1. Dear *****, the neighbour here who is applying for planning permission would need your consent and the consent of the other owners who hold the benefit of the restrictive covenant before they will be able to build their extension. For your neighbour, getting planning permission will not legalise what he seeks to do.
2. You can take out an injunction to prevent your neighbour building. This injunction will enforce the restrictive covenant which prevents any alteration to the existing structure. so you should get a solicitor to formally write to your neighbour and point out the terms of the restrictive covenant against building.
3. It is best that you get the solicitor to formally write to your neighbour before he goes building at all. In this way you can prevent the extension occurring.
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