Absences Abroad: Possible Effects on the Residence Permit
Longer absences should always be communicated to the Foreigners Office to find out, how or if they might affect your re-entry.

Breaks in residence may affect validity of residence title
Longer absences should always be communicated with the Foreigners Office before you leave Germany to find out, how or if they might affect your re-entry.
 If you stay abroad for a longer period of time, you need a re-entry certificate issued by the Foreigners Office. This certificate is necessary, because a longer break in residence may affect the validity of your residence title.
General rules:
Your residence title expires

if your reason for leaving Germany is not only temporary (for example to attend school, to work or to marry and settle abroad)

if you do not return within six months after leaving Germany or within a prolonged time-limit set by the Foreigners Office.

Please contact the Foreigners Office well in advance before you leave Germany for a longer stay abroad. Your re-entry certificate cannot be issued after you have left Germany.
You need to book an appointment and apply in person with the following documents:

written statement about the reasons for your stay abroad

proof of sufficient means of subsistence

proof of health insurance coverage for Germany

proof of expenses related to housing

In some cases additional documents may be required.
Special rules:

EU Blue Card
   Blue-Card holders and their family members are allowed absences of less than 12 months without affecting the Blue Card's validity.

Settlement Permit
   Your settlement permit stays valida) if you are the spouse or registered partner of a German national. In this case you have to make a joint statement, that your marriage or registered partnership continues to exist while abroad and after returning to Germany.b) if your are 60 years or older, have legally resided in Germany for 15 years and return to Germany within 12 months. The same applies to the settlement permit of your spouse or registered partner if they are also 60 years or older.c) if you have legally resided in Germany for 15 years and have sufficient means of subsistence after re-entry. In this case the settlement permit of your spouse or registered partner also stays valid.

EC Permanent Settlement Permit
   An EC Permanent Settlement Permit for Germany expiresa) if you live in the United Kingdom, Ireland, Denmark or outside the European Union for more than 12 consecutive months (for prior holders of a Blue Card: for more than 24 successive months)b) if you live in another EU Member State (with the exception of the United Kingdom, Ireland, Denmark) for more than six consecutive years.

Fee
  10 Euros