If you have several residences in Germany, the registration law distinguishes between a main residence and a secondary residence. Apartments abroad are not taken into account when determining main and secondary residences, even if they would fulfill the objective criteria of a main residence.
The main dwelling of single persons is their predominantly used dwelling. If you have several apartments, the apartment that is predominantly used by you in terms of time is the main apartment. The time calculation is based on one year. If the comparison of the periods of stay in the respective dwelling does not lead to a clear result, it depends on where the focus of your living relations lies. Living relationships include relationships with parents, partners, friends, ties to a job, clubs, church congregations or political parties, etc.
The main residence of a resident who is married or in a registered civil partnership is the dwelling predominantly used jointly by the family. If you have another dwelling in addition to the family dwelling, this is the secondary dwelling. The duration of residence is not taken into account for residents who are married or living in a civil partnership.
In the case of married residents or residents living in a registered civil partnership who are permanently separated, the same criteria apply as for single persons.
In the case of minors, the main residence is the home of the person with parental authority. If they live separately, the main residence is the residence of the custodian that is predominantly used by the minor resident. If a corresponding application is made, this also applies to persons up to the age of 25 who live in an institution for disabled persons.
In cases of doubt, the predominantly used dwelling is where the main focus of the resident's living relationships is located.