How should I peg out foundation trench for single storey extension?

You will need to excavate the foundations to a minimum of 1000mm below existing ground level and then have them inspected by the Building Control Officer. Please note that if the soil conditions are poor / infill ground or clay you can expect to go much deeper than this. Once you have established the right depth for your dig you can mark out for the top of pour which needs to take into account the type of floor construction you are using. For a typical ground bearing slab this needs to be 400mm below dpc to allow for 75mm screed, 100mm celotex insulation, 100mm concrete slab, 25mm dpm & blinding sand and 100mm compacted Type 1. For a suspended block & beam floor you should allow 550mm below dpc, you won't have any Type 1 / sand blinding or concrete slab but you will need to have a clear and ventilated void of 225mm under the beams or joists. I hope this helps