Write the application and state which partial building permit your application for extension relates to.
Submit the application to the responsible lower building supervisory authority.
If necessary, the lower building supervisory authority will ask you to make an advance fee payment.
Make the advance payment.
If there are any ambiguities, you will be asked to resolve them.
In this case, submit the clarification.
The lower building supervisory authority will check your application and involve the municipality and the necessary authorities.

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