Things that will affect planning permission and the overall viability of the site include whether or not it is sloping, sheltered, narrow or presents difficult soil conditions including underground water courses.
Flood prone areas should not be built on. Check if there is any contamination on the site (brownfield site) and also examine potential environmental issues such as whether the site may be affected by EU nature laws, or impacts upon protected areas such as areas of outstanding natural beauty or Natura 2000 areas.