When you have decided on your greenhouse you will need to think carefully about what you are going to put the structure down on, all greenhouses will need a firm level area prepared at the very least and many will require concrete or slabs laid, some modern greenhouses come with a manufacturers base which can be levelled onto soil, and secured down with metal feet in each corner, then the greenhouse sits onto this directly with special fixings, other greenhouses especially wooden greenhouses will need a concrete pad laid or a patio area prepared with slabs, this does not have to be the whole area if you wish to grow in the earth but just the outer perimeter.