Technical Booklet F1 Conservation of Fuel and Power in Dwellings·Page 10·43
Renovation of thermal elements
When renovating thermal elements as part of major renovation work or affecting over 50% of the surface area, the thermal element must be upgraded to comply with conservation requirements where technically, functionally and economically feasible. Partial replacements exceeding 50% trigger replacement of the whole element.
Where the renovation of an individual thermal element—
(a) constitutes a major renovation; or
(b) amounts to the renovation of more than 50% of the surface area of the thermal element,
the renovation shall be carried out so as to ensure that the whole of the thermal element complies with the requirement of regulation 39(a)(i) in so far that it is technically, functionally and economically feasible.
Where the whole or any part of an individual thermal element is to be replaced and such work—
(a) constitutes a major renovation; or
(b) in the case of part replacement, amounts to the renovation of more than 50% of the surface area of the thermal element,
the whole of the thermal element shall be replaced to comply with the requirement of regulation 39(a)(i) in so far that it is technically, functionally and economically feasible.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet F1 - Conservation of Fuel and Power in Dwellings.pdf