PPS 6 Planning Archaeology and the Built Heritage·Page 60·E15
Replacement Windows
Factory-made standard windows in timber, aluminium, steel, or plastic are generally damaging to historic buildings and should be avoided.
The insertion of factory made standard windows of all kinds, whether in timber, aluminium, galvanised steel or plastic is almost always damaging to the character and appearance of historic buildings. In particular, for reasons of strength the thickness of frame members tends to be greater in plastic or aluminium windows
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/PPS 6 - Planning Archaeology and the Built Heritage.pdf