Building your conservatory foundation
Excavate the conservatory foundation down to solid ground a minimum depth of 900mm (2′-6″) and cast the concrete foundation a minimum depth of 200mm (8″). This is the minimum depth of concrete. Once the concrete has set, normally overnight, build the brickwork up to the height of the existing house foundation. The next stage is to backfill any earth to fill all the voids and to hardcore the entire floor area to a minimum depth of 150mm (6″) and compact using a vibating plate compactor, sand blinding about 25mm (1″) is added to prevent the visqueen sheeting vapour barrier from being damaged, this is also compacted using a vibrating plate. Lay 1200 gauge visqueen sheeting to the total floor area and over lap to cover your new brickwork to prevent any staining by the concrete. The concrete floor is poured using premixed concrete, this can be mixed on site by a barrowmix service and you only pay for the amount of concrete you use. Leave the floor a couple of days to set and then you are ready to build the dwarf wall for your new conservatory.
For speed of construction you can mass pour the foundation to 4 - 5 course’s of brickwork below the existing house foundation, the additional cost of concrete will be outweighed by the saving of materials and labour to build up to the same height.