting until the business "gets big enough" is usually how people end up paying an accountant to untangle eighteen months of mess instead.
There's a similar discussion already running with some good answers on income streams and expanding too late:
What would you do differently if you had your time again in business?