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

The law of 6 April 2026 introduced a general capital gains tax on financial assets within the personal income tax and the legal entities tax. This capital gains tax is in force from 1 January 2026 and applies to capital gains realised on the transfer of financial assets for consideration. It concerns 3 types of transfer: internal capital gains (1), capital gains on a substantial interest (2) and residual category (3).  

A general rate of 10% applies to all capital gains with the exception of internal capital gains and capital gains relating to a substantial interest. For transactions that go through a Belgian intermediary, the law provides 2 possibilities: an automatic deduction of the tax via the financial institution at the time of realisation (opt-in) or declaring the capital gain yourself in the personal income tax return (opt-out).  


 

Opt-in: automatic deduction at source  

If the opt-in system is applied, the capital gains tax will be automatically withheld by the financial institution and transferred to the tax authorities. The payment of the withholding tax is done on an anonymous basis. In this case, the financial institution is not forced to communicate the identity of the account holders and the amount of the capital gains realised to the tax authorities. The withholding tax is final, which means that in general the capital gain does not have to be declared in the personal income tax return.  

However, the financial institution will not take into account the annual exemption of €10,000 (which may be increased to €15,000 under certain conditions) of capital gains. If the taxpayer wishes to apply this, the capital gain will still have to be reported in the personal income tax return and the overpaid tax can only be reclaimed after the assessment notice has been established (which will often only be one and a half to 2 years after the realisation of the capital gain). 

In addition, capital losses realised during the same taxable period are not automatically considered when determining the capital gains tax. On top of that, the acquisition value of the assets is not considered in the event it was higher than the value of the assets on 31 December 2025. In both cases, a correction must also be made via the annual personal income tax return. 

Due to the retroactive introduction of the capital gains tax as of 1 January 2026, it was not possible for financial institutions to withhold taxes before 1 June 2026. For capital gains realised up to and including 31 May 2026, a transitional arrangement has been provided. In doing so, the taxpayer may request the financial institution to make a withholding equivalent to the withholding tax on the capital gains. They should request this by 31 August 2026 at the latest, or earlier in case their financial institution set an earlier deadline.  

Opt-out  

If the taxpayer chooses to opt-out, the financial institution will not withhold taxes at the time the capital gain is realised. The financial institution will provide the tax authorities with an overview of the realised capital gains on an annual basis. The taxpayer will be responsible for reporting the realised capital gains in the personal income tax return and paying the tax at the time the assessment notice is sent by the tax authorities. 

When opting out, the tax exemption of €10,000 (which may be increased to €15,000 under certain conditions) can be applied immediately as well as any realised capital losses. 

For joint accounts, all account holders must agree to opt-out. If this is not the case, taxes will be withheld by default. 

In principle, the choice for the opt-out must be made at the time of opening an account or on a date determined by the financial institution and at the latest before the realisation of the first capital gain on the account. 

In addition to the transitional arrangement for the withholding and payment of the tax, a transitional arrangement is also provided for the opt-out. For capital gains realised from 1 June 2026 to 31 August 2026, the taxpayer must notify their financial institution of their choice to apply an opt-out by 31 August 2026 at the very latest, or sooner if the financial institution has specified an earlier deadline. 

Any choice for an "opt-out" after the above-mentioned dates will only take effect from the subsequent taxable period.

So, what is the right choice?

The choice between opt-in or opt-out will depend on the personal situation of the taxpayer. The opt-in is usually the best option for those who prefer administrative simplicity, while an opt-out can be interesting for those who wish to apply the exemption rules and set-off of capital losses and want to bear the administration and cash flow behind it themselves. With opt-out, there is no need for pre-financing of a tax, which may not be due in view of the exempted bracket and any capital losses. However, in this case, the financial institutions will provide an overview of the realised capital gains to the tax authorities, while in the event of an opt-in, the withholding tax will be paid anonymously.  

Every situation is unique and may require a different better choice depending on the concrete circumstances.

If you would like assistance in determining the most appropriate approach or if you would like more information, please contact the authors of this article or your trusted BDO advisor.