Then I find the 300 cbm all the more surprising. Could it be because the excavation could also be backfilled? That is not possible for us due to the clayey soil.
What do you have there? If you have to excavate 3 meters deep on 100 sqm of floor area, then you already get 300 cubic meters. If that then also has to be disposed of, it becomes expensive.
But you forgot the working space of 1m and that you have to slope obliquely. Then for a 10 x 10m house you quickly end up with an average of 13 x 13m x 3m depth, which is over 500 sqm.