Is this secondary construction cost calculation realistic?

The general contractor said that we do not have a rainwater shaft. The development plan states: The rainwater from the roof surfaces is to be infiltrated via near-natural infiltration systems, as long as it is not used as utility water.

Or does it always have to exist??

No, it does not have to. But it is the most cost-effective alternative. If there is no shaft but the requirement for natural infiltration systems, then these have to be built. The fact is: it is about connecting the downpipes to a place where the rainwater collects and then is naturally led away into the ground from there. However, I am not an expert in this. Either the downpipe water goes into a cistern and infiltrates there, or this cistern collects and leads it back to the house. Then there are these natural systems, usually a small basin/small pond (size is calculated)... it is a bit more expensive to install. One neighbor of ours has such a stone basin (a really nice highlight in the garden on a 650 sqm property), another has this basin under his wooden terrace, which saves space and is also an option for you. If, as with the OP, the rainwater is discharged into the sewer system, you pay monthly/annual fees for this disposal.

If there is no shaft, but the requirement for natural infiltration systems, then these must be built.

ok, then I should probably mention that again!

By the way, I received a list of the additional construction costs (excluding earthworks) from the general contractor, which includes all prices and the contractor named. For us, the "fixed costs," i.e. house connections, surveying, etc., will amount to about 15k.

We were at another contractor today and also prepared a statement of additional costs during the conversation.

What do you think about it?

The additional costs look okay.
The building costs, however, less so.
Have you studied the construction performance description carefully?

Yes, the construction service description has been thoroughly studied and is more than suitable so far.

Good day everyone,
do the incidental construction costs seem realistic, or should one rather plan an additional €5,000?

3. Incidental construction costs (experience values - dependent on the plot)
3.1 House connections €5,000.00
3.2 Drainage €3,500.00
3.3 Fees for building permit, acceptance, construction electricity, construction water, etc. €3,000.00
3.4 Surveying including building end surveying €3,900.00
3.5 Partial demolition costs of old building €10,000.00
3.6 Soil report €800.00
3.7 Partitioning costs €2,000.00

Total €28,200.00

I'll say something about the points I also know. 3.1 House connections ---- seems very little to me, or has this already been laid on the property due to existing structures? 3.3 does that include notary fees, deed of trust registration? --- then probably too little 3.4 you should be fine with that 3.6 tight but could be sufficient I find (almost) everything very tightly calculated. I would certainly plan an additional 5k. No further earthworks needed? Everything included in the demolition costs? However, the figures may look somewhat different for you than for me, due to the existing structures.