Common tax mistakes to avoid: individuals and small businesses

The ten most costly tax errors UK filers make are late filing, undeclared income, incorrect UTR or National Insurance numbers, poor record-keeping, missed pension reliefs, wrong expense claims, misunderstood payments on account, payroll misclassification, VAT slip-ups, and failing to notify HMRC of personal changes. Miss the Self Assessment registration deadline of 5 October and you are already behind; miss the 31 January payment deadline and an automatic £100 penalty lands before you have even opened the letter. In the next 48 hours: locate your UTR and National Insurance number, pull your most recent P60 or invoices, log into your HMRC online account to check for unread messages, and if anything looks unclear, speak to an accountant such as Concorde Company Solutions Limited before you file.

Quick-reference: highest-risk errors and your immediate action

• Late filing or late registration — check your 5 October and 31 January deadlines today
• Undeclared income (side gigs, savings interest, dividends) — list every income source before opening the form
• Incorrect identifiers (UTR, NINO, address) — verify them against your HMRC correspondence now
• Poor records — locate bank statements, receipts, and invoices going back at least five years
• Missed reliefs (pension, Gift Aid, marriage allowance) — check each one applies before you submit
• Wrong expense claims — confirm each item is allowable under HMRC rules
• Payments on account confusion — budget for the January double-payment if your bill is over the threshold for payments on account
• Payroll or employment-status errors — confirm worker classifications before the next RTI submission
• VAT and MTD non-compliance — check your VAT registration status and digital record requirements
• Failure to notify HMRC of personal changes — marriage, new rental income, or overseas work all affect your return

Pro Tip: Set two calendar reminders right now: 5 October (registration deadline) and 31 January (filing and payment). Missing either one costs money before HMRC has even looked at your figures.

Key takeaways

Avoiding the most costly tax filing errors comes down to three things: complete records, declared income from every source, and filing before the 31 January deadline.

Useful official resources

• Gov — the authoritative source for registration deadlines, filing requirements, and what records to keep; start here if you are unsure whether you need to file.
• Gov — explains how HMRC conducts checks, what lookback periods apply, and your rights and obligations during an enquiry.
• 10 tax return mistakes to avoid: Which? — a consumer-focused summary of the most common Self Assessment errors with concrete penalty examples.
• Self Assessment common mistakes: CalcHub — practical guidance on gross vs net box errors, payments on account, and pension relief claims.
• HMRC tax investigations: Simply Business — explains what triggers an HMRC investigation for self-employed taxpayers and what to expect during the process.
• What is Self Assessment? Concorde Company Solutions Limited — step-by-step local guidance on registering for and completing Self Assessment, written for UK individuals and small businesses.

This article is general information, not a substitute for advice from a qualified financial advisor. Consult a qualified financial professional about your own circumstances before acting on anything here.

Table of Contents

• 1. Poor record-keeping leaves you exposed
• 2. Maths errors and entering the wrong figures
• 4. Incorrect or missing identifiers
• 5. Incorrect expense claims
• 6. Missing pension reliefs and other allowances
• 7. Late filing, late payment, and missing registration deadlines
• 8. Not registering for Self Assessment when required
• 9. Misclassifying employment status and payroll errors
• 10. VAT accounting errors and Making Tax Digital
• 11. Ignoring foreign income and cross-border transactions
• 12. Failing to notify HMRC of personal changes
• Self-employed and small business pitfalls worth knowing
• What triggers an HMRC check and what to do if one arrives
• Your pre-filing audit checklist
• Concorde Company Solutions Limited: practical help before you file
• Sources

1. Poor record-keeping leaves you exposed

Incomplete records are the root cause of most filing errors. Without bank statements, receipts, and invoices to hand, figures get estimated, expenses get missed, and HMRC has no evidence to support your return if it asks questions. Self-employed taxpayers must keep business records for at least five years after the 31 January filing deadline for the relevant tax year. That means a return filed in January 2026 requires records kept until at least January 2031.

The practical fix is simple: one folder (physical or cloud-based) per tax year, labelled by category. Xero, QuickBooks, and FreeAgent all let you photograph receipts on your phone and attach them to transactions in real time. Doing it monthly takes twenty minutes; doing it in January takes days.

2. Maths errors and entering the wrong figures

Arithmetic mistakes are less common than they used to be now that most returns are filed online, but transcription errors are not. The most frequent slip is entering a net figure where the form expects gross, or vice versa. Pension contributions are a classic example: the box asks for the gross contribution, but many filers enter the net amount they actually paid, understating the relief they are owed.

Read the help text next to each box before you type. It takes thirty seconds and prevents a correction notice that takes thirty days to resolve.

4. Incorrect or missing identifiers

A wrong UTR (Unique Taxpayer Reference) or National Insurance number on a return can cause it to be processed against the wrong account, creating a phantom debt or a missed credit. Both are painful to unwind. Your UTR is a ten-digit number on any previous HMRC correspondence or accessible via your Personal Tax Account at gov.uk. Your National Insurance number is on your payslip, P60, or any letter from the Department for Work and Pensions.

Check both before you start the return, not after you have filled in twenty pages.

5. Incorrect expense claims

Claiming non-allowable expenses is one of the most common triggers for an HMRC compliance check. Commuting costs between home and a regular workplace are not allowable for employees. Client entertaining is not allowable for corporation tax or income tax purposes. Personal mobile phone bills claimed in full when the phone has mixed use are a red flag.

Equally costly is the opposite error: not claiming expenses you are entitled to. Home-office costs, professional subscriptions, tools and equipment, and mileage at the approved HMRC rate are all frequently missed. The Which? guide to Self Assessment errors highlights failing to claim reliefs as one of the ten most expensive mistakes a filer can make.

6. Missing pension reliefs and other allowances

Higher-rate taxpayers who contribute to a relief-at-source pension scheme receive basic-rate top-up automatically from the provider, but the additional relief for higher-rate or additional-rate taxpayers must be claimed on the Self Assessment return. Many filers simply do not know this and leave the money unclaimed year after year.

The same applies to Gift Aid donations, the Marriage Allowance, and Blind Person’s Allowance. None of these appear automatically on your return. You have to enter them.

7. Late filing, late payment, and missing registration deadlines

An online Self Assessment return filed even one day after 31 January triggers an automatic £100 penalty, regardless of whether any tax is owed. After three months, daily penalties of £10 per day can apply for up to 90 days. Interest accrues on unpaid tax from the payment deadline.

Registration is a separate obligation. If you became self-employed, started receiving rental income, or had any other new taxable income source during the 2024/25 tax year, you must have registered for Self Assessment by 5 October 2025. Missing that deadline is itself a penalty risk.

8. Not registering for Self Assessment when required

Many people simply do not realise they need to file a return. HMRC does not always send a prompt. You are required to register if your self-employment income exceeded £1,000, if you received untaxed income above certain thresholds, or if you are a company director. The Self Assessment overview on GOV.UK lists every trigger condition.

Registering late does not erase the obligation — it just adds a penalty on top of the original liability.

9. Misclassifying employment status and payroll errors

Getting worker classification wrong has consequences in both directions. Treating an employee as a self-employed contractor means PAYE and National Insurance contributions go unpaid, creating a liability that HMRC can pursue with interest and penalties. Treating a genuine contractor as an employee creates unnecessary payroll complexity and potential disputes.

Payroll errors go beyond classification. Late Real Time Information (RTI) submissions, incorrect tax codes, and wrong National Insurance category letters all generate correction notices and, in some cases, penalties. Concorde Company Solutions Limited’s payroll service handles RTI submissions, tax code management, and statutory payment calculations for small employers across Leeds and the surrounding area.

10. VAT accounting errors and Making Tax Digital

VAT-registered businesses must file returns accurately and on time. Common errors include reclaiming input VAT on non-business purchases, applying the wrong VAT rate, and missing the quarterly filing deadline. Since Making Tax Digital (MTD) for VAT became mandatory for all VAT-registered businesses, records must be kept digitally and submitted through MTD-compatible software. Filing a paper return or using a spreadsheet without a bridging tool is non-compliant.

If your taxable turnover is approaching the £90,000 VAT registration threshold, monitor it monthly. Registering late carries a penalty based on the VAT that should have been paid from the date you should have registered.

11. Ignoring foreign income and cross-border transactions

UK residents are taxed on their worldwide income. Foreign employment income, overseas rental income, foreign dividends, and gains on overseas assets all belong on your return. The foreign income pages of Self Assessment are frequently left blank by filers who assume overseas income is taxed at source and therefore not reportable in the UK. It usually is reportable, and double taxation relief is the mechanism that prevents you paying twice — not an excuse to omit it.

12. Failing to notify HMRC of personal changes

Getting married, separating, having a child, starting to receive rental income, moving abroad, or stopping self-employment all affect your tax position. HMRC does not automatically know about these changes unless you tell it. Failing to update your status can mean you are on the wrong tax code, missing allowances you are entitled to, or — worse — underpaying tax that HMRC will eventually reclaim with interest.

Update your HMRC Personal Tax Account whenever your circumstances change, and confirm the change has been reflected in your coding notice.

Self-employed and small business pitfalls worth knowing

Payments on account: the January cash-flow shock

If your Self Assessment tax bill exceeds £1,000 and less than 80% of your income is taxed at source, HMRC requires payments on account equal to half of last year’s bill, due on 31 January and 31 July. The January payment therefore combines a balancing payment for the year just ended and the first payment on account for the current year. For a sole trader seeing this for the first time, the bill can be double what they expected.

Budget for it from day one of trading. If your income drops significantly, you can apply to reduce payments on account, but do so carefully: underestimating and paying too little triggers interest from the original due date.

Allowable versus non-allowable expenses

The rule is that an expense must be “wholly and exclusively” for business purposes to be deductible. Mixed-use costs (a phone used for both personal and business calls, a car used for both commuting and client visits) must be apportioned. Common misclaims include:

• Commuting between home and a fixed workplace
• Client entertaining and hospitality
• Clothing that is not a uniform or protective gear
• Personal subscriptions unrelated to the trade

Missed allowable expenses are just as costly. Capital allowances on equipment, the Annual Investment Allowance, and the simplified expenses scheme for home working are all frequently overlooked.

Payroll and PAYE traps for small employers

Late RTI submissions attract penalties. Incorrect tax codes mean employees are under or over-taxed, and HMRC will eventually reconcile the difference. Statutory Sick Pay, Statutory Maternity Pay, and other statutory payments have specific rules about when and how they are reported. Getting any of these wrong creates a correction process that takes time and, sometimes, money.

Pre-filing self-audit for sole traders:

1. Confirm your UTR and National Insurance number are correct on the return
2. Check every income source is declared, including interest, dividends, and side income
3. Verify each expense claim is wholly and exclusively for business
4. Confirm you have not claimed both the trading allowance and actual expenses
5. Check your pension contributions are entered as gross figures
6. Budget for payments on account if your bill will exceed the applicable threshold
7. Confirm VAT returns are up to date and filed through MTD-compatible software

Pro Tip: Keep a single running spreadsheet or app log of every business expense as it happens. Reconstructing a year’s worth of costs in January from memory and a pile of bank statements takes hours and misses things.

What triggers an HMRC check and what to do if one arrives

Common red flags

HMRC investigations can be triggered by tip-offs, large income fluctuations between years, unexplained cash deposits, repeated late returns, and mismatches between what you declare and what third parties (banks, platforms, employers) have reported. A return that shows expenses as an unusually high proportion of income for your sector is also a flag. HMRC uses Connect, a data-matching system, to cross-reference returns against third-party data automatically.

Aspect enquiries focus on one specific area of a return. Full enquiries examine everything. Both require you to produce books, tax calculations, and returns.

Penalty and lookback summary

Sources: Which?; GOV.UK Self Assessment

HMRC lookback periods: Standard checks go back four years; careless errors extend to six years; deliberate behaviour can be investigated up to 20 years back. That is why record-keeping matters long after the filing deadline has passed.

What to do if HMRC contacts you

• Do not ignore the letter. HMRC sets a response deadline and missing it worsens your position.
• Authorise an agent immediately. HMRC will contact your agent directly once one is authorised, reducing the risk of missed correspondence.
• Gather records first. Locate bank statements, invoices, receipts, and the relevant tax return before responding.
• Be cooperative but precise. Answer what is asked; do not volunteer information beyond the scope of the enquiry.
• Seek professional advice. An accountant experienced in HMRC enquiries can negotiate timelines and present records in the most favourable light. For businesses in Leeds and the surrounding area, Concorde Company Solutions Limited provides HMRC compliance support.

Your pre-filing audit checklist

Documents to have ready before you file

1. P60 or P45 from every employer in the tax year
2. P11D (benefits in kind) if applicable
3. Bank statements for all accounts, business and personal
4. Sales invoices and receipts for all business income and expenses
5. Pension statements showing gross contributions
6. Dividend vouchers from any company shareholdings
7. Rental income records (rent received, allowable costs, mortgage interest)
8. Capital gains disposals (sale proceeds, acquisition cost, dates)
9. Gift Aid donation records
10. Foreign income documents (payslips, bank statements, tax certificates)

Record retention periods

Source: GOV.UK Self Assessment; GOV.UK Tax compliance checks

Accountant’s note: The checklist above covers the documents most commonly missing when a return is challenged. Gathering them before you open the form — not after — is the single habit that most reduces filing errors and HMRC enquiry risk. If you would like Concorde Company Solutions Limited to run through this checklist with you before you file, get in touch at Concordecompanysolutions.

Pro Tip: Scan or photograph every document as you collect it and store copies in a named cloud folder. If HMRC requests records two years from now, you will not be searching through shoeboxes.

Article authored by David, Concorde Company Solutions Limited.

A practical perspective on what actually goes wrong

The conventional wisdom on tax errors focuses on arithmetic. In practice, the most expensive mistakes are not calculation errors — they are omissions. Self Assessment penalises what you forget to enter far more than what you enter incorrectly. A filer who enters the wrong figure usually gets a correction notice and pays the difference. A filer who omits an income source entirely can face penalties for careless or deliberate behaviour, interest from the original due date, and a lookback that reaches back years.

The habits that genuinely reduce risk are unglamorous: a monthly reconciliation of bank statements against your records, a single folder where every receipt goes immediately, and calendar reminders set twelve months in advance for every HMRC deadline. None of this requires specialist knowledge. It requires consistency.

The payments-on-account issue deserves particular mention. A sole trader in Garforth came to Concorde Company Solutions Limited after receiving a January tax demand that was nearly double what they had budgeted. They had not been told about payments on account when they registered, and nobody had flagged it during the year. The tax owed was correct; the shock was entirely avoidable with a single conversation in April. That is the kind of thing a good local accountant catches before it becomes a problem.

Concorde Company Solutions Limited is the number one accountancy firm in Garforth, Leeds, and that standing comes from exactly this kind of practical, year-round support — not just from filing returns accurately, but from making sure clients are never surprised by what arrives in the post.

Concorde Company Solutions Limited: practical help before you file

Filing a Self Assessment return accurately is straightforward when your records are in order and you know what to claim. When they are not, the cost of getting it wrong — penalties, interest, and the time spent responding to HMRC — far exceeds the cost of getting professional help before you submit.

Concorde Company Solutions Limited, based in Garforth, Leeds, offers Self Assessment preparation, bookkeeping clean-up, managed payroll and PAYE support, VAT and MTD compliance, and representation during HMRC enquiries. Whether you need a one-off pre-filing audit, monthly bookkeeping, or ongoing tax and payroll management, the firm works with sole traders, limited company directors, and individuals across Leeds, Garforth, and Sherburn in Elmet. As the leading local accountancy practice in Garforth, Concorde Company Solutions Limited brings the kind of hands-on, year-round support that prevents the mistakes this article describes from happening in the first place. To book a pre-filing audit or speak to the team, visit Concordecompanysolutions or call directly.

Sources

HMRC receives data from banks, investment platforms, and employers. If your return does not match what those third parties have reported, a correction notice is almost automatic. Side-gig income from platforms such as Etsy, Airbnb, or Deliveroo must be declared. So must savings interest above your Personal Savings Allowance, dividends above the dividend allowance, and any capital gain above the annual exempt amount.

The annual CGT exemption has fallen considerably in recent years, so modest disposals that once sat safely below the threshold now need reporting. Use a capital gains tax calculator to check whether a disposal creates a taxable gain before you decide to leave it off the return.

Pro Tip: If your side income is under the trading allowance, you may owe nothing and have no reporting obligation — but you cannot claim both the allowance and actual expenses. Pick one and apply it consistently.

• Gov
• Gov
• 10 tax return mistakes to avoid this January – Which?
• Calchub

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