If you are over 18 years old, you can turn to PRV to apply for your foster parents' surname. Your application must include a consent form from your foster parents and birth certificates for both you and your foster parents. This is on the condition that you have actually been placed with these foster parents for a long time. A good way of proving this is to send in a certificate from social services stating the placement period. A further important factor is how long you have been registered at the foster family's address.
Per has been placed with the foster family Lund since he was two years old. The Lunds have two biological children. Per still uses his biological parents' surname, Johansson. Per is about to finish upper secondary school and wants his certificates to be in the same surname as the family he regards as his own, Lund. The two biological children even say that Per is their brother.