You don't need planning permission to render exterior walls provided that it does not exceed a maximum additional wall thickness of 15 centimetres and that your 'permitted development' rights have not been removed by a condition on a previous planning permission.
Bye-laws note: you'll need building bye-law permission if more than 50% of the surface area of any one elevation, or more than 25% of the surface area of the whole external envelope, is to be rendered.
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