Cost estimation for interior finishing, any experiences?

So our gas connection costs exactly €1720, not much saved there
Take a look at the construction service description regarding civil engineering:
Is a multi-utility connection included?
Are inspection shafts included?
Are KG pipes included?
Is the connection of the rainwater pipes included?
Filter gravel layers if needed?
Trench excavation included?
Just here, there are hidden additional costs of €5,000-10,000. These are all items that are often forgotten because no builder knows about them until they sign.

The heat pump would have been €10,000 plus drilling more expensive for us than solar and gas boiler.

Separating labor and material rarely makes sense. Recently, I have increasingly dealt with damages due to poor planning and self-purchase of materials. The blame is very difficult to determine. Example: The client buys natural stone slabs from the dealer - stonemason / tiler installs them. Since there is no planner / consultant involved, there is an imprecise contract and no written correspondence. All changes are made verbally. If damage occurs, for example water under the slabs, the assignment of blame is uncertain. The material may be unsuitable, the installer may have been careless, or the entire construction may be incorrect. With a supervised complete service, liability is clearer. Since disputes drag on for years, the saved money quickly disappears and one remains on a construction site until the verdict or settlement.