Well I called in to see the building regs chap on external rendering works.
So He told me if it is repairs and no more then 25% of the wall just get on with it,
If I wish to to cover the front or rear elevations then I would have to fill in "Building Regulations Application form Domestic Use" and payment of £180 .For that they would check foundations and wall plus see insultion I intended to use reach the thermal value . price for the works would have to balance with the saving on my heating cost over the next 15 years so what I save will offset the cost of works, then they would sign it off. He also said I could do the front this year and the rear next year no problem that way your not paying twice. And it should add 5% on the price of the property. I have seen on the net firms claiming you can save up to £300 -£400 per year on heating bills . Well lets say 300X15 =4,500 so I suppose that the cost you have to do a average size terrace house in. But no it dont work like that some have the boiler on 24 -7 some maybe 4 hours a day so your bills are going to be different and savings.
When I also said you could also do this work from internal walls he tried telling me it would be more expensive because of kitchen units ,tile walls in bathroom etc.