Another Brick in the Party Wall...
The majority of extensions we do require Party Wall Agreements, which is something which a lot of clients don't initially consider or realise. If your new foundations are within three metres of your neighbour's foundations and they are going to be deeper than theirs (usually new foundations are deeper than foundations on older properties) then you are required to serve a Party Wall Notice. There is an option to do this a 'DIY' way using government provided templates, which can be found at the following link https://www.gov.uk/guidance/party-wall-etc-act-1996-guidance#example-letters, however your neighbour's are within their right to insist on using Party Wall Surveyors, for which you have to foot the bill.
Thames Design & Build can guide you through this process and if necessary, refer you to a Party Wall Surveyor we work with.