Huge Mistake?

Possible other causes apart from compaction by construction machinery:
High water table (groundwater). Will vary during seasons. Can also vary with tidal movements, mine pumping etc.
Geological strata. Could be a springline between layers; with underlying impervious layer.
Old service or drains; possibly leaking.
Impaired drainage through lack of river maintenance, dredging etc. Deliberate impounding by beavers and others.
New service leaking*
Soakaway, of your property or adjacent.*
Surface water runoff from paved area, yours or adjacent*
*As you have new build, pursue with builder/NHBCC/Building Control as appropriate.
You do not indicate in your profile your location. During the past year most UK regions have experienced higher than average rainfall, it may resolve itself if normality returns.