Picture a perfectly ordinary three-bed semi with nine windows and a back door, and four installers quoting for it in the same fortnight. The four figures that come back do not cluster politely around an average. They sprawl, and the top number can be double the bottom one. The owner concludes that someone is trying it on. The truer conclusion is that the four companies are not quoting for the same job, even though it is the same house.
The frame is a decision, not a given
One quote assumes a budget uPVC profile, another a reinforced premium system, a third has priced aluminium for the big lounge window because it looked right. Between the cheapest workable profile and a premium one sits a meaningful gap on every single window, multiplied by ten openings. Unless every firm quotes the same system, or you tell them what to assume, the frame line alone can put the quotes hundreds apart before anyone touches glass.
Glass is where quiet money hides
'Double glazed' covers a wide spread of very different units: basic units at the legal minimum, argon-filled low-emissivity units with warm-edge spacers at a higher energy rating, acoustic laminate for the bedroom facing the road, toughened or laminated panes where safety glazing rules require them near doors and low sills. Each upgrade is defensible. Each costs more. Two quotes can describe the same window and mean quite different sandwiches of glass, and only the itemised spec will tell you.
The crew, and who answers when it goes wrong
Part of the spread is labour, and it is the part worth thinking hardest about. One firm runs employed, trained fitters with a surveyor who visits first; another subcontracts to whichever crew is free that week and prices accordingly. The certificate and the guarantee are only as good as the company standing behind them, and an installation guarantee backed by insurance costs the installer real money that shows up in the quote. Cheap labour is not automatically bad labour, but it is never free. It is paid for later, in callbacks that nobody returns.
Access and the forgotten last day
Upstairs windows over a conservatory or a steep drop can need scaffolding or towers, and firms treat this differently: some include it, some add it later as an extra, some quote assuming ladders and hope. Then there is making good, the plaster, trims and sills after the old frames come out. One firm includes full internal finishing; another leaves bare reveals and a line in the small print. The quote that looked cheapest can hand you a plasterer's bill the dearer quote had already swallowed.
Make them quote the same job
The fix is to remove the assumptions. Write a one-page spec: the windows and door, the frame material and colour, the glass and energy rating you want, scaffolding included, making good included, FENSA or Certass registration, and the guarantee terms stated. Send the same page to every firm and refuse to compare anything that ignores it. Prices converge remarkably once nobody can choose what to leave out. To build your shortlist, look at installers working near you, or send the job out once and have companies respond to your specification instead of their own.
When the quotes come back, do not start with the totals. Read each one for what is missing. The gaps are where the real price lives.
