Site preparation or merely just digging the foundation doesn't count as contruction starting, since digging a hole is just that, digging a hole. And plus any construction taking place within a foundation that pertains only to the excavation doesn't count either, since that's not construction for the actual building itself.
* Foundation work includes the implementation of permanent foundation components such as the drilling of caissons, etc.
* Top down construction involving piling doesn't require a foundation to have been dug for a building to be considered under construction.
A tall building is considered to be ‘under construction’ when site clearing has been completed and foundation / piling work has begun.
http://forum.skyscraperpage.com/showthread.php?t=151141