Onward Tickets from Cologne to Istanbul

Do I need an onward ticket to enter Germany?

The following documents are required for any Germany visa application:

• Application Form – duly completed with correct and honest answers that comply with the information in the other required documents.
• Passport – according to the passport requirements of the German embassy, among which a validity of three months beyond you planned stay in the Schengen.
• Photo – which must be taken within the last three months, and according to the other photo standards set by the German embassy.
• Health Insurance
• Travel Itinerary – that includes proof of your round trip flight/travel reservations or other documents that show intended transport, as well as proof of what you are planning to do in Germany within the period that you wish to Remain