Building loan calculations not realistic

I write in disappointment at how Standard Bank does their building loan calculation. We have bought land via the same bank in order to build our dream home! We have managed to finance the foundation and now approached the bank to finance the rest of the project! Our NHBRC builder who has been building for so many years said we need R800 000 to finish the house! His calculations were spot on for the foundation also! Standard bank came back to us and says we need R2.8m to build this house! How is it possible? This amount plus what we spent so far sounds like a market value and not a building value! Who does calculations for this bank coz surely they inflated the prices to suit them and not the owners of land? Also so now we are stuck with land and a foundation? We are truly not yet Uhuru in our own land ??