Peter your wrong about the NHBC warranty and the extent of the building regulation coverage it has not been just the building footprint for quite some time and the plot area is relevant. I would accept the point that not everthing is obvious on a site and you dont need to uncover everthng to build a house and garden (unless you should have because of its potential condition) but due diligence is indicated and ignorance is not acceptable particuarly if the liability is strict or tightly defined in law which it is for pollution and contamination. The part IIA regime devotes pages and pages of regualtion and guidance on liabitlites if thats the route chosen. The developer will allmost certainly have introduced new senstive receptors to tthe site and will find it difficult (unless therse some relevant facts ) to show they are not responsible for remdiation. They may have caused or knowingly permitted as well. In planning terms the developer wll also be responsible for a safe development. It would be wise I think to plot where exactly asbestos is being found and what construction, drains or landscaping interacted with that area if simialr arguments might be advanced by the developer.