A driveway isn’t a foundation slab. It has to come up and a proper foundation dug.

This is not a driveway. The is just a slab of concrete I poured about 5 years ago behind my garage like I mentioned keeping in mind that I might expand the garage and reinforced it about 1/3 thicker with 1/2 inch rebar. I don't plan on using the existing slab. I would be pouring a footing around the perimeter and center of it approx 10 inches deep and adding 2 inches of base on top of of existing slab there and pouring a min of 4 inches thick on top of that. Seems like the existing slab would be stronger then 8 inches of compacted base rock since it hasn't move at all

Do you trust your friend’s dad (with 35+ years in the biz I probably would)? If so, then it sounds like you have your answer.

A slab on grade needs deeper edges and independent footings wherever load bearing elements are located throughout the floor plan.

I think the OP is planning on footings AROUND the original slab and then will go OVER the original slab with the new slab. If so - then I agree. If we've not had settling, then of you pour proper footings, and do all the other steps (vapor barrier, etc) you are simply using the slab as "compacted base"

What did your friend's dad say about the unconcreted area around the existing concrete slab that will now get incorporated under new slab? Would it not have the potential to settle at a different rate/direction than the existing slab, making the new slab more prone to cracking around the perimeter of the old slab?

I ask because we've asked concrete people similar questions before on various projects and that was their concern. Ours specifically was for a porch/patio extension and we were told to be prepared to have to go back with some pavers over the entire thing later once the settling and inevitable cracking occurred. We haven't gotten to that point yet in our remodel so I'm curious to know if there is a solution to keep the cracking from occurring.

What does your municipality say? What does the structural engineer/soils engineer/geologic engineer say about the idea? Someone has to physically design your foundation based on your specific site conditions. That foundation design has to get the approval of your municipality.

A slab on grade must have a foundation perimeter and load points that goes into undisturbed subsoil. A concrete contractor should understand that, even if he is not a structural engineer. Or undisturbed subsoil with approved and inspected compaction material. A concrete slab isn’t an approved and inspected compaction material. Not without core samples proving what is under it, and the compaction of the fill that’s under it, and the depth at which you reach undisturbed soil. It also will complicate the required termiticide treatment required under slabs.

Doing all of the work and reports to prove that you can use it may cost you more than demolition and rebuilding. Time is money in that regard. Working around the old parts of a building is what makes remodeling cost more than new construction. The input of a concrete contractor is valuable, but he is limited to a single perspective that revolves only around his job portion. He doesn’t understand the interconnected complexity that goes into making his job “arrive and pour”. There’s a lot more engineering that happens before that happens. And the GC is the one that coordinates all of those moving parts that the concrete contractor does not see.

Talk to your municipality.

D Walker : That's exactly what we want to do. I was leaning on doing it way to save time and money. Thank you!

The Cook's Kitchen: All the build plans are done. A soils report is not required for a small slab foundation for a 1200SF ADU. You have a lot of information there but most of that doesn't apply to the project that I have been approved for by city hall. This build doesn't require an engineer as long as we follow the specif city guidelines and put the sheer walls where they are supposed to be on each corner and no windows within 4ft of each corner which is fine with me.

shead : He told me the existing slab appears to be in perfect condition and solid without settlement after 5 years. If he removes it to put 8 inches of base rock, he can't get his compaction as good at that 7-inch slab that is there. The footings that will go around the permitter and center will be supporting the entire structure anyway and would tie into the existing slab. He said it should be bulletproof.

just_janni : Your exactly spot on!

Virgil Carter Fine Art : That's all in the build plans currently

Hi John. You said, "Just want to know what everyone thought about that." From your responses, it appears that you wanted to know what everyone thought about that only as long as they agreed with what you wanted to do.

Your choice, of course. Good luck with your in-law slab. :)

I hope it works out and your new slab doesn‘t crack. I still have reservations about it since the new area being incorporated won’t have the same base, compaction, and settling that the existing slab has.

I think I read on here once that there’s two types of concrete: concrete that has cracked and concrete that’s going to crack. That’s always been my experience with it anyway ;)

For $3,500 it doesn’t seem worth the risk.