12cm step from hallway to basement rooms

Hello,

according to statements from our builder, there will be a step of 12 cm from the hallway to the basement rooms, as only screed without insulation (4 cm high) will be laid in the basement rooms. The basement rooms are not within the thermal envelope, but the hallway is.

He offers to also install floor insulation in the basement rooms to reach the same height of 16 cm. This would cost an additional 4090€.

Is this justified and is there possibly a more sensible way to avoid this step?

"Installation of insulation without underfloor heating on the floor slab in the basement, total floor structure 160 mm, consisting of:
Composite board compact EPS-DEO 032 at 30 mm
Thermal insulation EPS 035 at 60 mm
Edge insulation strip
Supply and installation"

He offers us to also install floor insulation in the basement rooms to reach the same height of 16cm. This should cost an additional €4090.

Is there an even more sensible way to avoid this paragraph?

Go to the hardware store and get the insulation yourselves (although I wonder why a 32mm one has to be included - but whatever). This way you easily save €3,000.

With underfloor heating, maybe just a normal GU price. Without underfloor heating, that is an outrage for which you could have challenged someone to a duel 200 years ago. That is incredibly bold. Do it as Jens says.

According to statements from our house builder, there will be a step of 12cm from the into the basement rooms, since only screed without (4cm high) is laid in the basement rooms. The basement rooms are not within the thermal envelope, but the hallway is. [...] Is this justified and is there possibly a more sensible way to avoid this step?

Even though you unfortunately did not show a cross-section and the question probably refers to the additional cost, in my view, it sounds above all highly questionable. If you want to let the (presumed) nonsense happen as it is, I would simply blend it over with a wedge-shaped ramp here.

I have attached a section of the basement corridor floor.

I would also like to lay the EPS boards in the basement rooms myself. It’s not rocket science after all.
However, this is unfortunately not logistically feasible, since the screed installer lays the cold welding membrane and then directly applies the screed.

In general, I consider the basement floor construction to be questionable. The perimeter insulation shown in the section has already been dropped because the basement was incorrectly calculated as a residential basement instead of a utility basement in the Building Energy Act certificate.
However, the basement corridor is still supposed to receive the two EPS boards, since it is allegedly within the thermal envelope, while the basement rooms are not. This then results in a 12 cm step down into the basement rooms.

How are the walls and doors between the basement corridor and the basement rooms constructed? In the attached floor plan, this appears to be standard and not walls for the closure of the thermal envelope.