Verbal promises are not enough, and registering a company in one spouse's name makes no difference. A marital (prenuptial) agreement must be in writing and registered with the District Commissioner — otherwise the main rule of equal division applies.
Under Article 80 of the Icelandic Marriage Act No. 31/1993, a marital agreement must be in writing. It is not valid between the spouses or against third parties unless it is registered, cf. Article 82 of the same Act. Registration takes place in the District Commissioner's register of marital agreements.
This is the reminder highlighted by Court of Appeal (Landsréttur) case No. 370/2026: separate property that people believe they have agreed on does not come into being through words alone. Without a registered marital agreement, the asset forms part of the marital property when the division takes place.