Why DIY Tax Planning Could Be Costing You Thousands

With more financial information available online than ever before, it’s no surprise that many investors attempt to manage their own tax planning. A quick search can produce guides on everything from pensions to ISAs to capital gains. So, at first glance, it can seem perfectly manageable.

But the reality is that tax planning isn’t always straightforward. Small inefficiencies, a missed allowance here, an untimely sale there, can quietly add up to thousands of pounds over time. And the higher your income or the more complex your finances become, the greater the potential cost of tax planning mistakes.

So ask yourself honestly: are you confident your strategy is as tax-efficient as it could be?

What DIY tax planning usually looks like

For many investors, DIY tax planning tends to involve a few familiar behaviours:

• Managing investments independently through online platforms
• Filing tax returns without professional advice
• Relying on blogs, forums or general financial articles
• Making pension or ISA decisions without a long-term strategy

None of these things are inherently wrong. In fact, for people with very simple finances, they may work reasonably well. But DIY approaches often focus on individual decisions rather than the bigger financial picture.

“Most people think about tax only when they have to,” says one First Wealth financial planner. “Effective tax planning is about thinking ahead and structuring your finances so you’re paying only what you need to – not what you accidentally end up paying.”

That difference is where many opportunities are missed. Have you ever looked at your investments and asked whether they’re arranged in the most tax-efficient way?

Why tax planning is more complex than it appears

The UK tax system is famously complicated. Investments can be affected by several different taxes at once, including:

• income tax
• capital gains tax
• dividend tax
• inheritance tax

On top of that, allowances and thresholds change regularly. For example:

• The personal allowance is currently £12,570, but can be amended by the Government at each budget
• The capital gains tax annual exemption has fallen to £3,000
• The pension annual allowance is £60,000 (for most people)

What’s vital to remember is that each of these changing parts also interacts with the others. Income can affect your tax band, which can then affect the tax on your investments. Pension contributions can reduce taxable income. Capital gains decisions can influence future tax exposure. For tax planning for high earners, these interactions become even more significant. So, without a clear strategy, it’s easy to miss opportunities or make costly tax planning mistakes.

Common DIY tax planning mistakes

Not using tax wrappers effectively

One of the most common tax planning mistakes is failing to use tax wrappers fully. For example:

• leaving investments outside an ISA
• underusing pension allowances
• holding dividend-producing assets in taxable accounts

ISAs allow investments to grow completely tax-free, with an annual allowance of £20,000. Pensions offer tax relief on contributions and tax-deferred growth. Yet many investors leave substantial assets outside these wrappers even though, over time, taxes on income, dividends and gains can significantly reduce returns.

Missing pension tax relief opportunities

Pensions are one of the most powerful tools available for tax planning. Contributions typically receive tax relief at your highest marginal rate. They can also reduce your taxable income. For tax planning for high earners, this can be particularly valuable. In some cases, pension contributions can:

• restore lost personal allowance
• reduce higher-rate tax
• help avoid the additional rate band

Yet many DIY investors contribute sporadically or only towards the end of the tax year, missing the strategic benefits of long-term planning.

Poor capital gains planning

Capital gains tax planning is another area where DIY investors often lose money. Common issues include:

• selling assets without using the £3,000 annual CGT allowance
• failing to transfer assets between spouses
• selling multiple investments in the same tax year

Spouses and civil partners can transfer assets tax-free, potentially doubling available allowances. Without careful tax planning, investors may pay capital gains tax unnecessarily.

Ignoring the 60% tax trap

One of the most surprising tax pitfalls in the UK affects people earning just over £100,000.

Once income exceeds £100,000, the personal allowance begins to taper away. For every £2 of income above this level, £1 of allowance is lost. This creates an effective 60% tax rate on income between £100,000 and £125,140. Many individuals fall into this trap without realising it. However, strategic pension contributions can often reduce taxable income and restore the allowance.

For those focused on tax planning for high earners, this is one of the most valuable planning opportunities.

Focusing only on investments rather than tax efficiency

DIY investors often focus heavily on investment performance: choosing funds, tracking markets or trying to improve returns. But the structure of your portfolio can be just as important. Two investors holding identical investments could end up with very different after-tax outcomes.

That’s why tax efficient investing plays such a key role in long-term financial planning. After all, returns only matter after tax.

How much poor tax planning can cost

Consider a simplified example.

A professional earning £120,000 per year does not make pension contributions beyond their workplace scheme.

As a result, they:

• lose part of their personal allowance
• pay higher-rate tax on more income
• miss additional pension tax relief

A £20,000 pension contribution could potentially reduce their taxable income and restore their personal allowance, saving several thousand pounds in tax in a single year. Now imagine similar inefficiencies repeated over a decade. This is why even small tax planning mistakes can have large long-term consequences.

Why tax planning should be part of financial planning

Good tax planning isn’t just about reducing this year’s tax bill. It’s about aligning tax decisions with your wider financial strategy.

That includes:

• retirement income planning
• tax efficient investing strategies
• estate and inheritance planning
• long-term wealth goals

“Tax planning works best when it’s integrated with the rest of your financial plan,” explains one adviser. “When everything is connected, you can often create much greater long-term efficiency.”

When DIY tax planning may still work

DIY tax planning may work for individuals with:

• relatively simple finances
• lower income levels
• limited investments
• straightforward tax positions

But complexity tends to increase over time. Higher income, multiple investments, pensions, business ownership and approaching retirement can all create additional layers of tax exposure. At that point, DIY approaches may start to leave money on the table.

Have you reviewed whether your tax planning strategy still fits your financial situation?

How professional financial planning can help

Professional advisers often look at finances in a more integrated way.

This may include:

• structuring portfolios for tax efficient investing
• managing pension contributions strategically
• planning capital gains across multiple tax years
• identifying opportunities for tax planning for high earners

Importantly, advisers also stay up to date with changing tax rules, allowances and legislation.

That insight can help identify opportunities that DIY approaches might miss. Missed allowances, poorly structured investments and inefficient pension contributions can all add up. Reviewing your strategy regularly and seeking professional guidance where needed can help ensure you’re not paying more tax than necessary.

If you’d like to explore whether your current approach to tax planning, tax efficient investing, or tax planning for high earners could be improved, our advisers would be happy to help. Get in touch today to arrange a conversation and discover whether your financial plan could be working harder for you.

The Financial Conduct Authority does not regulate tax planning.

This article does not constitute tax, legal or financial advice and should not be relied upon as such. Tax treatment depends on the individual circumstances of each client and may be subject to change in the future.

This document is marketing material for a retail audience and does not constitute advice or recommendations. Past performance is not a guide to future performance and may not be repeated. The value of investments and the income from them may go down as well as up and investors may not get back the amount originally invested.