Bumblebee123

I am considering rebuilding my semi-detach to become a detach house and so far I had spoken to a few builders and architects. The price varies quite greatly! If I were to maximize my GFA, I am probably looking at a built up of 6800 to 7000 sq feet. (2.5 story or 12 m mezzanine style). Architects has quote me $50k to $280K for their service! ( all are registered architects) Builders have quote me $180 psf to $400 psf. ( all of them are licensed BCA builders from class 2 and class 1). I understand that the it depends on the type of finishes and materials. I had indicated my preference of materials which is rather standard these days, and honestly not super premium. (i.e mid range marble flooring for ground floor and paquet flooring for rooms and common area. Mixed homogeneous tiling and ceramic tiles. Normal lightings. Based on what I had gathered, it can cost me from $1.3 mil to slightly over $3 mil. The difference is glaring! Thus it has made me wondered what the fair construction value psf. if it is cheap, why? If it is expensive why? Can anyone advice?