Comparison · Savings & Investing · 2026
ISA Platform Fees: Flat Fee vs Percentage Fee 2026: Which Costs You Less?
Stocks and Shares ISA platforms typically charge either a flat fee or a percentage of your portfolio. Which is cheaper depends entirely on how large your ISA is -- and how large it is likely to become. This guide compares both for 2026/27.
TL;DR -- 30-Second Summary
- • Flat fee: fixed cost regardless of portfolio size -- better value as your ISA grows
- • Percentage fee: scales with your balance -- often cheaper for smaller, newer ISAs
- • There is always a crossover portfolio value above which the flat fee wins
- • Dealing charges often pair inversely with account fees -- check both, not just one
- • The £20,000 annual allowance caps new money in, not how large your total ISA can grow over years
Side-by-Side Comparison
| Feature | Flat-fee platform | Percentage-fee platform |
|---|---|---|
| How the fee is charged | Fixed £ amount per month/year | % of total portfolio value per year |
| Cost as portfolio grows | Stays the same | Rises in line with balance |
| Best for small portfolios | Often relatively more expensive | Often cheaper |
| Best for large, long-held portfolios | Often cheaper | Often relatively more expensive |
| Typical dealing charge pattern | Fee per trade common | Free/reduced dealing more common |
Worked Example: Finding the Crossover Point
Compare a flat-fee platform charging £150 a year against a percentage-fee platform charging 0.35% a year, across different portfolio sizes.
| Portfolio value | Flat fee (£150/year) | Percentage fee (0.35%/year) |
|---|---|---|
| £10,000 | £150 | £35 |
| £42,857 (approx. crossover) | £150 | ~£150 |
| £100,000 | £150 | £350 |
| £250,000 | £150 | £875 |
Below roughly £42,857 in this example, the percentage fee costs less in pounds terms. Above it, the flat fee is cheaper -- and the gap widens the further the portfolio grows beyond the crossover point, since the flat fee never moves while the percentage fee keeps scaling with the balance.
When a Flat-Fee Platform Wins
A flat fee wins for anyone with a portfolio already above their personal crossover point, or who expects to accumulate a large ISA over years of contributions and compound growth. Long-term, buy-and-hold investors with infrequent trading needs benefit most, since the low dealing-charge downside of flat-fee platforms matters less if you rarely trade.
When a Percentage-Fee Platform Wins
A percentage fee wins for smaller or newly opened ISAs still below the crossover point, where the pounds cost of a percentage of a modest balance is genuinely lower than a fixed annual charge. It can also suit investors who value free or reduced dealing charges for frequent trading or rebalancing, even once the account fee itself becomes relatively more expensive as the balance grows.
Frequently Asked Questions
Frequently Asked Questions
What is the difference between a flat-fee and a percentage-fee ISA platform?
A flat-fee platform charges a fixed amount, either a set monthly or annual fee, regardless of how large your Stocks and Shares ISA portfolio grows. A percentage-fee platform charges a proportion of your total ISA value each year, commonly somewhere in a range depending on the provider, so the pounds-and-pence cost rises automatically as your portfolio grows, even though the percentage rate itself stays the same.
Why does portfolio size matter so much when choosing between the two?
Because a percentage fee scales directly with your balance while a flat fee does not, there is always a crossover point above which the flat fee becomes cheaper in pounds terms, and below which the percentage fee is cheaper. For a small, newly opened ISA, a percentage fee often costs less in absolute terms than a flat annual fee; as the portfolio compounds and grows over years, the same percentage fee can end up costing considerably more than a flat fee would have.
Do dealing/trading charges also differ between the two pricing models?
Often, yes -- providers commonly pair their account fee structure with a different approach to dealing charges. Many flat-fee platforms charge a modest fee per trade, which can add up if you trade or rebalance frequently, while some percentage-fee platforms offer free or reduced-cost trading, effectively building some of that cost into the ongoing percentage charge instead. Always check both the account fee and the per-trade dealing charge together, not just one in isolation.
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How do I estimate the crossover point for my own ISA?
Divide the flat annual fee by the percentage rate to estimate the portfolio value at which both options cost roughly the same amount per year. For example, a £150 flat annual fee against a 0.35% percentage fee gives a rough crossover around the point where 0.35% of the portfolio equals £150 -- above that portfolio value, the flat fee tends to be cheaper; below it, the percentage fee tends to be cheaper. Your own numbers will differ based on the specific fees quoted by the platforms you are comparing.
Are percentage fees ever capped?
Some providers cap the pounds amount a percentage fee can reach, which effectively turns the fee into a flat fee once your portfolio passes a certain size, removing the scaling disadvantage above that cap. Not all providers offer a cap, so check the specific terms of any percentage-fee platform you are considering rather than assuming a cap applies.
Does the type of investment held affect which fee structure is better?
It can. Investors holding a small number of large, buy-and-hold positions such as index funds may find a flat-fee, low-dealing-charge platform particularly cost-effective since they trade infrequently. Investors who prefer holding many smaller positions, or who rebalance frequently, may find a percentage-fee platform with free or cheap dealing more cost-effective, even at a higher headline percentage rate, because the dealing-charge savings offset it.
Should I switch platforms once my ISA passes the crossover point?
Not automatically -- switching involves a transfer process that can take time and, depending on the platforms and investments involved, may briefly move you out of the market or involve selling and rebuying. It is worth periodically reviewing whether your growing balance has passed the crossover point, but weigh any potential saving against the effort, any exit fees the current platform charges, and the risk of transfer delays before deciding to move.
Do flat-fee platforms ever charge more for larger, more complex accounts?
Most flat-fee platforms keep the same fee regardless of balance, which is the main appeal of the model, but some tier their flat fee by account type or add supplementary charges for services like paper statements, certain fund types, or additional accounts. Read the full fee schedule, not just the headline flat fee, before assuming the total cost is fixed.
Is a cheaper platform always the better choice?
Not necessarily -- fees are one factor among several, including the range of investments available, the quality of the platform's tools and customer service, and whether it supports the specific ISA type (Cash, Stocks and Shares, Lifetime or Innovative Finance) you want. A slightly more expensive platform that offers investments you actually want to hold, or a better experience for a large portfolio you plan to manage for decades, can be a reasonable choice even if it is not the absolute cheapest by fee alone.
Does the £20,000 annual ISA allowance affect which fee structure I should choose?
Not directly -- the £20,000 annual ISA allowance for 2026/27 limits how much new money you can add each tax year, but it does not limit how large your total accumulated ISA portfolio can grow over many years of contributions and investment growth. Because a large, long-held ISA portfolio is exactly the scenario where a percentage fee becomes most expensive relative to a flat fee, long-term ISA savers in particular should pay close attention to how their chosen platform's fee structure will behave once their portfolio has grown well beyond a single year's allowance.
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Disclaimer: This comparison is general information for 2026/27 using illustrative fee figures. It is not financial advice. Actual platform fees, dealing charges and caps vary by provider and change over time -- check current published fee schedules before choosing a platform.