The selection of a site is very important since the site represents a fixed form, that will radiate positive as well as negative energies depending on its location, orientation, shape, slope and soil quality. While selecting a Site these five major points are to be kept in mind.
Normally a Vaastu Expert is not involved at the time of buying a Site. But as a Vaastu Expert, I would suggest that it is good to involve one, at the time of selecting a Site, as his inputs could be very valuable in deciding the right Site. Generally, the points a Vaastu Expert will keep in mind while selecting a site, will by default be in accordance with Vaastu Shastra.
If you already own a site for construction of house or office, you should still refer to the Vaastu Guidelines prescribed for the selection of suitable Site.