Ethernet is good up to 350 feet, so if the parking lot is ~50 feet, unless
both buildings are really huge I would think that the distance would be well
within range.  The other thing that you could try and do if the buildings
are indeed too big is put in a repeter or small hub at the edge of the
buildings to boost your signal a little on either end.  Thats what we had to
do at the school to get ethernet from the front corner building all the way
to the back corner building where the plug meetings are held.

Brian Cluff