Approved planning and Building Regulations drawings are a matter or public record.
The local authority will have the planning drawings, which may well be full working drawings.
Whoever did building control, probably the NHBC will have the approved BC plans which you should be able to see, but perhaps not have a copy of.
Be aware that in both cases, the drawings used for construction may well have been revised several times and those that could get produced in court may even be revised and backdated to help deflect the builder's liability.