After 31 October: the online Self Assessment checklist for the 31 January deadline

The 31 October paper deadline has passed, and for anyone who missed it, the practical question is what happens next. The answer is straightforward: online filing is still open, and the online deadline is 31 January. Missing the paper date does not mean you have missed Self Assessment altogether, but it does mean the clock is now running on the online return, and late filing penalties apply after 31 January, not after 31 October. If you were planning to file on paper and the date slipped past, switch to online now rather than posting a late paper return. A paper return sent after 31 October can still be processed, but HMRC may treat it as late, and the online route gives you an immediate submission receipt, which paper never does.

Start with your Government Gateway account

The first check is access, not figures. Make sure you can sign in to your Government Gateway account and that it is linked to Self Assessment. If you have never filed online, you need to register for Self Assessment online and wait for an activation code by post, which can take up to 10 working days in the UK. That wait is the single most common reason an online return becomes a late return, so start it this week, not in January.

If you already file online, check that your sign-in details work and that your personal details, address, and Unique Taxpayer Reference are correct. A locked account or a wrong address in January is a problem you can solve in November for free.

Gather the income, not just the headline figure

Self Assessment asks for each source of income separately, so gather the documents by source before you start typing. For employment, that is the P60 and any P45s from the year. For self-employment, it is your business income and expenses records, not a single profit estimate. For property, it is rents received and allowable costs. For savings and investments, it is interest statements, dividend vouchers, and any capital gains records.

The most frequent error is not fraud, it is omission: a small second income stream, a savings account opened mid-year, or a dividend from a company you forgot you held shares in. Work through your bank statements for the tax year, 6 April 2025 to 5 April 2026, and ask of every regular credit whether it belongs on the return. That one pass catches more mistakes than any other check.

Expenses: claim what is allowable, and keep the evidence

For self-employed filers, allowable expenses are costs incurred wholly and exclusively for the business. That includes office costs, travel for business, professional fees, and a reasonable proportion of home working costs where applicable. It does not include personal spending that happens to pass through a business account, and it does not include a round estimate with no records behind it.

Keep the evidence even though you do not send it with the return. HMRC can ask to see records for several years after filing, and a claim with no supporting invoice or receipt is a claim you may have to repay. A simple folder, physical or digital, with one file per expense type, is enough.

Payments on account: check before you are surprised

Many filers are caught out not by the tax bill itself but by the payment on account. If your tax bill for the year is more than a threshold amount and less than 80 percent was collected at source, HMRC usually asks for two payments on account toward the following year, each half of the previous bill, on 31 January and 31 July. Your 31 January payment is therefore often one and a half times the balancing payment alone.

Check the calculation when you submit, and plan the cash now. If your income has fallen, you can apply to reduce payments on account, but that claim must be reasonable: if you reduce too far, interest applies on the shortfall.

The January checklist, in order

First, confirm your online access and activation. Second, collect every income source and match it to a document. Third, list allowable expenses with evidence for each. Fourth, check pension contributions, Gift Aid, and student loan details, which are easy to miss and change the bill. Fifth, submit well before 31 January, then pay by the same date, and keep the submission receipt and the calculation.

Where outside help fits

Self Assessment is designed to be filed by the individual, and many people do it themselves every year. The difficulty is rarely the form itself. It is the record keeping through the year, the expense coding, and the payment planning that turns a routine return into a January rush. Working with a team that provides accounting outsourcing services keeps those records in order through the year, so the return is assembled from clean books rather than reconstructed from a year's bank statements. For filers who want that discipline in place before the next tax year starts, tax compliance services cover the preparation and review that make the January deadline a formality instead of a fire drill.

One action for this week

If you take one action from this article, make it this: sign in to your Government Gateway account today and confirm it works. If it does not, start the fix now. Everything else, income, expenses, and payments, can be gathered over the coming weeks. Access cannot be improvised on 31 January.