A marriage contract is an agreement concerning what will be private property in a marriage. It can be written at any time during a marriage or before it.
What characterises private property is that it is not included in any future division of joint property, i.e. in the property that spouses divide between them if they get divorced, for example. What is not private property is called marital property.
It is always possible to write a new marriage contract making the property that you had made private property marital property again.
In order for a marriage contract to be valid, it must be in writing. You must also register it with the Swedish Tax Agency. The Tax Agency then enters the information on the registered marriage contract in the Marriage Register.
Do the following to register a marriage contract with the Swedish Tax Agency:
Skatteverket
Äktenskapsregistret
871 87 Härnösand.