A greenhouse can be mighty handy in a cool climate especially if it is linked (integrated) to the house. The link can be as simple as a clear shelter attached to a window or back door. If the greenhouse is attached to an outside door it can also serve as a thermal air-lock, reducing the risk of cold air getting into the house. Attaching a greenhouse to the house also works to strengthen the walls of the greenhouse as well as provide heat to the house during cool days.
If the greenhouse gets too hot in summer, you can cover the greenhouse with a recycled canvas sheet (ie. from an old tent), heavy shade cloth, and/or vines both deciduous and annual vines (such as pumpkin). If the greenhouse is at the rear of the home, you can also use this space to hang your washing during wet periods. If it is attached to the front, add sculptures and welcoming scented plants and use it as a feature of the house. You would locate the greenhouse on the side of the house that receives the most sunlight.
http://www.inspirationgreen.com/pit-greenhouses.html explains how pit greenhouses work.