Debunking common tax myths and what business owners get wrong is something we find ourselves doing in almost every new client conversation. That is not a criticism of business owners — tax rules are genuinely complex, they change regularly, and much of what circulates online is either oversimplified or flatly incorrect.

The myths that concern us most are not the obvious ones. They are the plausible-sounding beliefs that feel like they should be right — and that quietly create problems with HMRC because no one challenged them early enough. Cash-in-hand income that goes unreported. Expense claims that fail the wholly and exclusively test. Assumptions about Making Tax Digital that lead to non-compliance from day one.

We have picked five of the most common below, with a clear account of where the reality sits and why it matters.

This one persists at every scale of business — from sole traders taking weekend market payments to directors running a side operation with no card reader. The assumption is that cash is somehow invisible to HMRC, or that small amounts are beneath their notice.

Neither is true. All business income must be declared — cash sales, bank transfers, online payments, and overseas income alike. HMRC cross-references data from payment processors, business bank accounts, Companies House filings, and even third-party platforms such as eBay and Etsy. The idea that cash transactions fall outside their visibility has not been accurate for some years.

The relevant standard is straightforward: if money came in as a result of your business activity, it is taxable income and it belongs on your return. There is no minimum threshold below which you can quietly ignore a revenue stream.

We have seen enquiries triggered by discrepancies between declared turnover and bank deposits that look innocuous in isolation. The correction process is stressful and invariably more expensive than simply declaring the income in the first place. If in doubt about how to categorise a particular income stream, ask before you file — not after HMRC does.

This is a comfortable assumption and an understandable one, but it is legally wrong and it can leave business owners badly exposed.

The taxpayer — the individual or company named on the return — is always ultimately responsible for the accuracy of the information submitted to HMRC. Your accountant prepares the return based on the records and information you provide. If those records are incomplete, inaccurate, or missing entirely, the return will reflect that — and the liability for any resulting error sits with you, not with the accountant.

In practice, this means that handing your accountant a bag of receipts in January and assuming everything will be fine is not a risk-free approach. A good firm will ask questions, flag inconsistencies, and push back when something does not look right. But they are working from your data. If expenses are fabricated, income is omitted, or records have not been kept, the consequences fall on the business owner.

HMRC takes a firm line on reasonable care. Where they determine that a taxpayer has not taken reasonable care over their return, penalties are considerably higher than where a genuine mistake is made on otherwise well-kept records. Knowing your numbers — and being honest about them — is not optional.

The actual rule is more demanding than this. HMRC requires that expenses are incurred wholly and exclusively for the purposes of the business. Costs that have a dual purpose — personal and professional — generally cannot be claimed in full, and sometimes cannot be claimed at all.

A few examples of where this bites:

• Working from home: you can claim a proportion of household costs against business use, either via HMRC's flat-rate allowance or by calculating the actual cost apportionment. You cannot claim your entire mortgage or rent as a business expense.
• Client entertainment: generally not allowable for Corporation Tax purposes, even if it is a legitimate business activity.
• A vehicle used for personal journeys: only the business mileage element is claimable. HMRC's approved mileage rates apply for sole traders and directors using personal vehicles.
• Clothing: a suit worn to client meetings is not claimable — it is ordinary clothing you could wear elsewhere. Specialist protective gear or a uniform with a company logo typically is.

The test is not whether the expense was business-related. It is whether it was exclusively for business purposes. The distinction matters. Overclaiming expenses is one of the most common triggers for an HMRC compliance check, and the resulting adjustments routinely come with interest and penalties attached.

With Making Tax Digital for Income Tax (MTD for IT) becoming mandatory from 6 April 2026 for sole traders and landlords with qualifying income above £50,000, this particular myth is timely. A large number of business owners we speak to believe that MTD will require them to complete and submit a full tax return four times a year. It will not.

The quarterly submissions under MTD are updates — essentially a digital snapshot of your income and expenses for that quarter. They are not tax calculations, they do not result in a tax payment being due each quarter, and they are not the same as a Self Assessment return. The annual tax liability is still calculated and settled through an end-of-period statement and a final declaration, broadly equivalent to the current Self Assessment process.

The more significant change is the record-keeping requirement. MTD mandates digital records — you cannot continue keeping accounts in a paper ledger or a spreadsheet without proper digital links in place. Spreadsheets are not banned outright, but they must be connected to HMRC-compatible software via a bridging solution. A spreadsheet maintained in isolation and manually keyed into a filing does not meet the standard.

For most clients, the practical answer is a cloud accounting platform that is already MTD-compatible. The quarterly updates then largely happen as a by-product of keeping records up to date — which is exactly how your accounts should be managed anyway. If you are unsure where you stand, our post on why the latest MTD updates matter to your business covers the mechanics in more detail.

We hear this regularly from business owners in early-stage or loss-making years. The logic runs: if there is no profit and no tax due, why would HMRC be interested? The answer is that HMRC enquiries are not solely — or even primarily — about collecting tax on declared profits.

HMRC can open an enquiry into any return, for any year, within defined time limits. They do not need to suspect underpayment to trigger a compliance check. Enquiries are frequently opened to verify that income has been correctly reported, that expenses are legitimate, and that records meet the required standard. A loss-making business that cannot produce adequate records, or whose declared turnover looks inconsistent with external data, is just as likely to be scrutinised as a profitable one.

There is also the question of loss claims. If you are carrying forward losses to offset against future profits, HMRC has a legitimate interest in the accuracy of those figures. Inflated losses claimed in a loss-making year create a problem further down the line when the business becomes profitable and the offset is applied.

The takeaway is simple: the quality of your record-keeping matters regardless of your profitability in any given year. Good records protect you; poor records create risk.