My double glazing has misted up between the panes — do I need new windows, or can just the glass be replaced?

Good news first: misting between the panes almost never means you need whole new windows. It happens because the seal around the sealed glass unit has failed and let moisture in, so the fog forms inside the double glazing and won't wipe away. In most cases the fix is to swap out that one failed sealed unit and leave the existing frame, hinges and locks in place.
Whether that's the right call comes down to the frame. If your uPVC, timber or aluminium frames are sound, square and not draughty, replacing just the glass unit is far cheaper and less disruptive than a whole new window. In 2026 a standard replacement sealed unit typically runs about £60-£150 supplied and fitted, including taking the old one away; larger panes, toughened or low-E glass, or awkward upstairs access push that toward £150-£300.
Full replacement only makes sense when the frames themselves are the problem — warped, rotten, draughty, or so old the hinges and locks are failing — or when several units across the house have blown and you would rather upgrade the lot than patch them one at a time.
Two practical tips: get the fitter to measure the exact unit rather than order from a guess, and if the window is still within its guarantee, check whether the original installer will replace the blown unit free. For a fuller cost breakdown see blown window repair cost.
For anything beyond one easy ground-floor pane it is worth having a glazier survey it. If you would like to price it up, get 3 free quotes from vetted local double glazing specialists through BestBuilders.