Attic extension + individual measures ground floor _ how best to promote?

Hello everyone,

Currently, we are planning the expansion/extension of the upper floor of my parents' house. It is currently only partially developed. The owners are still my parents, but the building/land will be transferred to me as part of the construction project.

In addition to the expansion of the upper floor, various measures are planned on the ground floor, which makes it difficult for me to find the right approach.

Measures ground floor:

[*]Thermal insulation of walls / external insulation
[*]Renewal of windows
[*]Renewal of heating system

Measures upper floor:

[*]Expansion of the upper floor by dismantling the existing upper floor and building a new residential unit.
[*]New staircase with ground-level age-appropriate access
[*]Expansion of existing photovoltaic?

So. Now I have the following questions:

[*]Upper floor expansion together with ground floor measures as one project or better separate measures?
[*]KfW funding for the entire project or tax credit for individual measures on the ground floor?
[*]If there is a tax credit on the ground floor, can KfW be applied for the upper floor, or are the requirements for possible funding too high?
[*]How should ownership be clarified or when should/must the ownership change in case of KfW funding or tax credit?

As mentioned, the case is somewhat more complex and I would be grateful for advice!

Best regards, Sven