Write the application and indicate in it which partial building permit your application for extension relates.
Submit the application to the competent lower building supervisory authority.
If necessary, the lower building supervisory authority will ask you to pay a fee in advance.
Make the advance payment.
If there are any ambiguities, you will be asked to eliminate these ambiguities.
In this case, submit the clarification.
The lower building supervisory authority examines your application and involves the municipality and the necessary bodies.

You will then receive the extension of the partial building permit as well as a fee notice. Pay the fees.