ᐅ Extending Living Space onto the Terrace: Is a New Concrete Slab Required?

Hello! I wanted to increase the living space a bit and already have the necessary approval. The plan is to remove the current windows and install a glass facade two meters (6.6 feet) further out, spanning a width of 8 meters (26 feet), thereby enclosing the terrace roof. However, I have received two different opinions regarding the implementation. One says that the entire terrace slab needs to be replaced, while the other argues that due to the thickness of the current slab (over 30 cm (12 inches) of concrete plus 12 cm (5 inches) possible structure on top), this is not necessary. Is there a professional here? The interior floor structure is planned with 1.5 cm (0.6 inches) of finish flooring, 3.5 cm (1.4 inches) of screed, and 3 cm (1.2 inches) of underfloor heating, so in theory only up to 4 cm (1.6 inches) more can be used for additional insulation. Should I renew the slab to be safe or go for the more cost-effective method?