Self-Assessment: The Most Common Last-Minute Mistakes (and How to Avoid Them)

The run-up to 31 January is one of the busiest periods of the year for self-employed individuals, landlords and directors. Early preparation can help avoid last-minute stress and mistakes, making the process smoother.

With the deadline fast approaching, now is the perfect time to make sure your return is accurate, complete, and submitted on time. At Numeric Accounting, we support clients through this period every year, and we’ve seen firsthand how a little preparation can save some hassle.

Below, we’ve outlined the most common last-minute errors we encounter and how you can avoid them.

 

  1. Missing or incomplete records

When a tax return is rushed, record-keeping is often the first area to suffer. Incomplete income logs, missing expense receipts or gaps in bookkeeping can lead to incorrect figures being reported. Ensuring your records are thorough now can give you peace of mind and confidence that your submission is correct.

What to do:

  • Gather all your bank statements, invoices, receipts and dividend vouchers now.
  • Review your bookkeeping software to make sure no entries are duplicated or missing.
  • If you discover gaps, deal with them immediately, not on the final weekend of January.

 

Accurate records not only protect you from HMRC queries; they also help you claim the full reliefs and allowances you’re entitled to.

 

  1. Claiming the wrong expenses

One of the most common last-minute mistakes is claiming expenses that aren’t allowable or failing to claim those that are. When you’re rushing, it’s easy to either over-claim (a compliance risk) or under-claim (a lost tax opportunity).

Typical problem areas include:

  • Home office claims
  • Mileage vs actual vehicle costs
  • Phone and broadband use
  • Equipment purchases
  • Subscriptions and professional fees

 

Tip: If you’re unsure whether an expense qualifies, check the guidance or ask an accountant; guessing is never worth the risk.

 

  1. Forgetting additional income streams

When people file in a hurry, they often overlook income from:

  • Rental properties
  • Dividends
  • Capital gains
  • Foreign income
  • Side projects or ad-hoc freelance work
  • Interest from savings

 

Declaring all income accurately is essential. HMRC already receives significant data directly from banks and investment platforms so omissions are increasingly easy for them to spot.

 

  1. Misreporting payments on account

Payments on account catch many people off guard. If your tax bill was over £1,000 last year, you may need to make advance payments for the current year, which is often missed when returns are completed in a hurry.

This can lead to unexpected tax bills and cash-flow strain.

Avoid this by:

  • Checking your last HMRC statement
  • Budgeting early for payments on account
  • Confirming whether your circumstances have changed (you may be eligible to reduce them)

 

  1. Not checking personal allowances and reliefs

When you rush a return, it’s easy to overlook reliefs that could reduce your tax bill. Examples include:

  • Marriage Allowance
  • Pension contributions
  • Gift Aid donations
  • Trading loss relief
  • Capital allowances

 

A quick review with an accountant can often identify reliefs you’re entitled to claim, saving you money before you hit “submit”.

 

  1. Submitting late due to HMRC login issues

Every year, countless people attempt to file on the final day, only to discover they’ve forgotten their HMRC login details or never set up an account in the first place.

Account recovery can take several days. If you haven’t checked your login credentials yet, do it now to avoid penalties.

 

  1. Rushing the final checks

Last-minute submissions often contain simple errors:

  • Wrong figures
  • Incorrect UTR number
  • Mistyped National Insurance number
  • Misclassified income
  • Missed declarations

 

Even minor errors can delay processing or trigger HMRC enquiries later.

Take time to review your return carefully or ask an accountant to check it before filing. This step can help you feel more in control and confident that your submission is accurate, reducing the risk of delays or queries later.

 

  1. Leaving tax planning until after the deadline

January is the deadline for filing taxes, not for planning for them. By the time you file, many tax-saving opportunities for the year have already passed.

Working with an accountant throughout the year allows you to plan proactively rather than reactively. That means lower stress and better financial outcomes.

 

Need help before the 31 January deadline?

If you’re concerned about last-minute mistakes or want peace of mind that your tax return is accurate, we’re here to help.

At Numeric Accounting, we support clients with:

  • Self-Assessment filing
  • Catching up on incomplete records
  • Identifying tax-saving opportunities
  • Payments on account review
  • Digital record-keeping and cloud accounting
  • Planning ahead for next year’s return

The sooner you get in touch, the more we can do for you.

Salisbury: 01722 334888 | Southampton: 023 8084 4242

info@numericaccounting.co.uk

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