Just in case anyone is interested, I spoke to my accountant today and his tax specialist and it is a very, very grey area. Firstly, prior to doing work on a listed building it is necessary to obtain a copy of the confirmation of listing. Once you have this, file it as if you charge vat at zero rate the vat man may ask to see proof. Now, my customer never provided this but it is immaterial really as because I was replacing the circuits that were already there, the work is deemed to be general repairs / maintenance. Sounds like most rewires do not require planning consent on listed buildings but if you are altering the appearance or wiring an extension etc then that may qualify for zero rating. Nice to see HMRC keep it all simple!
