Tax is one of the areas new founders worry about most, yet paradoxically it is often the area given the least attention until a deadline forces the issue. The mistakes that follow from this pattern are rarely deliberate; they are simply what happens when a founder is stretched thin across every part of a new business and tax planning quietly slips down the list. Here we look at the tax mistakes London founders make most often, and how affordable accountant support helps you avoid them from the very start of trading.
Not Setting Aside Money for Tax
The single most common mistake is treating all income as available to spend, without setting anything aside for the tax that will eventually be due on it. This leaves founders facing a bill they cannot comfortably pay, sometimes forcing them to borrow or delay other payments. An accountant helps establish a simple, ongoing habit of setting aside a sensible proportion of income as it arrives, so the eventual bill is never a shock.
This mistake is particularly common among founders whose income grows quickly, since the habit of spending everything that comes in was formed when the business was small and the resulting tax bill was correspondingly modest. As turnover grows, that same habit becomes far more dangerous without anyone actively noticing the shift.
The fix for this is rarely complicated at all once someone genuinely points it out clearly: a separate savings account, a fixed percentage set aside automatically, and a rough sense of what is owed at any given moment are usually enough to prevent this particular mistake from ever happening again.
Missing Registration Deadlines
Whether it is registering for Self Assessment, corporation tax or VAT, each of these has a specific window in which registration must happen, and missing it can result in penalties even if the tax itself is eventually paid correctly. New founders, unaware of these windows, sometimes register late simply because nobody told them the deadline existed. An accountant tracks these dates from the very start of your business relationship, removing this entirely avoidable risk.
What makes this outcome particularly frustrating for founders is that the penalty applies regardless of whether any tax was actually owed, so even a business that made no profit in its first year can still be penalised for registering late, purely as a matter of process.
Claiming Expenses Incorrectly
Founders tend to fall into one of two traps: claiming too little out of caution, leaving legitimate deductions unused, or claiming too much by including costs that are not genuinely allowable, which risks trouble if HMRC ever queries the return. Both mistakes are genuinely common and both are avoidable with proper guidance. Reliable business planning and financial advice from an accountant familiar with small business taxation helps you claim accurately, capturing what you are entitled to without overstepping into riskier territory.
The line between what is and is not allowable is genuinely not always obvious, particularly for costs that serve both personal and business purposes, such as a phone or a vehicle, which is exactly the kind of grey area where professional guidance saves both money and unnecessary worry.
Getting VAT Wrong
VAT catches out many new founders, whether that means registering later than required once the threshold is crossed, choosing an unsuitable VAT scheme, or simply making errors in how VAT is charged and reclaimed on transactions. These mistakes can be costly to correct retroactively. An accountant helps you understand your VAT position well before it becomes urgent, and ensures your invoicing and record-keeping are set up to handle VAT correctly from the point of registration onward.
Correcting VAT errors retroactively is rarely straightforward, often involving amended returns and, in some cases, additional charges, which is exactly why getting the initial setup right is so much more valuable than fixing a mistake after several quarters have already passed.
Choosing between the standard, flat rate and cash accounting VAT schemes is a decision that genuinely depends on your specific business, and getting expert financial advice tailored to your circumstances here can make a measurable difference to how much administration and tax you ultimately face.
Mixing Up Salary and Dividends
For company directors, the balance between salary and dividends has genuine tax implications, and getting this wrong, whether by drawing too much informally or structuring payments in a way that is not properly documented, can create real problems at year end. An accountant helps set up a clear, compliant approach to how you pay yourself, reviewed carefully and periodically as your income and circumstances genuinely change over time.
Dividends in particular must be properly documented with board minutes and paperwork at the time they are paid, not reconstructed months later when the accounts are being prepared, and this is a small but important detail that genuinely catches out even otherwise careful, diligent founders who are simply unaware of the requirement.
Leaving Everything Until the Deadline
Perhaps the most avoidable mistake is simply leaving tax matters until the filing deadline is imminent, which increases the risk of errors, missed deductions and unnecessary stress. Founders who engage with their tax position throughout the year, with an accountant keeping them informed along the way, consistently produce more accurate returns and face far less last-minute pressure than those who treat tax as a once-a-year emergency to be endured.
Leaving everything until the very last few weeks also removes any real opportunity for genuine tax planning, since most legitimate ways to reduce a tax bill need to be actioned before the end of the relevant tax year, not after it, once the numbers are already fixed and cannot be changed.
Not Planning for Growth-Related Tax Changes
As a business grows, its tax position often changes, whether through crossing the VAT threshold, taking on staff and becoming responsible for PAYE, or simply earning enough that different reliefs and allowances become relevant. Founders who are not looking ahead can be caught out by these changes. An accountant keeps an eye on your trajectory and flags upcoming changes in good time, so nothing arrives unexpectedly at the worst possible moment.
These transitions rarely happen overnight, which is precisely why they are so easy to miss entirely if nobody is actively watching for them; a growing business can drift gradually toward a threshold over several months without the founder ever consciously noticing the approach at all.
- Setting aside tax consistently and proactively rather than reactively
- Registering for the right taxes at exactly the right time
- Claiming expenses accurately, neither too little nor too much
- Reviewing your salary and dividend approach regularly each year
The Bottom Line
Most tax mistakes made by new founders share a common root: a lack of ongoing attention rather than any deliberate error. Affordable accountant support addresses this directly, keeping tax matters on track throughout the year rather than leaving them to build up into a stressful once-a-year event. For London founders balancing every other demand of a new business, this ongoing support is often what turns tax from a genuine source of anxiety into a manageable, genuinely predictable part of running the business day to day, freeing up mental space for the things that actually grow it.
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