Reinforcement basement floor slab

Hello,

I have received the following offer from my basement builder:

"With the 25cm thick floor slab with a double layer of reinforcement using Q 424 mats, the following costs would occur.

Approximately 20 reinforcement mats are needed in the basement floor slab, the weight difference between a Q188 mat and a Q424 mat is approximately 42.7 kg per mat, so we come to an additional steel content of a total of 854 kg.

More steel in the floor slab = 854 kg * €2.00/kg = €1708.00 Additional concrete (increase from 20cm to 25cm) = 88.95 m² * 0.05m = 4.45 m³ * €188/m³ = €836.60

That makes a total of €2544.60 for the desired execution of the basement floor slab"

Unfortunately, I have no knowledge in this area at all. Is this a good offer? Or should I take the 20 cm floor slab without extra charge?

->>> under the floor slab is slate :-(

It wouldn't be bad if you had someone who knows their stuff... the amounts you just spend here, or don't, definitely justify that. The static calculations have to specify what needs to be installed, don't they?

Thank you for your help, but it does not help me further. The data is from the basement builder, who only commissions the static calculations once I have decided which [Bodenplatte] I want.

The statics of the basement must be coordinated with the rest of your house, right, or are you only building a basement? I am a bit confused. Who is coordinating the individual trades? What has been approved?

Determine the foundation slab/basement on site, as is done for example by Heinz von Heiden.

You are putting the cart before the horse.

A structural engineer must tell you, after knowing about the soil conditions and the construction project, what thickness and reinforcement you need.

The other way around makes no sense at all.