UK Self-Assessment tax return filing, done properly.

UK Self-Assessment tax return filing is the annual process of declaring income that was not taxed at source and paying what is owed on it. The online filing deadline is 31 January after the tax year ends on 5 April, with payments on account falling due on 31 January and 31 July.

Self-Assessment in the UK looks deceptively simple from the outside, fill in the SA100, declare your income, pay what you owe. In practice, it's where most personal tax mistakes happen: missed reliefs, mis-classified income, miscalculated CGT, dividend tax bands applied incorrectly, foreign income disclosed badly, child-benefit clawback ignored.

We prepare Self-Assessment returns for directors of our company clients (extracting salary, dividends and benefits-in-kind from payroll and ledger data), for sole traders running unincorporated businesses, for landlords with property income, for freelancers and the self-employed, and for high earners with complex income mixes.

Our process starts from your existing records (we don't ask you to fill in a spreadsheet you don't understand), runs through tax-planning checks (have you used your CGT allowance, ISA allowance, pension annual allowance), and ends with the SA100 filed via our HMRC Agent gateway with full audit trail.

Where Making Tax Digital for Income Tax begins phasing in from April 2026 for sole traders and landlords with income above £50,000, we're fully MTD-ready, and will transition you onto the new quarterly reporting cycle without drama.

The dates and the money attached to them, from HMRC:

• 5 April: the tax year ends. Anything you wanted to do about it had to be done by this date.
• 31 October: paper return deadline. Almost nobody should be using it.
• 31 January: online return, the balancing payment for the year just filed, and the first payment on account for the next.
• 31 July: second payment on account. Required where the last bill was over £1,000 and less than 80 percent of your tax was collected at source.
• Miss 31 January and the penalty is £100 immediately, even if no tax is owed. From three months late it is £10 a day up to £900, then 5 percent of the tax due (minimum £300) at six months and again at twelve.

The arithmetic is rarely what goes wrong. It is a director drawing dividends past the point where the £500 dividend allowance and the basic rate band stop absorbing them, and finding a January bill nobody budgeted for. It is income crossing £100,000, where the personal allowance tapers away at £1 for every £2 above the threshold and the effective marginal rate on that band is far higher than the headline. It is a landlord still deducting mortgage interest as an expense rather than claiming the basic rate tax reducer. It is a capital gain reported without using the annual exempt amount, or a residential property gain missed entirely, which carries its own 60-day reporting deadline separate from the return.

A worked example. A director on a £12,570 salary taking £60,000 of dividends has £500 covered by the dividend allowance, part of the remainder taxed at the ordinary rate and the balance at the upper rate once total income passes the basic rate band. Shift £10,000 of that into an employer pension contribution before 5 April and the company saves Corporation Tax on it while the personal bill falls. Do the same thing on 6 April and you have waited a year for the benefit. This is why the planning conversation happens in February and March, not in the following January.

Making Tax Digital for Income Tax changes the rhythm for sole traders and landlords above the income thresholds, moving them to quarterly digital updates instead of one annual return. HMRC publishes the phased start dates, and we have written up what the change means in practice in our guide to MTD for Income Tax Self Assessment.

Most of the returns we file are for sole traders, landlords and property SPV owners, freelancers and contractors, and directors of companies we already act for. Where several years are outstanding, the catch-up bookkeeping service rebuilds the underlying records first. Company-level filings sit on the VAT, GST and tax compliance service. Fees are fixed and published on the pricing page. The annual return is one item in our full accounting service list, and most clients pair it with monthly bookkeeping from the same list.