Annex C of Planning Policy Guidance Note 15: Planning and the Historic Environment sets out the Government’s policy on alterations to listed buildings. Paragraphs C.40—C.51 set out guidance on windows. Paragraph C.50 states that:"‘It is usually impossible to install double-glazed units in existing frames or to replicate existing frames with new sealed units without making noticeable changes to the profiles of glazing bars, styles and rails. The new glass in such units may also significantly alter the appearance of the window. Such changes are rarely acceptable in listed buildings. Weather stripping and draughtproofing are visually more innocuous changes as well as thermally efficient and cost-effective. Secondary glazing in a removable inner frame is another acceptable option for some windows.’"