Is demolishing old buildings really that expensive?

Thanks already for the many helpful answers here! We will definitely get more quotes. We already have one over €40,000, but unfortunately it is relatively "rough" and some items are missing that will probably be added later (especially the toilet already mentioned here :p). In general, though, we will soon have an architect and site manager who will support us with some questions. Especially with the question of how best to handle filling the excavation pit and building the new structure in a slightly different location. You can either demolish and fill and compact and then leave it like that until autumn (possibly beneficial for soil settling? We can’t build anyway before because of the building permit and tree protection) or alternatively combine demolition and excavation of the new pit (probably cheaper).

Note that from August the EVB (Substitute Building Materials Ordinance) largely replaces the Laga and different requirements are set for the analytics.
This should be taken into account depending on the time frame.

One of the demolition companies mentioned something like this around August but was very vague and I thought it was just a rip-off. Do you expect problems/cost increases from this legal change so that demolition should definitely be done before that?

Problems/cost increases so that it should definitely be demolished beforehand?

As it is currently proceeding, it will definitely be more expensive since it is something new again. If possible, complete everything beforehand.

Thanks already for the many helpful answers here! We will definitely get further quotes. We already have one over €40,000 but unfortunately it is relatively "rough" and some items are probably still missing (especially the toilet already mentioned here :p)...

Don’t be bothered if a comparatively inexpensive quote doesn’t include such small details. The toilet for 3 weeks is not a major cost driver. Important are the large, expensive items as well as later cost drivers. As said, the toilet is not one of them; it was just symbolic for the "builder-provided debate" afterwards. €40k is already in a region that sounds reasonable if backfilling is included. A neighbor will soon be tearing down a roughly 120 sqm solid house from the 50s/60s and according to her own statements received a quote of €26k plus €5k for gutting and about €8k for backfilling. So that matches your price quite well. Good luck with obtaining further quotes and a steady hand in awarding the contract.