Most entrepreneurs must add VAT to the prices of their goods & services. You are classified as an entrepreneur if you independently carry out work and have income from it.
The Dutch tax office will determine when you must submit your income tax declaration. Make sure to regularly check your mail and Mijn Belastingdienst to never miss a deadline.
If the Dutch tax office requests you to submit an income tax declaration, you must do so. Alternatively, you can also submit an income tax declaration without being required to do so. This is [...]
Income tax declaration is a yearly declaration of a person's earnings. In the Netherlands, the tax authorities will inform you if you are required to declare your income taxes.