There are a number of factors to be considered when selecting a site for your polytunnel. Here in Shetland, one of the primary considerations is wind, and more specifically shelter from that wind.
Shelter can also bring shade. A polytunnel heats up rapidly when the sunlight is shining directly on it, cooling down to ambient rather quickly once the tunnel is shaded.
Drainage and soil condition may also be considerations. At Da Barn we don't grow directly in the ground, but rather everything is in pots. Depending on what you want to grow and what methods you want to use, you may need to select a site on good growing soil. If you don't want to grow directly in the ground, you could choose raised beds or smaller containers. Hydroponic growing would benefit from a solid concrete slab.