New choice for investors
As from 1 January 2026, Belgium has introduced a capital gains tax on financial assets. This has not only created a new tax liability, but also a practical choice for investors: should the tax be withheld automatically by the Belgian bank or broker, or should the investor report the capital gains in their personal annual tax return instead?
In simple terms, this is the choice between opt-in and opt-out.
Opt-in: simplicity first
Under the opt-in system, the Belgian bank or broker withholds the 10% capital gains tax (CGT) at the moment the gain is realised. For many investors, this will feel like the most straightforward option. The bank calculates, withholds and transfers the tax, meaning that the taxpayer does not need to report the transaction separately in the tax return.
This reduces the administrative burden and limits the risk of mistakes. It may also offer a certain level of discretion, since the tax is transferred without the taxpayer having to report the capital gain in detail in the tax filing afterwards.
However, this simplicity comes at a cost. The withholding is made immediately, without necessarily taking into account any exemptions or capital losses. In practice, this may lead to pre-financing: tax is withheld first, and any correction or refund must then be claimed later through the personal tax return. This may take up to two years, depending on the timing of the tax filing process and the issuance of the tax assessment.
Opt-in is also not always available. Foreign brokers, crypto assets, physical gold, currency transactions and certain specific structures fall outside the withholding system. For more active investors, or investors with several accounts, the automatic withholding may therefore not reflect the full picture.
Opt-out: more control, more work
Under the opt-out system, the taxpayer reports the realised capital gains in the personal income tax return. This requires more active follow-up, but it also gives the investor more control.
The main advantage is that capital gains and capital losses can be offset immediately. Exemptions can also be applied directly, without any prior withholding. This can create a clear cashflow benefit, since the tax is only paid after receiving the tax assessment. Opt-out may therefore be more appropriate for investors with several accounts, foreign brokers or more complex portfolios.
The downside is that the taxpayer must take responsibility for the calculation, documentation and reporting. This means keeping proper records, applying the correct valuation method and ensuring that the personal tax return is completed correctly. In many cases, this will require additional reporting assistance.
There is also less discretion. The tax authorities will receive a nominative overview of the declared capital gains. The taxpayer must also ensure that sufficient liquidity is available when the tax assessment arrives.
Practical reality
In practice, the choice will not only depend on tax considerations. Much will depend on what Belgian banks and brokers are technically able to offer in the months to come.
Some financial institutions may apply withholding retroactively for the period between 1 January 2026 and 1 June 2026 (when the opt-in was not yet in place due to the absence of any legal basis). Others may not. Certain banks may only provide full reporting of capital gains and losses as from 2027, meaning that for income year 2026 investors may still need to rely on bank account statements and their own calculations.
Where no retroactive withholding or full reporting is available, the opt-in system may lose much of its practical value. In such cases, investors may effectively be pushed towards an opt-out approach.
No clear-cut answer
At first sight, opt-in looks simple and safe. For many investors, it may indeed be the most comfortable starting point. But this will not always be the best solution.
Opt-out offers more flexibility and may be more efficient for investors with losses, exemptions, several accounts and/or foreign brokers. But it also requires proper administration and careful tax reporting.
The best choice will therefore depend on the investor’s portfolio, the financial institutions involved and the quality of the available reporting. In transition year 2026 especially, this should not be treated as a purely administrative ‘box-ticking’ exercise. Each situation will require its own assessment.
For that reason, a growing number of software developers and fintech providers are now investing heavily in specialised CGT compliance tools. The expectation is that dedicated CGT accountancy software will become increasingly important in the coming years, especially for internationally diversified investors and portfolios spread across multiple financial institutions.