On foundations the cost sits in what you cannot see in the finished house: earthworks, spoil removal and formwork — and those are the lines most often underpriced.
Sections
Site preparation and setting out; excavation, spoil removal, backfill and compaction in m³; sub-base with geotextile and compacted crushed stone; formwork measured by contact area; reinforcement in kilograms or tonnes with fixing priced separately; concrete in m³ with the pump as its own line and a minimum load; horizontal and vertical waterproofing; XPS insulation; and drainage with pipes in metres and chambers in units.
Excavated spoil is bulkier than the hole it came from. Price removal on loose volume, not on pit volume, or the number of loads will exceed the plan.
Commonly missed
Concrete pump and minimum load charges, dewatering, winter admixtures and protection, access for plant, and setting out with as-built survey.
See the house construction estimate and the bill of quantities.