Change heating after building application or during construction phase

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1. Can the heating system still be easily changed during the construction phase and after obtaining the building permit?

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Of course, no building permit is set in stone.

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2. What documents are needed for an amendment at the building authority? Exterior views, data sheets of the air heat pump, and what else?
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also the corrected building energy law calculation

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3. Could there possibly be problems with the already approved KfW55 application and would a new energy calculation have to be made regarding the subsidy?
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With almost certain probability, the application is lost, since the basis for the calculation is no longer given – the execution will no longer correspond to the approved status. You need to clarify the details with your energy efficiency expert who prepared your application. In this case, the nice money will probably no longer be available for known reasons, even if the type of heating is an improvement in terms of the building energy law.

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4. Are there any other topics and/or points that should be considered?
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Cancel the gas connection if already applied for ;)

With an almost certain probability, the application is lost, because the basis for the calculation no longer exists - the execution will no longer correspond to the applied version. You need to clarify the details with your energy efficiency expert who prepared your application. In this case, the nice money will probably no longer be granted for known reasons, even if the type of heating is an improvement in terms of the Building Energy Act.

We had this topic before, it was about the windows. The consensus back then was that a recalculation is necessary, but improvements are not a problem.
In case of doubt, just call KFW/BAFA and ask. They can give definitive answers.

We had the topic before, it was about the windows. The consensus back then was that recalculation is necessary, but improvements are not a problem. In case of doubt, just call the KfW/BAFA and ask. They can give definitive answers.

Were there any feedback on how it turned out? I really hope that my assumption is wrong.

I have now found the thread. had changed that and it was not a problem
https://www.hausbau-forum.de/threads/kfw-foerderzusage-Änderungen-am-haus-danach-moeglich.42473/#post-554538