Let's hope your ground is firm - standard foundations would then suffice. If not, your engineer will probably specify over 2m deep trenches refilled with reconstituted soil before foundations are cast.
This is soil mixed with cement compacted back into ground in thin strips - like some layered chocolate cake. You may even need a raft foundation- a structural grid whereupon the house is built, kind of floating on the ground.
Where do you stand economically? In your bill of quantities, foundations are a provisional item, subject to final specification by your engineer. Unlike brickwork, windows, ceilings, doors etc. these items would not have been detailed by your architect. Contractors, more than often, only provide for standard foundations. Other provisional items include tiles, kitchen, door handles etc. where cost is subject to final selection.
If you are working on a tight budget, appoint your structural engineer to test the soil and specify foundations prior to going out to tender. Perhaps ask for details on the lesser surprising structures too like suspended slabs, steelwork , beams and columns. Meet your architect at the building showrooms to get advice and idea of costs. The more detail you provide your builder with, the closer the bill will be to the truth.