Speculating with bitcoins is hugely popular.
Despite the fact that the price varies enormously almost from day to day, many people are purchasing the virtual currency. This should be declared in your tax return in box 3.
The bitcoin’s value fluctuates enormously. Fluctuations of dozens of percent per day are no exception. For your income tax return, however, only the value on the reference date of box 3 is important. For the coming return for 2017, this is the rate on 1 January 2017. This was around €950 (for the exact value, please contact your stockbroker). The fact that the current value is many times higher is only important for your 2018 return.
Bitcoin transactions are in principle anonymous. However, you are obliged to declare your possession of them. If you fail to do so and the tax inspector discovers your possession of bitcoins, an additional assessment will be imposed on you plus a hefty fine.
The probability that the Tax and Customs Administration will trace your possession is by no means small because the tax authority has a whole range of control methods at its disposal. For instance, in almost all cases you will pay for your purchase of bitcoins through a bank. In the Netherlands, banks are required to give the tax authority access to transactions like these. And there are also agreements about this with foreign banks.
Additionally, the tax authority has insight into purchases of a considerable sum. So if, for example you spend part of your bitcoins profit on a new car, the tax authority can ask you to explain how you financed it. If undeclared income or hidden assets, like bitcoins, is suspected, you can also be sent an additional assessment and be fined.