Cost saving/basement/affordable tiles/sealed screed?

Correct - there isn't even a basement yet where cost reduction possibilities could be sought. Current status: offers are currently being obtained for the individual trades, most have been received, but some essential ones are still missing. And a discrepancy between desired equipment and budget naturally exists - this needs to be resolved. The reserve for unforeseen expenses should not be touched yet.

As mentioned, how about cement screed instead of anhydrite screed? It would probably save a few thousand euros quickly in terms of price as well

Of course this will be discussed - all points that would result in significant savings must be considered. Discussion with the construction engineer of my partner company as well as the individual trades will follow in February.

As I said, how about cement screed instead of anhydrite screed? It would probably save a few thousand euros quickly in terms of price

The difference cement <--> anhydrite screed for me would have been exactly €0.9/m², so there isn't much to save there (personally, I would still opt for cement screed in the basement). In my opinion, a proper screed sealing won't become much cheaper than an inexpensive fine stoneware tile (of course, it depends on the installation price). For a ready-to-use screed, there are naturally different requirements for flatness, which also increase the installation cost (or require subsequent leveling).

If it should be cheap: Why not lay a click floor (laminate, cork, parquet, vinyl...) yourself? There are quite a few nice options available, also suitable for underfloor heating (which might have to be laid a bit closer together).

90 cents/m²? I'll check that again with me today... but I think it was about a 30% price difference.

How many square meters are we actually talking about? I think the savings probably aren't worth the discussion?! Whether sealing for 2€ per m², laminate for 5€, or tiles for 8€. The costs are ridiculous in the grand scheme of things. The labor costs are the highest. If you want to save money, doing it yourself is where you save the most. So, carpet or laminate is the cheapest option. But you keep everything open so you can still change it later. Are you painting the walls yourself? Then you can also do the sealing, it's no different.