The Hidden Cost of Holding Too Much Cash in Your ISA or SIPP
Providers vary wildly in how they treat cash held within investing portfolios, making it crucial for investors to understand their specific platform's policies. With markets experiencing swings between rallies and setbacks, and savings rates remaining relatively attractive, many individuals are retaining more cash than usual. However, this discussion focuses not on high-street savings accounts, but on the uninvested cash sitting inside ISAs, personal pensions (SIPPs), or general investment accounts.
This cash may accumulate from selling funds, receiving dividends, or waiting for a perceived "better time" to invest. In most DIY investment accounts, the amount of cash held is entirely at the investor's discretion. Yet, holding excessive cash for prolonged periods can quietly diminish the potential growth of your money.
When Cash Makes Sense in Financial Planning
Cash plays a clear role in any robust financial plan. James Norton, head of retirement and investments at Vanguard, advises, "Keep enough cash for emergencies—three to six months is recommended. If you have a short-term goal, such as buying a house where absolute certainty is needed, cash makes sense." Importantly, this emergency cash should not reside in your investment account, as easy access is key.
Timescale is also critical. Matt Lewis, a chartered financial planner at EQ Investors, adds, "Where money is likely to be used within the next three years, or forms part of a rainy-day reserve, holding cash in a competitive interest-bearing account is sensible." Beyond short-term needs, cash within a long-term investment portfolio should generally be modest. Lewis notes, "Provided funds are invested with a medium or long-term objective, cash held within an investment portfolio can reasonably be low, often around the two per cent mark."
Instead, he suggests maintaining an appropriate personal cash reserve outside your investment account to provide the "sleep-at-night factor," allowing the remainder of your funds to stay invested and focused on achieving longer-term growth objectives.
The Cost of Sitting on Cash for Too Long
While holding some cash for flexibility is prudent, retaining large amounts over years can be costly. Norton explains, "Cash saving has historically generated lower returns than investing and often struggles to keep pace with inflation. If you invested £10,000 in global stocks 20 years ago, you would have a portfolio worth £82,500 today, accounting for inflation. In contrast, leaving that £10,000 in cash would have seen its value eroded to just £4,100 due to inflation."
Lewis warns that the impact becomes noticeable once cash exceeds what is genuinely needed for security. "Cash begins to create a drag on returns once it exceeds an investor's required safety buffer," he says, emphasising that leaving money uninvested for extended periods can mean missing out on potential growth compared to staying fully invested.
The Risks of Trying to Time the Market
With markets periodically reaching record highs, hesitation is natural. Norton observes, "You may feel uneasy investing when markets are rallying or at all-time highs. But history shows markets usually go on to reach new highs." Attempting to jump in and out of cash based on headlines rarely proves effective. He advises, "Stay invested over the long term and don't try to time when to move in and out of cash."
For goals more than five years away, Lewis states that investors are typically better off allowing their investments to grow over time, rather than holding large cash balances that can limit overall returns.
Not All Platforms Treat Cash Equally
A key wrinkle many investors overlook is how their platform handles uninvested cash. Some investment platforms, including AJ Bell, Freetrade, Hargreaves Lansdown, Lightyear, Trading 212, Vanguard, and XTB, pay interest on cash balances held within ISAs and SIPPs. However, policies vary significantly.
Some providers pass on most of the interest they receive from partner banks, while others retain a portion. Rates may be tiered, capped, or require investors to activate a specific feature. Conversely, some platforms pay little or no interest on default cash balances. Lewis recommends, "First confirm whether your platform pays interest on uninvested cash, and at what rate and under what conditions. Charges and taxation can materially reduce the effective return received." On five-figure balances, these differences can amount to hundreds of pounds annually.
Tax Implications of Cash Holdings
Interest earned within ISAs and SIPPs is tax-free, providing a significant advantage. However, cash held in a taxable general investment account counts towards your personal savings allowance—£1,000 for basic-rate taxpayers and £500 for higher-rate taxpayers—above which interest becomes taxable. Lewis notes, "Interest earned on cash is taxed as income," and it contributes to the total interest earned across all savings accounts.
Determining the "right" amount of cash is specific to individual circumstances and goals. Long-term investors with stable income may only need a small working balance, while those closer to retirement or planning to draw money soon may sensibly hold more. Cash can steady a portfolio, but left unchecked, it can also slow it down. The key is ensuring cash is held for a clear purpose, not merely because markets feel uncomfortable.
When investing, your capital is at risk and you may get back less than invested. Past performance does not guarantee future results.
