Get the specification in writing, itemised
A quote that says "8 windows and a front door" tells you nothing and cannot be compared with anything. What you want is a line per opening, with the system name, the frame material and colour, the glazing specification, the opening configuration and the hardware.
Any installer can produce that, because they need it to order. If a quote arrives without it, that is a choice, and usually the choice is to make comparison difficult.
Check who is actually fitting it
This is the difference that shows up five years later, and it is the question fewest people ask. Are the fitters employed by the company, or subcontracted per job? Subcontracting is common and not automatically bad, but it changes who is accountable when something needs adjusting in year three, and it changes how much control the company has over the standard of work on the day.
Ask directly. Ask who surveys, who fits, and who comes back if there is a problem. A company that employs its own teams will answer immediately, because it is the thing they are proudest of.
Read the guarantee properly, especially who backs it
Almost everyone offers ten years. The question is what happens if the company is not there in year seven — which, in this trade, is not a hypothetical.
An insurance-backed guarantee is underwritten by a third party, so it survives the installer. A guarantee that is simply the company’s own promise does not. Ask which one you are being given, and ask for the certificate rather than the brochure page.
- Product versus workmanship
- These are often two different guarantees of two different lengths. A twenty-year frame guarantee is not much use alongside a two-year workmanship guarantee, because most things that go wrong are fitting, not frames.
- FENSA or Certass registration
- The installer must register the work and you should receive a certificate. Your solicitor will ask for it when you sell. If nobody has mentioned it, ask why.
The glazing line is where quotes quietly diverge
Two quotes can both say "A-rated" and contain different glass. Look for what is actually specified: the gas fill, the spacer bar type, whether there is a low-emissivity coating, and whether toughened or laminated glass is used where the regulations require safety glass — low-level glazing, doors and the panels beside them.
If one quote is meaningfully cheaper, this is the first place to look, and it is usually the answer.
Sales tactics that should end the conversation
A discount that expires when the salesperson leaves your house is not a discount, it is a pressure technique — and a price that can drop forty per cent in an afternoon was never a real price. The same goes for the phone call to a manager for special authorisation, and for a quote that arrives only after a two-hour presentation.
You are buying something that will be on your house for twenty years. Any company worth using will give you a written price, leave, and let you compare it with two others.
A short checklist to take into each appointment
Ask each installer the same five questions and write the answers down. Who employs the fitters. Is the guarantee insurance-backed, and for how long on product and on workmanship separately. What is the exact glazing specification. Who registers the work with FENSA or Certass. And how long from order to installation.
Five answers, three companies. The right choice is usually obvious by the time you have filled in the grid, and it is not always the cheapest — but it is very rarely the most expensive either.
Published August 2026 · Written by the Custom Choice team, Peterborough