Financing new construction KfW70 prefabricated house

Hello, we have decided on a prefabricated house, which now needs to be financed. The situation is as follows: Ownership: 7-room condominium with an outstanding loan of 80,000 EUR (value approx. 170,000 EUR) 2-room apartment, purchased in 2011 and rented out permanently - costs: 65,000 EUR Single-family house - debt-free, however, a mortgage was registered for the 2-room apartment. (Value approx. 300,000 EUR including land) Costs for new construction including land and all calculated additional costs approx. 380,000 EUR My way of thinking is this: I take the full 380,000 I sell the 7-room apartment after moving into the house - residual value after sale 90,000 EUR This should flow directly into the financing. Is that then a special repayment, or how is that done? Also, the single-family house should be sold after the death of the father. Proceeds of approx. 300,000 EUR should also flow into the financing as described above. Then that should work, right, or am I seeing something wrong? Income net - stable (civil servant): 5800 EUR Who can give me tips on this?