Thicker concrete floor slab costs

Dear House Construction Forum Team, We are currently in the construction phase and wanted to increase the planned concrete floor slab thickness from 20 cm to 25 cm for soundproofing reasons. Our planner said that would cause extreme additional costs. Now we are very uncertain. Does anyone have more specific information? Regards Anna

Doesn't the upper floor lose 5cm of ceiling height because of that? Or has no building application been submitted yet?

25% more material alone. But it doesn't stop there. The static calculations also have to be redone and designed. It's a real headache, especially if you're already in the construction phase. What kind of ceilings are they exactly? What kind of floor is going on top? I suspect with a good impact sound insulation you'll be significantly cheaper and above all better off.

for soundproofing reasons

What kind of noise is it?

That is not worth it. Not much less noise passes through a 25cm concrete ceiling compared to a 20cm ceiling. The sound mainly comes through the stairwell, and it makes more sense to install a door there. For impact sound, there is impact sound insulation.

Our ceiling had to be 2cm or 4cm thicker (than calculated in the offer & contract) after the static calculation was completed. It was an additional cost under 500€ and the only point over which we actually wanted to argue with the general contractor. But let's be honest: they would have just added it to another item, e.g. the earthworks, drainage piping, etc., because another soil class was found somewhere. So we had a bit of a discussion and then just paid.