5 feet depth length Roof tile is not really heavy.  if wall crack is your concerns just ask your contractor install a row of ground beam just below  the new wall.

Actually I didn't engaged any contractor yet and I don't really know what is the renovation cost per sf now. But I did asked one of the Indonesia contractor who just started renovation my neighbor home and he said about RM200 per sf if normal tile roofing and additional RM30 if RC roof.  This cost is only covered RC structure , brick wall, door , windows, concrete top, paint, internal floor and wall tiling. Wiring and plumbing work is not included. My extension area is 160sf there for about 32k to 36k.

I guess this quotation from chinese contractor.