The ISA fee question looks simple: how much does the platform charge per year? But the answer changes entirely depending on how much money you have in the account, and most comparison tables never show you the crossover point.
How percentage fees scale — and when they hurt
A platform charging 0.25% per year feels cheap at £10,000 — that is £25. At £100,000, it is £250. At £500,000, it is £1,250 per year, every year, permanently. If your money compounds at 7% annually, you are handing away 3.6% of annual gains to the platform before you receive a penny.
The flat-fee case
AJ Bell Dodl charges £1 per month for a Stocks & Shares ISA — £12 per year regardless of balance. At £10,000, that is 0.12%. At £50,000, it is 0.024%. At £200,000, it is 0.006%. The crossover from percentage to flat-fee favourability generally occurs around £20,000–£30,000 on most platforms.
What comparison tables miss
Fund fees are separate from platform fees. A platform charging 0.25% can still give you access to ETFs at 0.07%. A 'free' platform offering only high-cost funds (0.75% OCF) may be more expensive in total. Always calculate platform fee plus fund TER as a combined figure. Never compare platform fees alone.