Question: I have an exterior service door on my garage I'd like to remove and wall off. The exterior of the garage is cedar siding. Replacing that and framing the doorway in is not the issue. My concern is pouring concrete over the existing floor. There is a foundation on the exterior wall, maybe 8" as the garage is attached to the house

Can I just pour new concrete over the existing concrete without leaking issues? I plan on drilling into the floor and insides of the foundation doorway walls, and adding lengths of rebar to tie it all in. Will this be enough? Thanks ![enter image description here](https://i.stack.imgur.com/Ehl8t.jpg)

Answer: 
I understand the question to be about filling in the foundation wall through the former doorway opening over the top of the floor slab. I would not do so because the slab will have a tendency to move with seasonal soil movement, etc. 

Presuming that the slab is poured over a lowered area in the foundation wall, I'd cut back the slab and pour your foundation wall repair directly on the original wall. This will provide the most stability. 

If you'd rather not do that, consider simply framing in the opening with wood, using pressure-treated lumber where there's concrete contact. Construction adhesive would secure everything just fine. If it's exposed below the siding on the outside, paint it with an appropriate gray paint to match the foundation. Caulk it in and it'll become nearly invisible.
If I understand what you are asking it is only about the slab, can you pour inside the foundation, yes this is done all the time adding some rebar pins around the perimeter will help lock everything together I do this when repairing areas I had to cut out usually in a slab repair I will use heavy wire to keep the patch in place. Since you have an existing floor I would heavily acid etch the existing floor and flush it out this will help the new layer to bond. There are also adhesion promoters that can be mopped on the existing floor but for them to work the floor needs to be cleaned a good strong etch with muratic acid and water will provide a fresh rough surface for the new layer to attach to. Remember always add acid to water (AAA). Muratic acid can be found in most big box stores or pool supply stores.