Cost estimate renovation single-family house

Is there a current estimate of the price per square meter we can expect, possibly differentiated between KfW 55 (EE) and KfW 70 (EE)?

The difference can be €0 or very large. It depends on the house. For KfW55, proof of equivalence for thermal bridges usually has to be provided; otherwise, the specified values are not achieved. This can cause extra effort with basement exits and work on the plinth, or maybe 2-3 cm more insulation thickness is necessary. In the building services engineering, there is usually no difference. What makes sense cost/benefit-wise can be told to you by your energy consultant during the planning process.

For the EE standard, you also need a central ventilation system (9,000-13,000).

No. The KfW requires a ventilation system with heat recovery that is sufficient to provide the minimum air exchange. There is no obligation to equip every room with it and no obligation for a central system. The necessary air exchange can be determined by the energy consultant; based on that, a suitable device is selected. Usually, a decentralized device with heat recovery per floor is sufficient (e.g., living room and adjoining WC connected as a secondary room, and on the upper floor, the bathroom and corridor/staircase connected as a secondary room) or one set of swing ventilators per floor.