Living space expansion on the terrace, is a new floor slab necessary?

Hello! I wanted to enlarge the living space a bit, and I already have the permit for this. The plan is to remove the current windows and install a glass facade two meters further out, spanning 8 meters in width, thereby closing off the terrace roof. However, I have received two different opinions regarding the implementation. One person says that the entire terrace slab needs to be renewed, while the other believes that due to the thickness of the current slab (over 30 cm concrete slab + 12 cm possible build-up), this is not necessary. Is there a professional here? The interior build-up is planned with 1.5 cm topsoil, 3.5 cm screed, and 3 cm underfloor heating, so actually only up to 4 additional cm can be used for further insulation. Should I renew the slab as a precaution or opt for the more cost-effective method?