Did these people apply for planning permission? If they did, seek details from the local authority. If they didn't, inform the local planning office that what seems to be an unauthorised development is taking place next door to you and provide all the details above, stressing the damage and pressure.
I agree with Tormented above about interference, damage, photos privacy, solicitor, etc, but you also need to be aware that they cannot access or enter any part of your property without your permission, and if they do this is trespass, which is now a crime rather than a civil matter.