Don’t wait until January: build your Self Assessment tax reserve month by month

A profitable year does not guarantee that the cash for Self Assessment will still be in the bank when HMRC needs it. For sole traders and landlords, the 31 January payment can arrive after months of supplier costs, Christmas trading, slower customer payments or personal drawings. The answer is not to predict the tax bill perfectly in October. It is to make tax part of the cash-flow plan before January.

For the 2025–26 tax year, the usual online Self Assessment filing and payment deadline is 31 January 2027. Depending on your circumstances, the amount due can include a balancing payment for the year just ended and the first payment on account towards the next tax year.

Why January can be bigger than the tax bill you expected

HMRC payments on account are advance payments towards the next Self Assessment bill, including Class 4 National Insurance for the self-employed. They are usually paid in two instalments: one by 31 January and the second by 31 July. Each instalment is normally half of the previous year's relevant Self Assessment tax amount.

Payments on account do not apply in every case. GOV.UK says they are generally not required if the previous year's relevant tax was less than £1,000 or if more than 80% of the tax was collected outside Self Assessment. Check the amount shown in your HMRC account rather than assuming the rule applies or does not apply.

Turn the expected bill into a monthly tax reserve

Start with the best current tax estimate, then convert it into a reserve target. If your estimated remaining liability is £6,000 and there are six months until the payment date, a simple planning baseline is £1,000 per month. That is a cash-flow target, not a tax calculation: the tax estimate should still be updated as income, expenses and reliefs change.

For a sole trader, the reserve can be refreshed whenever bookkeeping is reconciled. If year-to-date profit rises, increase the reserve. If legitimate expenses or tax already paid reduce the estimated liability, update it. The benefit is visibility: you can see the difference between money in the bank and money that is genuinely available to spend.

Add tax to a 13-week cash-flow forecast

A 13-week cash-flow forecast can make the January deadline operational rather than abstract. Start with opening cash, expected customer payments and other cash in. Add recurring costs, supplier payments, payroll where relevant, VAT and tax. Then calculate the expected closing balance each week and identify where the minimum cash position occurs.

This is why profit and cash are not the same thing. You can show a healthy accounting profit while customer invoices remain unpaid or while a large tax amount is already economically committed. A tax reserve makes that commitment visible before you make another spending decision.

HMRC also allows payments before the deadline

If you prefer to pay HMRC gradually, GOV.UK says eligible taxpayers who are up to date with previous Self Assessment payments can set up a Budget Payment Plan and make weekly or monthly Direct Debit payments towards the next bill. HMRC also allows one-off payments before the deadline. These payments reduce the amount left to pay later.

That is different from simply holding a reserve inside your business cash forecast. Some people prefer to keep the money reserved until closer to the deadline; others prefer to make payments to HMRC during the year. Whichever method you use, keep a clear record so you do not treat the same cash as available twice.

A practical October-to-January routine

1. Reconcile income and expenses now instead of waiting for the year-end filing deadline.
2. Estimate the 2025–26 Self Assessment amount using current records and include any payments on account already made.
3. Check your HMRC statement for the actual payments-on-account position and dates.
4. Set a monthly or weekly tax-reserve target and include it in the business cash-flow forecast.
5. Review the estimate after large invoices, seasonal income changes, major expenses or changes to other taxable income.
6. Before paying dividends, drawings or discretionary costs, compare the bank balance with the protected tax reserve and other committed cash.

If the January amount looks unaffordable, act early

Do not wait until the payment deadline to discover a shortfall. Update the books, check the HMRC statement, confirm whether the estimate is correct and explore HMRC's published payment options. If your expected income has genuinely fallen, HMRC also provides a process for claiming to reduce payments on account, but reducing them too far can lead to interest if the eventual liability is higher.

The practical goal is simple: know what you owe, know what is already covered and know what cash is actually safe to spend. A tax reserve turns Self Assessment from a January emergency into an ordinary part of running the business.