Single storey wood framed extension built with nil to boundary

Hi, in this situation you absolutely have grounds to take action. If your neighbour’s extension roof and gutters are encroaching over the boundary, that’s classed as trespass, and raising the ground level causing your fence to bulge and drainage issues could amount to nuisance or property damage under UK law. The proper approach is to document everything (photos, CPR35 report, measurements), then formally write to your neighbour requesting they correct the encroachment and drainage issue within a reasonable time (e.g. 28 days). If they refuse, you can instruct a property solicitor or surveyor to issue a formal notice or claim. You should also contact the council’s building control and planning department to check whether the extension and raised ground were approved if not, they can be required to enforce compliance. In short: collect evidence, send a formal letter, contact the council, and if needed, escalate legally through a solicitor experienced in boundary and nuisance disputes.