You don't have enough room to put anything on top of the existing slab, in a thickness that will last, so it's going to need to come out. I would not use it as fill, I'd just excavate the area as needed down to the correct level.

I don't know what your finished floor level is, relative to the existing slab. I don't know if you have the possibility of putting the new slab in at a lower level than what is there now (a step down from the inside floor level) which might minimize the excavation needed.

You don't have good drainage away from the house so consider your soil type. You may wish to add a bed of sand under the new patio. If it's really bad, a perimeter grade beam might be a good idea.

Use #4 rebar on 12' centers, each way, in the new slab. Keep the concrete warm and wet while you cure it. Use an expansion joint between the house and patio and caulk it well.

JR

I vote for pavers. We took out a deck and had a slate looking paver patio put down. We were very pleased with the result.--Dave

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