When space is limited a lean to takes up less room and comes in many shapes and sizes. There are many lean to greenhouses available in the UK starting at small 4×2 or 6×2 structures which are effectively little wall gardens right up to big buildings that you can sit in and use like a conservatory. One of the main considerations should be the wall you are putting it up against. The wall must obviously be high enough to accommodate the lean to, but also take a look at the condition and check how you will seal the greenhouse to the wall. Also check that the wall is at right angles to the base area as this will need to be pretty good to allow proper installation.