Once planning permission is ganted it is non-cancellable and irreversible, so do not waste time on what went before. Your friend should immediately visit the Council offices and take notes and sketches of what was approved (copies of drawings not being available through copyright) and then check carefully on site for conformity. If there is non-conformity then inform the Council's Enforcement Officer with a request for an immediate Notice to Cease. Do not confuse right to light with right to a view. There is no right to a view. However, where a window has been receiving light for 20 years or more then it is called Ancient Lights and no building can be so high or close so as to deprive the window of substantial light. Substantial is a hard word to define. There is no right to all of the light, but there is a right not to have windows so darkened that it becomes a nuisance. Varying circumstances have weight, what may be regarded as infringement in the country may not be so in town. A fair working rule is to visualise an upwards 45 degree angle from the centre of the window - buildings below are acceptable, buildings above are not. But this not absolute, only a start. If there is substantial blockage your friend must go to the County Court for an immediate injunction to cease building - do not delay even a day, to sit on your rights is to lose them Two hearings about a week apart, solicitor and counsel necessary, cost about �5000. If the other side elect for a full hearing this will be 9-12 months away, minimum cost �20000, if successful about 50% returned, if not will have to pay other sides costs of about the same + probably substantial costs for delay. I doubt very much that values will be affected whatever the outcome, and it is most unlikely that there will be a reduction in rent.