6 inch concrete because it is a floating slab holding the whole house together, you must be either on thick soil, in an earthquake area, or have a high ground water table, so that the whole house has to move as a unit on the soil. The architect did the job right there at least, or there is an outer lip only and the rest is 3in in the middle of the slab.
I am so glad I got the 350mm SDS drill bit I did, as most common long length masonry bits are 300mm overall, and the 350mm one is 350mm usable bit before you run the chuck into the work. Helps in drilling slabs, as the typical concrete slab plus floor screed is 330mm, on a nominal 300mm (1 foot) thick slab. Before it was drill down with the 300m bit, then get out the 4 pound hammer and the steel rod, and spall off a cone underneath to get the hole, and fill it again afterwards. I did borrow a 110mm hole bit for a few holes, to feed network cabling through, as I really was not going to like drilling 20 plus 25mm holes there. 2 holes took 10 minutes, drilling up a ladder for one, and using a vacuum cleaner to keep the dust down.
Another was drilling for a new mains cable, 4 core 32mm, which was done with a rental 70mm bit and a massive 30kg Hitachi drill from the hire place. Funny how they never have the 50mm bit in stock, but the 70 was brand new, and 10 years old. I just had to get a second drill after a half hour, first one stopped turning in the one hole. Easy to drill, just let the drill weight do the work, only control torque. I did hit rebar near the end, but that half inch bar only was polished a bit, the smaller ones just came out in the slug. Other cores I contracted out, it was easier, plus the slab there was nearly a meter thick, they broke a diamond thin wall core bit in there and took an hour extra to get the shattered chunks out.