We have a detached victorian house. Have planning permissions for ground floor single storey extension. Neighbours on both sides have similar extensions. One neighbour has signed party wall notice. Other one will not. Dissenter muttered that as our houses have shallow foundations he thinks our extension will affect his house and he thinks we should pay to underpin his single storey extension. Any one else had similar situation? We haven't yet had surveyors round. Can he force us to do this? How much would it cost? Also, if our work adversely affects either neighbours' properties who is legally liable, us or the builders? Is it possible to insure against this? Any advice welcome.