Important facts to consider when choosing a site for your Greenhouse are Sun, Water access, Slope, Drainage and Wind. Sun or lack of, including shadows make a big impact on the success of you growing. Your site must be level for the base, you will need to get a hose to your Greenhouse to water your plants. Think about if you'll need power in your Greenhouse for lighting or heating in Winter. If wind is a factor then consider building a trellis wall (then grow passion fruit on it!) on the prevailing side to protect your Greenhouse. Below is a diagram on how to assess the shadows in Summer and Winter to assist with your positioning.