Starting a complete home remodel and want to convert an existing garage to living room space. It was built in the 1950s and the existing slab foundation is as deep as 8" and as thin as 4" in some places (slightly sloped yard, the rest of the house is built on a crawl space).

I have been advised it might be smart to dig up (!!) the existing concrete slab foundation and add a 5" one with a moisture barrier. This seems excessive and expensive. Is this really necessary? Are there other options to protect from moisture that can be added on top of the existing slab?

For additional background, we are in a not very rainy area of California and have never had moisture issues in the garage at all in 20+ years.