Flat-fee vs percentage-fee investment platforms

Why the fee model changes as the account grows

Flat-fee and percentage-fee platforms are not rival slogans; they are different ways to charge for custody and account access. A percentage fee usually feels fair when the balance is small because the pound cost starts low. A fixed monthly subscription can feel inefficient at that same point because the investor pays the same amount even before the account has much value. As the account grows, the maths can reverse. A percentage fee keeps rising with assets, while a flat or capped fee can become easier to justify.

Investornet treats fee choice as an account-design problem rather than a race to one winner. The right answer depends on wrapper, balance, holding type and trading behaviour. Our editorial boundaries are set out in About Investornet and the evidence rules are described in the Editorial Policy. This guide uses public fee pages such as Interactive Investor charges, Vanguard Investor fees explained, Hargreaves Lansdown charges and interest rates, AJ Bell charges and rates, and the FCA Consumer Investments Strategy as source anchors, but final numbers should be checked directly before acting.

Percentage fees: where they usually work

A percentage platform fee can suit an investor who is building slowly, especially in an ISA or general investment account where the balance is still modest. The attraction is proportionality. Paying a percentage of a small account often feels more reasonable than paying a fixed monthly amount that eats into the first few years of contributions. For a reader who is still learning, investing monthly and holding a small number of funds, this can be a sensible way to keep platform cost aligned with account size.

The weakness appears later. If the account grows and the fee is not capped, the annual pound cost can become material even though the percentage looks small. A reader with a larger ISA, a consolidated SIPP or several wrappers on the same platform should not assume the original fee model remains appropriate. The percentage model needs periodic review, especially after transfers, employer pension moves, inheritance contributions or a long period of investment growth.

Flat and capped fees: where they usually work

A flat-fee or capped-fee model deserves attention once the account balance is large enough for predictable pound cost to matter. Interactive Investor is the common example because it uses a subscription-style approach, while some other providers cap selected charges depending on wrapper or holding type. The appeal is clarity: the investor knows the platform charge will not rise in a straight line with the portfolio.

The weakness is timing. A fixed fee that is efficient on a large account can be clumsy on a small one. It can also be undermined by dealing costs, FX, add-on account charges or the wrong wrapper. A reader should never switch to a fixed-fee platform simply because fixed sounds disciplined. The fixed cost has to be modelled alongside how often trades happen, whether included trades are useful, and whether the platform supports the investments and paperwork the account requires.

The balance bands that usually matter

Account stage	Percentage-fee question	Flat-fee question
Small starter ISA	Is the pound cost low enough while habits form?	Does the monthly fee take too much of each contribution?
Growing ISA or GIA	When does the percentage charge overtake a fixed alternative?	Are dealing costs and investment range still suitable?
Larger SIPP	Does the platform cap charges for the holdings used?	Does the fixed plan include pension administration needs?
Consolidated family accounts	Does each account compound the same percentage fee?	Can one subscription or plan support multiple accounts cleanly?

The table is deliberately qualitative. Published prices change, and the right breakpoint depends on account value and behaviour. Build a simple spreadsheet with today's balance, planned annual contributions, expected number of trades, fund versus ETF holdings, foreign-exchange use and any transfer cost. Then run the same comparison again at a larger future balance. If the winner changes, the account needs a review trigger rather than a one-time decision.

Wrapper differences can overturn the result

ISA, SIPP and general investment accounts do not always behave the same way on fee pages. A platform can be attractive for an ISA but less compelling for a SIPP once administration, drawdown, transfer paperwork or pension-specific charges enter the decision. A general investment account can add tax-document and dividend-reporting considerations that matter less inside an ISA. For wrapper-first decisions, use ISA vs SIPP platform choice before comparing fee models.

SIPP investors should be especially careful with fixed-fee comparisons. A low platform charge is not enough if the account lacks the pension features needed later. Beneficiary details, retirement options, transfer acceptance and administration timelines can matter more than a narrow fee difference. ISA investors may have a simpler decision, but they still need to consider transfer times, cash treatment, fund availability and how the platform handles tax-year activity.

Dealing fees and FX can beat the custody fee

Custody cost is only one part of the platform bill. A fund investor buying monthly through regular-investment tools may pay very little in dealing costs. A share or ETF investor making frequent trades can create a different cost profile. Foreign exchange is another swing factor: a platform that looks cheap for UK funds may become less attractive for overseas shares if FX terms are expensive or unclear.

Behaviour matters as much as published pricing. If a fixed-fee platform includes trades the reader does not need, the inclusion has limited value. If a percentage-fee platform charges separately for every intended trade, the apparent simplicity fades. The correct comparison is the account as it will be used, not the most flattering scenario in a table. Our investment platform shortlist uses the same principle when comparing brand fit.

When switching for fees is worth the friction

A fee-led transfer can be sensible when the old platform is clearly too expensive for the current balance and the new platform supports the same holdings, wrapper and service needs. It is less sensible when the saving is small, the transfer might force sales, or the receiving platform is unfamiliar in ways that could add future friction. Transfer incentives should be treated as a separate line in the model, not as proof that the new fee model is better.

Before moving, check whether the transfer will be cash or in-specie, whether any holdings are excluded, whether the old or new provider describes expected timelines, and whether the receiving platform imposes a qualifying period for any offer. The platform transfer offers guide covers promotion terms in more detail. A reader comparing specific providers should also read Interactive Investor review for fixed-fee context and AJ Bell review for a mainstream percentage-fee alternative.

Practical decision rule

Choose a percentage-fee platform when the account is still small enough that proportional pricing keeps annual cost modest and the platform has the wrapper, investments and support needed. Choose a flat or capped platform when the balance makes percentage pricing expensive in cash terms and the fixed package still supports the account's real behaviour. Recheck the decision after large contributions, transfers, pension consolidation, a move into overseas holdings or a change in trading frequency.

The strongest answer is rarely permanent. A platform that is efficient for a beginner ISA can become expensive for a mature portfolio. A fixed-fee account that looked unnecessary at the start can become logical after consolidation. Good platform management means setting review points before the costs drift, not waiting until a promotion or complaint forces a hurried move.

A worked way to compare without guessing

A practical comparison can be built with four rows and three balance columns. The rows are custody or platform charge, dealing and regular-investment charges, FX or overseas dealing costs, and wrapper-specific administration charges. The columns are today's balance, the likely balance after three years, and a mature balance if contributions continue. This does not require precision about market returns; it requires honesty about contribution size, trading behaviour and whether the account may receive a transfer.

Run the model for the actual holdings rather than for an idealised portfolio. A fund-only ISA, a share-heavy general account and a SIPP holding investment trusts can all produce different answers at the same balance. If a platform caps shares but not funds, or treats regular investments differently from ad hoc trades, the account composition matters. The same published fee table can create several outcomes.

The output should be a review trigger. If the percentage-fee platform is cheaper today but fixed fees win at a later balance, write down the approximate balance where the decision should be revisited. If fixed fees win only when the reader trades more often than planned, ignore that scenario. The goal is not to prove one model superior; it is to avoid staying with a fee model after the account has outgrown it.

FAQ

Is a flat-fee platform always better for large portfolios?

Not always. It often deserves attention, but dealing costs, wrapper support, investment range and service quality still matter. A fixed charge is only useful if the account can be run properly on that platform.

Should a beginner avoid fixed fees?

A beginner should model the annual pound cost against expected contributions. A small monthly subscription can be heavy on a low balance, but there may be cases where wrapper or investment access justifies it.

How often should fees be reviewed?

Review after major balance changes, transfers, new wrappers, pension consolidation or a shift from funds to shares and ETFs. Annual review is a sensible minimum for long-term accounts.