Costs for construction project city villa, 1 full floor

Regarding the subsoil, I do not expect major problems since a soil survey has already been conducted for the area. And as it is fully developed including supply, sewer, boundary stones, road, etc., only the stormwater drainage is at our expense. For this point, I have now calculated €2,500.

No, exactly not. Fully developed still means that the house connections still need to be made. Calculate here

Electricity approx. €2,000
Water + wastewater €5,000
Telecom €600
Gas? approx. €3,000-5,000

These costs are incurred in any case, even if fully developed!

No, not at all. Fully developed still means that the house connections have to be made. Calculate here

Electricity approx. €2,000
Water + sewage €5,000
Telecom €600
Gas? approx. €3,000-5,000

These costs will definitely be incurred, even if fully developed!

Yes, I know that, which is why €7,500 for the house connections (street to house) is also included in the additional costs. The sewage channel is already on the property.

What kind of sewage system do you have? What kind of sewage pump do you need? That can quickly cost a substantial amount. Connecting the sewage system is not free either. Depending on the situation, pressure tests may also need to be carried out. Then the pump shafts require a fairly deep hole. Are you doing that yourself? How do you heat? I wouldn’t have €7500 for that. Especially since you should also factor in construction electricity and water, as well as the rental/purchase of the electrical box.

What kind of sewage system do you have? What kind of wastewater pump do you need? That quickly costs a substantial amount. Connecting the sewage system is not free either. Depending on the situation, pressure tests may also have to be carried out. Then the pump shafts require a fairly deep hole. Are you doing that yourself? How do you heat? 7500€ would not be enough for me. Especially since you should also include construction electricity and water, as well as the rental/purchase of the electrical box.

As additional costs, as you can see in the initial post, I have calculated the following items

House connections 7,500€
Rainwater 2,000€
Building permit 1,000€
Surveying 2,000€
Building expert 2,500€
Construction water 1,000€
Construction electricity 1,000€
Construction insurance 500€

A gas heating system with solar panels & a ventilation system with heat recovery is to be installed.

Since we are still in the planning phase, I unfortunately cannot say anything about the sewage system yet.

Which wastewater pump you need to use can be told to you by the city or the wastewater yard/lower water authority. If you have pressure pipes, you are quickly at 5000€ for the pump (including installation).

You can get a building surveyor through the Bauherren-Schutzbund for 1500 (7 inspections).
For the surveying, calculate a bit more. Usually, the rough staking is not included, and that costs around 300€.
Then cautiously add a few thousand to the connections, so that the gas connection is included. Depending on the length of the line, that can quickly become expensive.
Check with the lower water authority how infiltration is regulated in your area. Depending on how pedantic they are, you may only be allowed to make one infiltration basin and you have to have it done by a professional. Again, this means costs.