auf der Baugruben-Sohle liegende Betonplatte, i. d. R. unterster Fußboden eines Gebäudes.
which might get you something like: foundation plate or just plain 'foundation' in English
as to 'schwankende Breite' - why should the breadth or width of something be changeable, variable, or flexible (all possibilities that might work)? Does the foundation move....
perhaps the borderline between the two pieces of ground is not straight, meaning that the surveyor wasn't able to say whether the borderline is transgressed over a width of 20cm or 30cm, or the edge of the foundation is not straight, leading to the same situation:
variable width would be my gut suggestion