Capital gains tax: opt-in or opt-out?

The new capital gains tax on financial assets has been in effect retroactively since January 1, 2026. Every investor faces an important choice: do I opt for automatic withholding through the bank (opt-in), or do I declare capital gains myself through my tax return (opt-out)?

In this article, we explain the difference and implications.

What is the capital gains tax?

From Jan. 1, 2026, capital gains on a wide range of financial products will be taxed. Specifically, the positive difference between the selling price and the acquisition value of assets such as stocks, bonds, funds, trackers, crypto-assets, physical gold, investment insurance and foreign exchange, regardless of whether they are apprehended in Belgium or abroad.

The tax applies to natural persons those at the personal income tax are subject and invest outside their professional activity but within normal management. The first tranche of 10,000 euros of capital gains per year is exempt. Above that amount, you pay 10% tax on the surplus.

Also read: Agreement on capital gains tax: an overview of the new ground rules

Two systems: opt-in or opt-out

The legislature provides two ways to pay capital gains tax. The default system is opt-in: your bank automatically deducts the tax from every transaction. Those who do not wish that, actively choose opt-out and handles the return itself through personal income tax.

When is it best to choose which system?

There is no universally right answer. The best choice depends on your personal situation: how actively do you invest, how many portfolios do you have, and to what extent are you willing to take on the administration yourself?

What is the timeline?

• Jan. 1, 2026: Capital gains tax effective date
  The tax applies to all capital gains arising from this date. Snap start values as of 31/12/2025 is critical.
• January - May 2026: Transitional measure: no automatic withholding yet
  During this period, there is no automatic deduction yet. Banks will contact you to register your choice.
• May 31, 2026: Opt-in/opt-out choice deadline
  Most banks ask you to submit your choice by the end of May. However, some institutions give you longer.
• As of June 2026: Final regulation opt-in active
  With opt-in, the bank will now automatically deduct capital gains tax from every transaction and you will receive an annual statement. The exact effective date varies from bank to bank and is of course related to the deadline for choosing opt-in/opt-out.
• AJ 2027: Personal income tax return
  Those who chose opt-out declare capital gains spontaneously. Those who opted-in can reclaim the 10,000-euro exemption through the tax return.

Important points of interest

The choice applies per account, not per person

An essential but often overlooked detail: the choice of opt-in or opt-out applies per account, not per account holder. Moreover, in the case of a joint account (e.g., two spouses), opt-out can only be chosen if all account holders agree.

The choice is also annual and cannot be changed during the current year.

Keeping history is always crucial

Even those who choose to opt-in do not escape the obligation to provide the acquisition history of its financial products. At a transfer of financial products to another bank or in the case of a donation, the history is lost: the new bank takes 0.00 euros as the acquisition value, which can later lead to a greatly increased taxable capital gain.

So always provide a accurate and organized documentation of your purchases and associated values, even if your bank handles the deduction automatically.

Not all assets are in the opt-in system

Certain assets are excluded from automatic deduction through the bank even if you choose to opt-in. Consider physical gold, crypto-assets and shares of the proprietary company. For these assets, you will always have to maintain and declare the necessary information yourself.

Foreign securities portfolios

Do you hold securities with a foreign bank or intermediary? If so, it pays to check Whether this institution supports an opt-in system or at least offers correct reporting.

If not, it is best to weigh the return on investment against the administrative cost Of keeping track of all transactions yourself.

What can/should you already do today?

• Enter your choice (opt-in or opt-out) in a timely manner to your bank. The exact deadline varies from institution to institution, but most banks ask for no later than May 31, 2026 pass on your choice.
• Document the value of your financial products as of Dec. 31, 2025. This is the reference value for future capital gains.
• Keep the complete acquisition history of your portfolio, even when opting in.
• Have you crypto, physical gold or shares in a proprietary company? Ensure proper valuation and documentation.
• Analyze whether holding foreign financial products at an institution without proper reporting is still desirable.

Conclusion

The choice between opt-in and opt-out is not a simple decision and depends on your personal investor profile, the number of transactions, the complexity of your portfolio and your willingness to take on administration. Opt-in offers peace of mind and automatism; opt-out gives you more control but requires rigorous personal follow-up.

Whichever system you choose: a Correct and complete documentation of all your financial products, their acquisition values and transaction histories is necessary. Those who organize this today will avoid unpleasant tax surprises tomorrow.