Removal of large patio doors

Re-enforcing existing foundations to your extension would only be required if you were adding additional weight. The existing weight of the flat roof will have to be supported by whatever you replace your doors and units with. There are many options of lintels and beams on the market of all different density’s, dependant on what frame and unit you use as a replacement, this will determine what extra support would be needed if any.

There are probably footings there already, as a wall will have been built up to the level of the concrete slab. The weight of the roof will be taken by the lintel or steel above the patio door. So you can remove the existing doors and block or brick it back to suit the size of a standard door. Be sure to tie the new block work to the existing wall with suitable stainless steel wall ties.