Being Prepared for Self-Assessment Deadlines: A Comprehensive Guide

In the fast-paced world of small business ownership and self-employment, few things strike as much dread as the approach of tax season.

For many in the UK, the Self-Assessment tax return is an annual ritual that, if not handled properly, can lead to stress, penalties, and unnecessary financial strain. As we find ourselves in December 2025, with the online filing deadline for the 2024/25 tax year looming on 31 January 2026, now is the perfect time to get organised.

At Suzanne Lock Business Services (SLBS), we’ve helped countless small businesses navigate these waters with ease. This article will guide you through everything you need to know about preparing for Self-Assessment deadlines, offering practical tips, insights, and strategies to ensure compliance and peace of mind.

Whether you’re a sole trader, freelancer, landlord, or director of a limited company, understanding and preparing for Self-Assessment is crucial. It’s not just about avoiding fines; it’s about optimising your tax position, claiming what you’re entitled to, and setting your business up for success in the year ahead. Let’s dive in.

What is Self-Assessment and Who Needs to Do It?

Self-Assessment is the system used by HM Revenue and Customs (HMRC) to collect income tax from individuals whose tax isn’t automatically deducted at source, such as through PAYE.

It applies to a wide range of people, including the self-employed, partners in a business, those with rental income, high earners (over £100,000), and anyone with untaxed income like savings interest or dividends.

If you’re new to this, you might wonder if it applies to you. HMRC provides a handy online tool to check.

Common triggers include starting a side hustle, renting out a property, or receiving child benefit while earning over £50,000.

For the 2024/25 tax year (running from 6 April 2024 to 5 April 2025), if your circumstances changed during this period, you may need to register for Self-Assessment by 5 October 2025 if you hadn’t already. Missing this registration deadline can result in penalties, even if no tax is due.

The process involves declaring all your income, calculating your tax liability, and paying what you owe. It’s more than a form-filling exercise; it’s an opportunity to review your finances holistically. For small businesses, this can highlight areas for improvement, such as expense tracking or investment strategies.

Key Deadlines for the 2024/25 Tax Year

Timing is everything in tax matters. For the 2024/25 tax year, the deadlines are straightforward but non-negotiable. If you opt for a paper return, HMRC must receive it by midnight on 31 October 2025. However, since we’re past that date in December 2025, most people will be filing online, with the deadline set at 11:59 pm on 31 January 2026.

But deadlines extend beyond filing. If you owe tax, payment is due by the same 31 January 2026 date, including any payments on account (advance payments towards the next year’s bill). For those with balances over £1,000, you’ll typically make two payments on account: one on 31 January and another on 31 July. Late payments incur interest at 7.75% (as of late 2025) plus penalties starting at 5% after 30 days.

Additionally, if you’re expecting a refund, filing early means getting your money back sooner, often within weeks via online filing. Last year, millions filed at the last minute, leading to system overloads and errors.

Don’t join that statistic; preparation now avoids the January rush.

Steps to Prepare Your Self-Assessment Tax Return

Preparation is the key to a smooth Self-Assessment process.

Start by gathering your documents well in advance. You’ll need your 10-digit Unique Taxpayer Reference (UTR), National Insurance number, P60 (from employment), P45 (if you left a job), and P11D (for benefits like company cars). For self-employed individuals, compile bank statements, invoices, receipts for expenses (such as office supplies, travel, or home office costs), and details of any capital allowances.

Organise your records digitally if possible, tools like spreadsheets or accounting software can make this easier. HMRC recommends keeping records for at least 22 months after the tax year ends, but for businesses, it’s wise to retain them for six years in case of audits.

Next, log into your HMRC online account. If you haven’t set one up, do so via the Government Gateway. The form itself (SA100) covers sections on employment, self-employment, property, foreign income, and more. Be thorough: declare all income sources, including side gigs on platforms like Etsy or Uber.

Claiming reliefs and allowances is where many save money. The personal allowance is £12,570 for 2024/25, but you can also claim for pension contributions (up to £60,000 annually with relief), charitable donations (via Gift Aid), and work-related expenses if unreimbursed by your employer. For example, if you work from home, you might claim £6 per week flat rate or actual costs.

If your affairs are complex, say, involving cryptocurrency gains or overseas income, consider supplementary pages. HMRC’s guidance is helpful, but for accuracy, double-check calculations. Software like HMRC’s own toolkit or third-party options can automate much of this.

One pro tip: reconcile monthly. Don’t wait until year-end; track income and expenses regularly to spot discrepancies early. This habit not only eases Self-Assessment but improves business decision-making.

Common Pitfalls and How to Avoid Them

Even seasoned filers make mistakes. A frequent error is underreporting income, perhaps forgetting bank interest or a one-off freelance job. HMRC cross-checks with third parties like banks and employers, so omissions can lead to investigations and fines.

Another pitfall is claiming ineligible expenses. For instance, entertaining clients isn’t deductible, but business travel is. Keep receipts as evidence, digital photos suffice.

Late filing is the biggest issue: an automatic £100 penalty applies even if no tax is due, escalating to daily charges after three months.

Procrastination is a killer. Many assume it’s simple until they start, then panic. Start early, even if just gathering docs. If you’re unsure, use HMRC’s helpline or webchat, but lines get busy near deadlines.

For higher earners, watch for the high-income child benefit charge or pension taper relief reductions. And don’t forget capital gains: selling shares or property over the £3,000 allowance triggers reporting.

To avoid these, create a checklist: list all income streams, review last year’s return for patterns, and seek advice if needed. At SLBS, we often see clients who’ve overpaid tax due to missed reliefs, professional review can reclaim thousands.

The Role of Professional Accountants in Self-Assessment

While many handle Self-Assessment themselves, enlisting an accountant like those at SLBS can transform the experience. We offer tailored tax planning, ensuring you minimise liabilities legally. For small businesses, our bookkeeping and payroll services keep records impeccable year-round, making tax time a breeze.

Accountants spot opportunities you might miss, such as R&D tax credits or SEIS investments. We handle filing, deal with HMRC queries, and provide forecasting to budget for tax bills. In a 2025 survey, 70% of small business owners using accountants reported less stress and better compliance.

If your business is growing, professional help scales with you. At SLBS, our remote services mean UK-wide support without the hassle of in-person meetings. Think of it as an investment: the time saved allows focus on what you do best: running your business.

Upcoming Changes: Making Tax Digital for Income Tax

Looking ahead, Self-Assessment is evolving. From 6 April 2026, Making Tax Digital (MTD) for Income Tax Self-Assessment mandates digital record-keeping and quarterly updates for those with business or property income over £50,000, dropping to £30,000 from April 2027. This shifts from annual to real-time reporting, using compatible software.

Prepare now by adopting digital tools. MTD aims to reduce errors and close the tax gap, but it requires adjustment. SLBS can guide you through compliance, from software selection to quarterly submissions.

Conclusion: Take Control Today

Being prepared for Self-Assessment deadlines isn’t just about compliance—it’s about financial empowerment. By understanding the process, meeting key dates like 31 January 2026, and organising early, you avoid penalties and maximise savings. Whether you go it alone or partner with experts like SLBS, the effort pays off.

Don’t wait until the new year; start today. Gather your documents, review your finances, and if needed, reach out for professional support. At Suzanne Lock Business Services, we’re here to make tax season stress-free. Visit our website or contact us to discuss how we can help your small business thrive.