pouring concrete over existing slab

I have a Slab on grade house, there was a 10 foot extension added on the rear of the garage which I want to add anotehr four feet, Also add second floor above for office. The extension and garage are lower than the main florr of the house, however I would like to have the extension to the rear of the garage and the addtional four feet that I am wanting to add be the same level as the rest of the house as I will need to remove all the originall roof and walls on the existing extension anyway.
What are the issues with having new slab foundation added and concrete pourded over existing slab assuming the existing slab foundation is suffiencient to support a second floor above ?
We are planning new laundry, bath and storage space in this new area on first floor.
Thanks, MC
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Call a contractor to look at the situation. Everyone would be guessing with your description
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