easy way to find out thickness of basement floor (conrete)?
Lowering the floor is not a trivial exercise. What has to happen is the foundation has to either be replaced entirely (propping up the house and putting a new one under it) or by underpinning it a section at a time. Either way, this is not an inexpensive endeavor. And it most definitely is not a total DIY job. There's a lot of engineering that has to go into it. Soil conditions, weight loads, local codes (earthquakes, etc), drainage, radon, and more.
Permits exist for a fair amount of time. It never hurts to actually check. Don't just assume you won't find anything useful. Sure, you're not likely to find everything back to 1918, but that's no reason not to at least inquire. Unless you want a printed copy many permitting offices will let you review a list of what they know about at no cost.