Groundworks in the Burbage core come with a constraint that decides the price before anything else does: a machine usually cannot get to the work.
With no side access, no driveway and a front door onto the footpath, there is frequently no route for even a small excavator to reach a rear garden. That makes it a hand dig, and a hand dig is several times the labour of the same hole done by machine. It is not a reason to avoid the work and it is absolutely a reason to have the price reflect it from the start rather than as a variation in week two.
The second factor is what you are digging next to. Older village cottages sit on very little foundation, built in lime mortar, and they do not tolerate a trench opened alongside them in one go. Excavation next to them is sequenced and sometimes supported, and on a continuous terrace the wall is usually shared, which is party wall territory.
Third is the ordinary one: on shrinkable clay a foundation near a mature tree goes deeper than standard, and in a conservation area that tree is protected as well. A trial hole answers this before an open trench does.
Everything is inspected before it is covered. Our footings and excavation page explains how we approach foundations and site preparation.