In the Netherlands you can make your relationship official by marrying or entering into a registered partnership. You and your partner will then have certain rights and obligations towards each other. Alternatively, you can have a cohabitation agreement drawn up to arrange, for example, how possessions and costs are divided between you.
There are a number of things you will need to take into account if you decide to marry or enter into a registered partnership. For instance, you and you partner must be 18 years or older. Also the municipality must be informed well in advance concerning your plans.
A checklist of all the requirements can be found on the website of the Dutch government.