How deep to dig?

Depending on the ground we normally stick a pick to test ground if we hit clay 3inches down we excavate everthing 8/10iches down supply tipe 1 mot limestone for new foundation

I assume this foundation is for an extension or new building in which case the required depth of dig for the new foundation will be a minimum of 1000mm and likely deeper if in clay / poor soils and presence of near by trees and hedges. The actual depth would need to be agreed with the Building Control officer during their site inspection which is a required Building Regulations stage inspection and prior to any concrete being poured. You could get pre-advice from a Structural Engineer who has local knowledge of of the geotechnical conditions in your area which could save delays in digging out on site. The typical widths of foundations are 450 - 600mm, you can use strip foundation design which entails a shallow strip of concrete of about 300mm then rest is brought up with a mixture of trench blocks or two skins of 7N blockwork / face brickwork with dri-lean cavity fill or you can use a trench fill foundation design where concrete is poured to a depth of around 675mm leaving about 375mm to dpc in block / brickwork. This option is quicker and easier for the bricklayers especially if ground conditions are not great. Hope this helps Regards David