Key takeaways
- Mixing personal and business money is the most common mistake; it makes records unreliable and, in a company, can create an overdrawn director's loan account.
- You must register for VAT within 30 days of the month end in which rolling 12-month taxable turnover goes over £90,000.
- Filing company accounts up to a month late costs £150, rising to £1,500 after six months, and the penalty doubles if you are late two years running.
- A late Self Assessment return costs £100 straight away, then £10 a day after three months, up to £900.
- Statutory late payment interest on business debts is 8% plus the Bank of England reference rate, which makes 11.75% for debts due from 1 July to 31 December 2026, plus £40 to £100 fixed compensation.
- Companies must keep accounting records for 6 years; failing to keep them can mean a £3,000 fine or director disqualification.
The most common financial mistakes small businesses make are mixing personal and business money, not forecasting cash flow, missing tax deadlines and thresholds, and chasing customers too slowly. Most are cheap to fix and expensive to ignore: HMRC and Companies House penalties are automatic, and a cash shortfall can end a profitable business. Here are ten mistakes, what each costs in 2026/27, and what to do instead.
Why do financial mistakes matter so much?
Small errors compound into penalties, interest and lost time. The Insolvency Service recorded 1,882 company insolvencies in England and Wales in September 2026, about one in every 200 companies over the year. Not every failure starts with a bookkeeping mistake, but poor records and weak cash control make problems harder to spot and fix.
The ten mistakes at a glance
| Mistake | Typical cost | Fix |
|---|---|---|
| Mixing personal and business money | Lost expenses, DLA tax | Separate bank account |
| No cash flow forecast | Missed bills, borrowing | 13-week forecast |
| Not saving for tax | Debt, interest | Tax pot each month |
| Missing the VAT threshold | Backdated VAT, penalty | Monthly turnover check |
| Late accounts | £150 to £1,500 | Diarise deadlines |
| Late tax return | £100 upwards | File early |
| Slow credit control | Cash tied up | Clear terms, chase fast |
| Underpricing | Thin margins | Know your break-even |
| Poor records | £3,000 fine | Software, receipts |
| Leaving advice too late | Costly fixes | Get advice early |
1. Mixing personal and business money
Use a separate business bank account from day one. Mixed accounts make it hard to prove expenses and easy to miss income. In a limited company, paying personal costs from the company account builds an overdrawn director’s loan account, which can trigger a section 455 charge; see director’s loan account.
2. Not forecasting cash flow
Profit is not cash. A business can be profitable on paper and still run out of money if customers pay late or a tax bill lands. Keep a rolling 13-week forecast of money in and out. Our guide to cash flow forecasting shows how.
3. Not setting money aside for tax
Move a share of every receipt into a separate savings account for VAT, PAYE and income or corporation tax. Sole traders are often caught by payments on account, which can mean paying 150% of a year’s bill in one January.
4. Missing the VAT registration threshold
You must register for VAT if your taxable turnover for the last 12 months goes over £90,000, within 30 days of the end of the month you crossed it. You must also register if you expect to exceed £90,000 in the next 30 days alone. Register late and you owe the VAT on sales since the date you should have registered, and a penalty may apply, according to HMRC’s registration guidance. Check the rolling total every month, not once a year.
5. Filing company accounts late
Companies House penalties are automatic and start the day after the deadline.
| How late | Penalty (private company) |
|---|---|
| Up to 1 month | £150 |
| 1 to 3 months | £375 |
| 3 to 6 months | £750 |
| More than 6 months | £1,500 |
The penalty doubles if your accounts are late two years in a row. See company accounts filing deadlines.
6. Filing a tax return late
A Self Assessment return filed after 31 January costs £100 immediately. After three months HMRC adds £10 a day for up to 90 days (£900), then 5% of the tax due or £300 at six and 12 months. Late payment adds 5% of the unpaid tax at 30 days, six months and 12 months, plus interest.
7. Letting customers pay late
Agree payment terms in writing and invoice promptly. If a business customer pays late, you can claim statutory interest of 8% plus Bank of England Bank Rate. The reference rate is Bank Rate on 30 June for debts due from July to December (3.75% in 2026, so 11.75% a year) and Bank Rate on 31 December for debts due from January to June. You can also claim fixed compensation:
- £40 for debts under £1,000
- £70 for debts of £1,000 to £9,999.99
- £100 for debts of £10,000 or more
See late payment interest and compensation.
8. Underpricing
Many new businesses set prices against competitors, not their own costs. Work out your break-even point and the margin each product needs to cover overheads, tax and your own pay. Try the break-even calculator.
9. Keeping poor records
Companies must keep accounting records for six years from the end of the financial year they relate to. HMRC can fine a company £3,000, or a director can be disqualified, for not keeping them, according to GOV.UK’s guidance on company records. Sole traders with income over £50,000 must also keep digital records for Making Tax Digital for Income Tax from 6 April 2026.
10. Leaving advice too late
Owners often call an accountant after a problem appears, such as an HMRC letter or a cash crunch. Advice at the start, on structure, VAT and how to pay yourself, usually costs less than putting things right later.
Checklist: a monthly money routine
- Reconcile the bank account in your software
- Update the 13-week cash flow forecast
- Move money into the tax savings account
- Check rolling 12-month turnover against £90,000
- Chase invoices more than 7 days overdue
- Check the next filing and payment deadlines
Frequently asked questions
What is the most common financial mistake small businesses make?
Mixing personal and business finances. It makes records unreliable, hides the real profit and, in a company, can create an overdrawn director’s loan account.
What happens if I register for VAT late?
You must pay the VAT due on sales from the date you should have registered, and HMRC may charge a penalty depending on how much you owe and how late you are.
How much is the penalty for filing company accounts late?
For a private company, £150 if up to a month late, rising to £1,500 if more than six months late. It doubles if you are late two years in a row.
How much interest can I charge on a late invoice?
8% a year plus Bank of England Bank Rate on business to business debts, using the reference rate set on 30 June or 31 December; for debts due from 1 July to 31 December 2026 that is 11.75%, unless your contract sets a different rate.
How long should a small business keep financial records?
Limited companies must keep records for six years from the end of the financial year they relate to, and longer if a return was late or HMRC has opened a check.
Sources
Rates, thresholds and rules in this guide were checked against HMRC and GOV.UK guidance on 6 November 2026. Tax rules change, so check the current position on GOV.UK or with a qualified adviser before acting. This guide is general information, not personal advice.