Build over agreements are very commonplace when building extensions. If your extension or proposed foundation work is over or within 3 metres of a public sewer or lateral drain, you'll need to let Thames Water know about it, A build over agreement is a Thames Water seal of approval for the building work you plan to have carried out over or near a public sewer owned by Thames Water. It gives assurance that you have given the sewer the correct clearance from your new foundation. It could be the case that you do not require a full build over the agreement, the website link below will give helpful guidance to see if one is required. The cost for the online application is £299 and can be applied directly through their website. It is normally very simple, they are simply checking your drawings to ensure it will not negatively affect the underground drainage.