How much do platform charges cost an ISA over time?
Platform charges come in two shapes: a percentage of the balance, or a flat annual fee. They cost the same at one particular balance and diverge sharply either side of it, so the right platform for a £15,000 ISA is frequently the wrong one for a £250,000 ISA — for identical service.
- Two shapes: a percentage of assets, or a fixed annual fee.
- The crossover: the balance at which the fixed fee equals the percentage fee.
- The other layer: fund charges, which sit on top of whatever the platform takes.
- The friction: transferring in specie avoids being out of the market during the move.
01 The two charging shapes
A percentage platform charge scales with the balance: 0.25% costs £50 on £20,000 and £625 on £250,000, for the same account and the same service. A flat fee charges the same amount whatever the balance.
The crossover is a simple division. A £120 annual flat fee equals a 0.25% percentage charge at £48,000 — below that the percentage is cheaper, above it the flat fee is.
Some platforms cap the percentage above a threshold, which produces a hybrid. The relevant comparison is always the total pounds charged at your actual balance rather than the headline rate.
Shows: the balance at which a flat platform fee becomes cheaper than a percentage charge, and what the difference costs you now. Ignores: fund charges, dealing costs, foreign exchange spreads, and any cap on the percentage fee.
On the defaults above, the worked example shows £330 a year. The two charging shapes cost the same at a balance of about £48,000.
Source: FCA consumer information
02 Fund charges sit on top
The platform fee is for holding and administering the account. The funds inside it charge their own ongoing charges figure, and both come out of the same money. A 0.25% platform fee with a 0.65% fund charge is a 0.90% total, and it is the total that matters.
That is why comparing platforms on their headline fee alone is misleading. A platform with a low fee and a restricted range of expensive funds can cost more than a dearer platform giving access to cheap index funds.
Dealing charges and foreign exchange spreads apply to some platforms and not others, and they matter more for someone trading frequently than for a buy-and-hold investor.
03 Moving, and when it is worth it
An ISA transfer moves money already inside the wrapper and uses none of your annual allowance. It should be done through the provider's transfer process rather than by withdrawing and re-subscribing, which would consume allowance.
Ask for an in-specie transfer where the same funds are available at both ends, so the holdings move without being sold. A cash transfer takes you out of the market for several days, and over the long run that timing risk is real even though it averages out.
The threshold for bothering is the annual saving against the hassle. A £400 a year difference is worth an afternoon; a £30 difference is not, and switching frequently for small savings is its own cost.
Source: MoneyHelper: ISAs
Platform choice is not a permanent decision and most people treat it as one. A percentage fee is right for a small balance and wrong for a large one, and the crossover is a single division you can do in your head: flat fee divided by the percentage. At 0.25% and £120 a year that is £48,000, and above it you are paying hundreds a year more for the same screen. Add up the platform fee and the fund charges together, because the total is what comes out of your money, and ask for an in-specie transfer so you are not out of the market while it moves.
FAQ
Is a flat fee always better?
Only above the crossover balance, which is the flat fee divided by the percentage rate. At £120 a year against 0.25% that is £48,000 — below it the percentage charge is cheaper.
Do platform charges include fund charges?
No. The platform fee is for administering the account and the funds inside it charge their own ongoing charges figure on top. The total of the two is what actually comes out of your money.
Does transferring an ISA use my allowance?
No. Transfers move money already inside the wrapper and are unlimited. Use the provider's transfer process rather than withdrawing and re-subscribing, which would consume the annual allowance.
Sources
Regulator references
- FCA consumer information · Financial Conduct Authority · 2025The regulator's own consumer guidance on the products discussed here.Last verified: 2026-09-07
- Individual Savings Accounts (ISAs) · GOV.UK · 2025The annual subscription limit and the rules on transfers between ISAs.Last verified: 2026-09-07
- MoneyHelper: ISAs · MoneyHelper · 2025The account comparison behind the wrapper choice discussed here.Last verified: 2026-09-07
Calculator unit tests · the assertions this page's worked example is checked against, and their last result
Changelog
- 2026-09-07 — initial publish (new format)
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Jordan Reeves
Founder of Talk Through Wealth. A software engineer for over a decade before turning to retirement planning, Jordan built the projection engine after watching family members get fragmented, country-by-country advice that never reconciled. He writes about retirement the way the engine computes it: month-by-month, lifetime-long, and skeptical of any rule of thumb that hasn't been run through the math.