ISAs (Individual Savings Accounts) are one of the UK’s most powerful vehicles for tax-efficient saving and investing. With generous tax advantages and a wide range of account types, they can help you build wealth over time, provided they are used wisely.
However, many savers and investors make costly ISA mistakes that can reduce returns, trigger avoidable tax, or even lose the benefit of an annual allowance.
In this Moneyfarm blog we will take a closer look at the most common ISA investing mistakes and how you can avoid them.
1. Failing to Use Your ISA Allowance Before the Deadline
Every UK resident over 18 is entitled to an annual ISA allowance of £20,000 for the 2026/27 tax year — the same amount that has applied since 2017/18.
The allowance operates on a ‘use it or lose it’ basis: if you don’t use your full allowance by 5 April, you can’t roll it over into the next year.
This is set to change, however: under ISA reforms confirmed in the Autumn Budget 2025 ISA reform factsheet (GOV.UK), from 6 April 2027 the cash component of the allowance will be capped at £12,000 for savers under 65, while the £20,000 cash limit is retained for those aged 65 and over. The overall combined ISA limit stays at £20,000 either way.
Example: Lisa, 34, planned to invest £5,000 in her Stocks & Shares ISA but missed the deadline. That unused allowance was lost permanently, along with the opportunity to protect those savings from tax.
Tip: Set a reminder for mid-March and review your finances in advance to make additional contributions where possible.
2. Selecting the Wrong ISA Type for Your Goals
ISISAs serve different purposes, and choosing the wrong type may lead to missed opportunities or unnecessary risk.
| ISA type | Best suited for | Key features |
|---|---|---|
| Cash ISA | Short-term savings | Interest rates are often low and may not keep pace with inflation |
| Stocks and Shares ISA | Long-term investing | Potential for higher growth but subject to market fluctuations |
| Lifetime ISA | First-time home buyers and retirement savings | Contributions capped at £4,000/year, with a 25% government bonus; available ages 18-39, contributions permitted until 50 |
| Innovative Finance ISA | Peer-to-peer lending | Higher risk, including the possibility of capital loss |
The Lifetime ISA in particular may not stay in its current form for much longer: the government has proposed replacing it with a new First-Time Buyer ISA focused solely on property purchases, removing the pension-savings option and the early-withdrawal penalty entirely. A public consultation on the First-Time Buyer ISA (GOV.UK) closed in mid-August 2026. Existing Lifetime ISAs will continue to operate as normal while the details are finalised, and current LISA holders are not expected to be able to transfer their savings into the new product.
Example: Alex, 40, kept all his long-term savings in a Cash ISA earning 1.5% interest. Over 10 years, he missed out on the compounding growth that a diversified Stocks & Shares ISA might have delivered.
Tip: Match your ISA choice to your time horizon and risk tolerance. For short-term goals, a Cash ISA may be appropriate. For long-term growth, a diversified Stocks & Shares ISA is often more suitable.
3. Overlooking Fees and Charges
While ISAs themselves are tax-free, investment platforms and fund managers may charge fees, which can significantly reduce returns over time.
Charges to look out for include:
- Platform fees
- Fund management fees (Annual Management Charges, or AMC)
- Exit charges (on transfers or withdrawals)
Example: Claire opened a Stocks & Shares ISA with a major provider but did not realise she was paying 1.2% in ongoing charges. Over 10 years, those charges reduced her returns by more than £5,000 on a £30,000 investment.
Tip: Always review the fee structure before investing. Even small differences in charges can make a substantial impact over time.
4. Not Reviewing or Rebalancing Your Investments
Many ISA investors adopt a “set and forget” approach to their portfolios. However, markets can be turbulent, and investment strategies should be reviewed and adjusted accordingly.
Example: David, 55, invested heavily in technology stocks in 2021. Following the market correction in 2022, his ISA fell by 18%. Regular portfolio reviews could have helped him reduce risk and rebalance his holdings.
Tip: Review your ISA portfolio at least annually. Rebalance where necessary to keep your investments aligned with your risk profile and long-term objectives.
5. Withdrawing Funds Without Understanding the Rules
Not all ISAs allow you to withdraw funds and replace them without affecting your annual allowance. Only Flexible ISAs permit this feature, and many ISAs are not flexible.
Example: Priya withdrew £3,000 from her ISA for an emergency expense. A month later, she attempted to replace the funds, only to discover she had already used her annual allowance and was unable to do so.
Tip: If flexibility is a priority, confirm that your ISA is designated as “flexible” before making a withdrawal.
6. Transferring ISAs Incorrectly
You can transfer ISAs between providers to obtain better rates or features, but this must be done through a formal ISA transfer rather than by withdrawing the funds directly.
From 6 April 2027, the rules around transfers will also tighten as part of the ISA reform’s anti-circumvention measures: transfers from a Stocks & Shares or Innovative Finance ISA into a Cash ISA will no longer be permitted, though transfers in the other direction will remain possible (Source: GOV.UK). This restriction will not apply to savers aged 65 and over.
Example: Tom wanted to switch providers, so he withdrew £20,000 from his ISA and opened a new account. As he had not requested a formal transfer, the new deposit counted towards his annual allowance, preventing him from making further contributions that year.
Tip: Always initiate ISA transfers through your new provider, who will manage the process while preserving your tax-free status.
7. Keeping All Savings in Cash Long-Term
Cash ISAs provide security and easy access, but over long periods their returns may fail to keep pace with inflation. This means that while your balance grows, the real value of your money could decline.
Example: Emma, 38, kept £15,000 in a Cash ISA for over a decade. With interest averaging 1%, her savings grew slowly, while inflation rose by more than 2% each year. In real terms, her purchasing power fell.
Tip: Use Cash ISAs for short-term savings or emergency funds, but consider Stocks & Shares ISAs for long-term goals where growth potential is important.
8. Ignoring Inheritance Tax Treatment
Although ISAs are tax-efficient during your lifetime, they usually form part of your estate for Inheritance Tax (IHT) purposes. This means they may be taxable on death, which can come as a surprise to many investors. The main exception is the Additional Permitted Subscription (APS), which allows a surviving spouse or civil partner to inherit the ISA allowance.
As things stand, the standard Inheritance Tax nil-rate band is £325,000, rising to as much as £500,000 when the additional residence nil-rate band applies to a main home left to children or grandchildren (Source: GOV.UK). Estates above this threshold are generally taxed at 40% on the excess, and ISA holdings count towards that total.
Example: After Mark passed away, his £50,000 ISA was included in his estate and counted towards IHT. His wife, however, was able to use the Additional Permitted Subscription rules to continue sheltering the funds in her own ISA.
Tip: If estate planning is a priority, factor in how your ISA will be treated on death and consider professional financial advice to explore available allowances.
Practical example: preparing for the 2027 cash ISA reform
Suppose a saver under 65 currently puts their full £20,000 annual allowance into a Cash ISA. From 6 April 2027, only £12,000 of that allowance can go into cash — here is how their options change:
| Before 6 April 2027 | From 6 April 2027 (under 65) | |
|---|---|---|
| Maximum in a Cash ISA | £20,000 | £12,000 |
| Remaining allowance | £0 | £8,000 — must go into a Stocks & Shares or Innovative Finance ISA to stay tax-free |
| Transfers from Stocks & Shares ISA into Cash ISA | Allowed | No longer allowed |
| Total tax-free ISA allowance | £20,000 | £20,000 (unchanged) |
This is a simplified illustration based on the rules confirmed in the Autumn Budget 2025 and is not financial advice. Savers who rely on the full £20,000 cash allowance may want to start thinking, well ahead of April 2027, about how much of their new savings they are comfortable moving into stocks and shares.
Making the Most of Your ISA
Avoiding common mistakes can make a substantial difference to the long-term growth of your savings. Whether you are preparing for retirement, saving towards a first home, or aiming to make your money work more efficiently, using your ISA appropriately is essential.
Tax-efficient growth is a valuable benefit, but it only delivers its full potential when combined with fully informed decisions and a disciplined approach.
If you are uncertain whether your current ISA arrangements align with your objectives, consider seeking guidance from a regulated financial adviser who will help ensure that your strategy remains effective and well-suited to your circumstances.
Key Takeaways
- Use your ISA allowance before the 5 April deadline — it cannot be rolled over.
- Select an ISA type that suits your savings goal, whether short-term security or long-term growth.
- From 6 April 2027, only £12,000 of the £20,000 allowance can go into a Cash ISA if you’re under 65 — plan ahead for where the rest will go (Source: GOV.UK).
- The Lifetime ISA may be replaced by a new First-Time Buyer ISA; existing LISAs continue as normal for now.
- Be aware of all charges, even small percentage fees compound into large sums over time.
- Review your portfolio annually and rebalance to stay on track with your risk profile.
- Confirm whether your ISA is flexible before withdrawing funds.
- Always transfer ISAs through the new provider to avoid losing tax benefits.
- ISAs do not usually avoid Inheritance Tax, except through spousal Additional Permitted Subscriptions.
Frequently Asked Questions
What is the ISA allowance for 2026/27?
The overall ISA allowance for the 2026/27 tax year is £20,000, unchanged from previous years. From 6 April 2027, however, the cash portion of this allowance will be capped at £12,000 for savers under 65, following reforms confirmed in the Autumn Budget 2025.
Is the Lifetime ISA being scrapped?
Not immediately. The government has proposed replacing it with a new First-Time Buyer ISA focused on property purchases, and closed a public consultation on the plan in mid-August 2026. Existing Lifetime ISAs continue to operate under the current rules while the details are worked out.
Do ISAs form part of my estate for Inheritance Tax?
Yes. ISAs are generally included in your estate for Inheritance Tax purposes and are taxed at 40% above the standard nil-rate band of £325,000 (up to £500,000 with the residence nil-rate band), except where a surviving spouse or civil partner uses the Additional Permitted Subscription to inherit the ISA allowance.
Can I pay into more than one ISA of the same type in a tax year?
Yes. Since 6 April 2024, savers have been able to open and pay into multiple ISAs of the same type in a single tax year, as long as the combined contributions stay within the overall £20,000 allowance (Source: legislation.gov.uk).
What happens if I transfer my ISA incorrectly?
If you withdraw funds and deposit them into a new ISA yourself, rather than requesting a formal transfer through your new provider, the deposit counts as a new contribution against your annual allowance and you lose the continuity of your tax-free status.
*As with all investing, financial instruments involve inherent risks, including loss of capital, market fluctuations and liquidity risk. Past performance is no guarantee of future results. It is important to consider your risk tolerance and investment objectives before proceeding.
