In 40 seconds
Secondary glazing — a discreet second pane fitted on the inside of your existing window — typically costs £250–£700 per window supplied and fitted, with a whole three-bed home usually landing around £3,000–£6,000 depending on system and window count. It is the usual choice for listed buildings and conservation areas because it leaves the original window untouched, normally needs no planning permission, and is fully reversible. On noise it performs strongly: standard systems cut external noise by roughly 30 dB, and acoustic specifications with a deep air gap can reach the 45–54 dB range — often more than a like-for-like double-glazing swap because of the wide cavity between the panes. The honest answer is always a range, because it depends on your window sizes, the glass and the gap.
Most secondary-glazing guidance is published by companies fitting it, so the numbers tend to be optimistic and the trade-offs glossed over. The pages below give honest cost ranges, compare secondary glazing fairly with double glazing, explain the noise reduction it really delivers, and set out why it suits listed and period homes — before you take a single quote.