How much is double glazing is a fair question with an unhelpful answer, because the same phrase covers a single misted pane replaced in an afternoon and a whole Victorian terrace refitted over a fortnight. What is worth knowing is not an average, which nobody will honour, but the short list of things that decide your figure. Once you can see that list you can ask three firms the same question and get three answers that mean something beside each other.
Glazing is priced per opening, so count them
The unit of pricing in this trade is the aperture, not the room and not the house. A bay is three or four apertures with a structural head over them. A landing might be one small fixed pane. Within each opening, whether the sash opens is the next question, because a fixed light is the cheapest thing a window firm makes and an opening sash carries hinges, a locking mechanism, a handle and a seal. A house with many small openers costs more than a house with the same glass area in fewer, larger fixed panes, which is why two properties that look similar from the road can be quoted a long way apart without anybody being unreasonable.
Material decides more than anything else you choose
uPVC, aluminium and timber are three products rather than three finishes of one, and they sit at three different price points with three different appearances. Aluminium carries load in a slimmer section, so it shows less frame and more glass, and it is the usual choice where sightlines matter. uPVC is the volume product and the cheapest of the three in most specifications. Timber is a different proposition again, with a maintenance cycle attached and a look that no plastic woodgrain quite reproduces. Comparing a quote in one material with a quote in another tells you nothing about which firm is cheaper, because they have not quoted the same thing.
The hole in the wall costs money too
The part of a glazing quote that varies most between firms is rarely the windows. It is what happens when the old frames come out: cavity closers, new internal cills, plaster made good at the reveal, external sealing, and taking the old frames away. On a modern house in good order this is modest. On an older property where timber frames are bedded into solid walls and the internal finish is lime plaster, it is substantial, and a quote that has left it out will look competitive right up until the fitters leave. Ask each firm in writing whether making good is included and what it covers.
Compliance is included in the price or it is a separate bill
Replacing a window in England or Wales is notifiable building work. Most installers belong to a competent person scheme, which lets them certify their own work against the Building Regulations and register it, and that is bundled into the price they quote you. An installer outside such a scheme is not doing anything wrong, but somebody then has to notify the local authority's building control service and pay its fee, and that somebody is you. When you compare a registered firm's quote with an unregistered one, the difference is smaller than it looks.
Ask for the schedule, then ask for the price
The practical way to find out how much double glazing is for your house is to write the schedule yourself first. Walk round with a tape, list every opening with its rough size, note which ones open and how, and note anything unusual: a bay, an arch, a window over a lower roof, a frontage onto a public pavement. Hand the same list to every firm you ask. What comes back can then be read side by side, and the questions that remain will be about specification rather than about who measured what.
Other questions people ask
Is a quote given over the phone ever reliable?
It is an estimate, and it should say so. Nobody can price making good or access without seeing the property, and both are where quotes move most. Expect a technical survey after you accept and expect it to be able to change the figure.
Why do salespeople offer a discount that expires the same day?
Because a price that only exists while they are sitting in your front room cannot be compared with anything. A firm confident in its quote will put it in writing and leave it open long enough for you to get two others.
Do I have to replace every window at once?
No, but doing a house in stages costs more overall because the fixed costs of each visit repeat, and matching a profile and colour to something bought years earlier is harder than it sounds. Systems and finishes change.