As the 31 January Self Assessment deadline approaches, a lot of people assume it’s too late to fix anything. That’s rarely true. Even in the final days before the deadline, there are still important changes you can make that reduce errors, avoid penalties, and put you in a stronger position with HMRC.
Here’s what can still be amended now, what really matters at this stage, and where people often get caught out.
First, what the 31 January deadline actually covers
The 31 January deadline is not just one thing. It usually covers:
- Filing your online Self Assessment tax return
- Paying any tax owed for the year
- Making your first payment on account, if applicable
Missing it triggers automatic penalties, but being close to the deadline doesn’t mean you’ve lost control. The key is knowing what can still be corrected before you file.
Income figures can still be reviewed and corrected
Until you submit the return, your income figures are not locked in.
You can still:
- Double-check self-employed income
- Add or correct CIS income and deductions
- Include side income, freelance work, or cash jobs
- Review online sales, commission, or one-off work
This is where many people rush and make mistakes. Over-declaring out of fear or under-declaring because something feels minor can both cause problems later.
If you’re unsure what counts, our blog What Counts as Income for Self Assessment (and What Doesn’t) explains this clearly.
Expenses can still be adjusted
Expenses are another area that can still be amended right up until submission.
You can:
- Remove expenses that don’t qualify
- Add allowable expenses you missed
- Correct business-use percentages
- Review mileage, home office, or tool costs
Getting expenses wrong is a common trigger for HMRC queries. It’s better to file accurately than quickly.
Employment and PAYE details can still be checked
If you’ve had employment income alongside self-employment, you should still review:
- P60 and P45 figures
- Tax deducted under PAYE
- Benefits in kind
- Employment start and end dates
Even small PAYE mismatches can delay processing or lead to incorrect tax calculations.
You can still file even if you can’t pay
One of the biggest misconceptions we see in January is this: people delay filing because they can’t pay.
Here’s the reality:
- Late filing triggers penalties automatically
- Late payment usually results in interest, not immediate penalties
- Filing now stops the situation getting worse
Our blog Late Self Assessment Filing vs Late Payment: What HMRC Penalises (and What It Doesn’t) explains why filing should always come first.
Payment options can still be explored
If tax is due and payment is a concern, you can still:
- Check if a Time to Pay arrangement may apply
- Plan partial payments
- Understand what HMRC expects next
Even if you don’t set up payment immediately, filing on time keeps options open and reduces pressure.
What cannot be amended after filing (without extra steps)
Once you submit your return:
- Penalties for late filing cannot be avoided
- Changes require an amendment process
- HMRC scrutiny may increase if figures change significantly
That’s why the final review before submission matters, even when time is tight.
What if you’ve already missed the deadline?
If you’re reading this after 31 January, the advice changes slightly, but action still matters.
You may want to read:
- Self-Assessment Not Filed Yet? What to Do Now to Avoid HMRC Penalties
- What Happens If You Miss the Self Assessment Deadline?
Filing late is never ideal, but delaying further only increases costs.
A quick last-minute checklist
Before you submit, ask yourself:
- Have all income sources been included?
- Are CIS deductions entered correctly?
- Are expenses reasonable and supportable?
- Are PAYE figures accurate?
- Have you reviewed the final tax calculation?
If the answer to any of these is “I’m not sure”, it’s worth pausing rather than rushing.
Final thought: accuracy beats speed, even in January
The 31 January deadline creates pressure, but mistakes made in a rush can cause long-term issues. The return you submit now sets the tone for penalties, payments, and HMRC correspondence later.
Whether you need a final sense-check or full filing support, acting now gives you the best outcome.
If you’re still making changes, you’re not too late. You just need to make the right ones.
Tax2u helps clients review and file Self Assessment returns correctly, even close to the deadline.