Meerwaardebelasting: the opt-out choice, the 31 August ceiling, and the bank cut-offs that came first

You have a choice about how your 2026 investment gains are taxed. The legal deadline is 31 August. Your bank may have closed the door in May, and it was allowed to.

Since 1 January 2026, Belgium taxes realised capital gains on financial assets at 10%. Realised means sold: paper gains on a position you still hold are not affected. Gains you built up before 2026 are not affected either — the value of your portfolio on 31 December 2025 is the starting line.

From 1 June 2026, banks and brokers withhold that 10% themselves. That is where the choice comes in.

The choice in one paragraph

Either your bank takes the tax off at source and the matter is closed — nothing about those gains appears in your personal tax return — or you tell the bank not to, and the gains go into your return instead, where you work out and declare the position yourself.

Doing nothing is a decision: it leaves you on the first route, with the bank withholding. To end up on the second one, you have to say so, and you have to say so in time.

Two things about that choice surprise people:

• It covers all of 2026, not just the months after you communicated it. And it stays in force for the years after, until you change it.
• Communicating it late does not fix 2026. A late instruction only takes effect from the next income year. There is no catching up.

The date on the news is probably not your date

The law says the choice has to reach your bank by 31 August 2026 at the latest. A Royal Decree from May then allowed every institution to set its own earlier cut-off, and several did — some as early as spring, several in June. If you walk in this week quoting 31 August, you may be told the window closed months ago.

The Council of State has said the decree should not have been able to do that, and that you ought to be able to make the choice known right up to 31 August. That view did not change the decree. It means the situation is genuinely contested, not that your bank will agree with you at the counter.

Practically:

• Look up your own institution's published cut-off before anything else. That is the date that decides what happens this week.
• If you are past it but still before 31 August, put the instruction in writing anyway, dated, through a channel that leaves a trace. If this ever ends up being argued, having asked in time and been refused is a very different position from never having asked.
• Do not spend the week waiting for a phone call to resolve it. Written request, keep the answer.

If your broker is not Belgian

Then none of this deadline applies to you, because there is no Belgian institution to instruct. Nobody withholds anything on your behalf, and the gains go into your return regardless.

That is less pressure this week and more work later. What you need is the same underlying material: what you sold in 2026, at what price, and what those positions were worth on 31 December 2025. Ask your bookkeeper what format they want it in, and get it before year end rather than in the spring.

The one item that catches people out

If an insurance product paid out to you between 1 June and 31 August 2026, the default is reversed. For those payouts you are treated as having chosen the return route unless you explicitly asked for tax to be withheld. So a summer payout you have not thought about since may have left you with something to declare.

If that describes you, say so to your bookkeeper this week rather than in April. It is not a large job; it is only a large job when it is discovered late.

The document worth chasing today

The value of every position you held on 31 December 2025. That figure is what your gain is measured from, and it is what protects everything you built up before 2026. Ask each bank and broker for a written valuation statement at that date, per position, and file it somewhere you will find it in three years.

Clients ask for this document at the point where the tax authority asks a question about the return — which is exactly the moment it is hardest to obtain and least persuasive when reconstructed afterwards. It costs an email now.

What to ask your bookkeeper for

Four questions, and they are all answerable in one sitting:

1. What did I actually realise in 2026 so far, per account, and how much of it falls before 1 June?
2. What was my institution's own cut-off, and did I already miss it?
3. Given my situation, is there a reason to prefer the return route over the bank settling it — or is having it finished at source the better outcome?
4. Do we have the 31 December 2025 valuations in the file, or do I need to request them?

The third question is the only one with judgement in it. Having the bank withhold is the low-effort route: it is final, and nothing about those gains reaches your return. Putting the gains in your return means you keep the computation in your own hands, which matters when your position across the year is more complicated than a single account at a single institution — because a bank only ever sees the positions it holds for you. Which of those is better is a question about your file, not a general rule, and it is the one thing here worth ten minutes of professional advice.

If you do nothing at all

For a Belgian account, the bank settles it at 10%, that settlement is final, and 2026 is closed on the first route. For most people with one account and a straightforward year, that is not a bad outcome — it is simply an outcome you chose by not choosing.

Where doing nothing does cost you is on the two edges: a summer insurance payout that quietly left you on the return route, and a foreign broker where nobody is withholding anything at all. Both of those turn into a declaration, and a declaration is much cheaper to prepare in September than to repair a year later.

A note on the second regime you may hear about

There is a separate set of rules for someone holding 20% or more of a company directly, with its own threshold and its own rate band. It has a longer runway — its valuation window runs to the end of 2027 — and it is a different conversation. If it applies to you, book it as its own meeting in the autumn. Do not let it crowd out this week, which is about a much simpler question: what has your bank been told, and when did it stop listening.