conception
The idea of building a structure receives its original statement, before an identifiable design is established.
A structure is conceived when the idea of building it is given its original statement. As this event occurs before its design, it is normally assigned to a design retroactively, as the structure has no status until it is designed or at least exact specifications exist.
The conception date can be assigned retrospectively once a specific design exists. The absence of a design at the time of the original statement does not erase that earlier event.
Basic design completed requires enough specifications to distinguish the structure from other designs. Project announced concerns a reasonably detailed design concept and its first public release or earlier recorded mention.