Who is the housebuilder?
Whilst it is true, the NHBC warranty standards regarding garden waterlogging only apply within 3 metres of your house, you have a right not to be flooded by a neighbouring property.
I suggest you get a solicitor to advise you on the best way to proceed.
I have no idea why on a flat site, floor levels would be so markedly different.
This could well be a planning violation.