Digging Basement on Flip to Increase Ceiling Height
I'm looking to dig out a relatively small basement (15'x20') to increase ceiling height. The current height from slab to ceiling joists is 6'4". Anything over 7' is the goal. It looks like a stone foundation with parging (pretty common in early 20th century Philadelphia rowhomes). My GC recommends that we first dig holes in the slab along each foundation wall and check the thickness. He believes there's a good chance we can remove the slab, pour a new 4" slab, and pick up 6-10" of ceiling height, all without underpinning. The cost is $6,000, but he says we'll know how deep we can go before committing the money. I'm not experienced in evaluating this, so I am throwing it out there for anyone to provide insight on. Basically, does this make sense? In all my research, it seems like people just recommend benching/underpinning. No one talks about shaving off thickness of the foundation slab. I'm guessing this is because I'm only looking to go 6-10" deeper and most people are inquiring about 2-3'. And my second concern is I don't see anyone saying that slabs are often 12" thick. It seems like slabs are more commonly 4-6" thick and sitting on the footers, so is it wishful thinking that my contractor is going to find a 12" thick slab, or am I misunderstanding what he's saying. Maybe he's saying dig out the slab entirely and then repour the new one inside the footers (so the top of the slab is flush with the top of the footer)...?
All insight appreciated. Thanks.