Unfortunately it is not possible to do the groundwork and the base within a day, hence some pre-preparation will be required by the customer. 
These buildings will require the customer to provide a flat and level base of either paving slabs or concrete, either on a DIY basis or by a local competent builder. Alternatively they can sit on sleepers or a deck floor, as long as they are finished so that the floor can sit level and be supported adequately so that it doesn't bounce. 
If choosing a concrete pad - you will require a 150mm hardcore, dpm membrane and 100mm concrete. There will be no requirement for a brick course (unless the building is larger than 3.6m x 6m).
If choosing a paving slab base, simply remove the turf and level with a sand and cement mix, before fitting 50mm thick paving slabs. 

If you have an existing base that would like to site the building on - please email a picture to if you need help in assessing whether it will be suitable.