The ideal position is traditionally said to be with the greenhouse in a West to East line so that the sun travels along the southerly aspect for the day.

But this is not always possible. The things to consider include: proximity to trees, shelter from the prevailing wind and from the North, access from the house and winter shadow from other buildings (the sun is 17 degrees lower in the sky over the winter). Ultimately, even if the sun can only get to the top of the greenhouse it will still heat the building up.