The 31 January Self Assessment deadline is one of those dates that looms large on every founder’s calendar, yet often remains a source of last-minute pressure. In our latest Startup Essentials article, we hear from Graham Davies, CEO of AIN partner Addition Finance, on the critical steps you need to take right now to avoid automatic penalties and ensure your financial house is in order for the year ahead.

With the 31 January Self Assessment deadline just days away, it’s natural to feel under pressure – especially if you’re running a business or juggling multiple income streams. If you’re not quite ready to submit yet, you’re not alone – but now is the time to act. Your actions over the next few days can make the difference between a smooth submission and unnecessary stress.

In this article I will explain what the deadline means, what happens if you miss it, and how to get your Self Assessment sorted quickly and confidently.

What is the Self Assessment deadline?

The Self Assessment deadline is 31 January following the end of the tax year. By 11:59pm on 31 January, HM Revenue and Customs expects:

• Your tax return to be submitted
• Any tax you owe to be paid

Submitting even a few minutes late can trigger penalties, so understanding your options before the deadline is key.

Who needs to submit a Self Assessment tax return?

You usually need to submit a return if you:

• Are self-employed or a sole trader
• Are a company director (unless all income is taxed via PAYE)
• Earn income outside PAYE (consulting, freelance work, side income)
• Receive rental income from property
• Earn significant savings or investment income
• Are affected by the High Income Child Benefit Charge
• Have capital gains to declare

Many individuals assume tax is “taken care of” elsewhere – until they realise too late that Self Assessment still applies to them.

What documents do you need to submit your return?

If you’re pulling this together this week, focus on the essentials:

• Details of all income (salary, dividends, freelance, rental, investments)
• Records of allowable business expenses
• Bank statements showing savings interest
• Dividend statements
• Pension contribution details
• Gift Aid donation records
• Your Unique Taxpayer Reference (UTR)

You don’t need immaculate records – but you do need complete and honest information.

What happens if you don’t submit by 31 January?

If your return isn’t submitted on time, HMRC applies penalties automatically. These are not discretionary and they escalate.

Here’s what typically happens:

• £100 late-filing penalty applied immediately after the deadline
• Interest charged daily on any unpaid tax
• After 3 months, additional daily penalties can apply
• Further penalties follow if the delay continues
  

The key point: penalties are based on lateness, not intent. Being busy, stressed, or unsure does not stop them accruing.

If you’re cutting it close, should you still submit?

Yes – but only if it’s accurate. 

Aim to file a return that’s correct to the best of your knowledge. If a figure is genuinely missing (for example, you’re waiting on paperwork), you may be able to use an estimated or provisional figure and then amend the return once you have the final numbers

If you act now, you can:

• Avoid late-filing penalties altogether
• Reduce stress and uncertainty
• Correct or amend details later if needed
• Get clarity on what you owe and plan payment

For many people, the biggest barrier is simply starting.

If you miss the deadline, here’s what to do immediately

If the 31 January deadline passes and your Self Assessment hasn’t been submitted, the most important thing is not to freeze. Missing the deadline triggers an automatic late-filing penalty from HMRC, but the situation is still recoverable – and acting quickly can prevent it from getting worse.

If you miss the deadline, you should:

• Submit your tax return as soon as possible
  This stops further late-filing penalties from escalating.
  
• Pay what you can, even if it’s not the full amount
  Interest is charged on unpaid tax, but paying early reduces how much builds up.
  
• Use provisional figures only where genuinely necessary
  If you’re waiting on final information, you may be able to submit with estimated figures and amend the return later. Do not guess.
  
• Set up a payment plan if needed
  If you can’t pay the full bill immediately, HMRC may allow a Time to Pay arrangement.
  
• Get help if you’re unsure
  Acting quickly with support can help limit penalties, reduce errors, and bring the situation back under control.
  

The worst option is doing nothing. Even a late submission is far better than continued delay.

Should you do it yourself or get help?

At this stage, most founders don’t struggle because Self Assessment is impossible – they struggle because they’re trying to fit it into an already overloaded schedule.

If you’re still preparing your return in the final days before the deadline, the risk isn’t effort – it’s mistakes, missed income, or avoidable penalties caused by rushing.

Getting expert support makes sense when:

• Time is tight
• Your income comes from more than one source
• You want certainty it’s done correctly
• You don’t want to risk errors under pressure
• You’d rather hand it over and focus on your business

For many founders, outsourcing isn’t about convenience. It’s about reducing risk and buying peace of mind when it matters most.

Getting your Self Assessment submitted without the stress

For founders who still need to submit and don’t want to risk mistakes or penalties, Addition Finance helps individuals get their Self Assessment prepared and submitted accurately, even close to the deadline.

Their process focuses on speed and clarity – confirming what’s needed, handling the submission, and ensuring nothing is missed when time is tight. To kick start your Self-Assessment with Addition submit your details HERE. 

The key takeaway

If the Self Assessment deadline is looming and you’re feeling underprepared, you still have options. Acting now – even in the final days – can help you avoid penalties, reduce stress, and move forward with confidence.  Once it’s submitted, the relief is immediate, and you can put better systems in place for the year ahead.

The important thing is not to panic – just to take the next step.

Graham Davies is the Founder and CEO of Addition Finance. Addition combines smart technology with real human support to give ambitious SMEs clarity, speed, and confidence in their numbers.