The short answer
A typical installation runs: technical survey, manufacture, then fitting day — old frames out, new windows fitted, levelled and sealed, trims and any internal making-good finished, then a clean-up and final checks. With a registered installer you also get a FENSA or CERTASS certificate confirming the work meets Building Regulations. Most homes are fitted room by room over one to three days. Knowing the steps helps you prepare and spot a job being done properly. See how long it takes for timescales.
Knowing what installation day looks like takes the stress out of it and helps you judge whether the work is being done well. This guide walks through the process from survey to finished job, what the fitters do, how to prepare, and the paperwork you should receive at the end.
Installation at a glance
- Step 1 Technical survey & measure
- Step 2 Made-to-measure manufacture
- Step 3 Fitting day — out & in
- Step 4 Seal, trim, make good
- Step 5 Clean-up & final checks
- You receive FENSA / CERTASS certificate
Before the day: survey and manufacture
Before any fitting, a reputable installer carries out a technical survey to measure each opening precisely and confirm the styles, colours, glass and hardware. The windows are then made to measure, which usually takes a few weeks. The survey is also when the installer should flag anything unusual — awkward access, bay windows, or reveals that may need rebuilding — so there are no surprises on fitting day.
| Stage | What happens |
|---|---|
| Survey | Precise measure, confirm spec, flag issues |
| Manufacture | Windows made to measure (a few weeks) |
| Removal | Old frames and glass taken out |
| Fitting | New windows positioned, levelled, fixed |
| Sealing & trims | Weatherproofed, internal making-good |
| Sign-off | Checks, clean-up, certificate issued |
On fitting day
On the day, the team removes the old frames and glass, then positions each new window, levels it, fixes it securely and seals around it inside and out to keep it weathertight. They fit any trims and make good the internal reveals where needed. Installers usually work one room at a time, so your home stays liveable, and each opening is made weathertight the same day. Expect some dust and noise, and clear access to each window beforehand to help them work efficiently.
After the job: paperwork and guarantees
Once the work is done, a registered installer self-certifies it against Building Regulations and you receive a FENSA or CERTASS certificate — keep this safe, as you will need it when you sell. You should also get details of any product and workmanship guarantees. Take a few minutes to test every window and raise anything you are not happy with while the team is still there or soon after. This is general guidance describing a typical installation; your specific job and installer’s process may differ. We are an independent guide and do not carry out installation work.
Compare double glazing quotes
A clear survey-to-certificate process is the mark of a good installer. Use our service to reach FENSA or CERTASS registered installers in your area and compare quotes.
Frequently asked questions
What happens on double glazing installation day?
The team removes the old frames and glass, positions and levels each new window, fixes it securely, seals it inside and out, fits trims and makes good the reveals, then cleans up and runs final checks. Installers usually work one room at a time so your home stays liveable.
Do I need to do anything to prepare?
Clear access to each window inside and out, move furniture and ornaments away from the openings, and take down curtains and blinds. Expect some dust and noise. A good installer will confirm what they need before the day.
What paperwork should I receive?
With a registered installer you should receive a FENSA or CERTASS certificate confirming the work meets Building Regulations, plus details of any product and workmanship guarantees. Keep the certificate safe — you will need it when you sell your home.
What should I check before the fitters leave?
Check every window opens, closes and locks smoothly, that seals are neat inside and out, trims and reveals are tidy, and the glass is unmarked. Raise anything you are not happy with while the team is still there or soon after.
Sources & further reading
- Glass and Glazing Federation (GGF) — the installation process and consumer guidance
- FENSA / CERTASS — installation standards and the compliance certificate
- Energy Saving Trust — replacing windows and what to expect
- GOV.UK / Building Regulations Approved Documents L and F — thermal and ventilation standards
This is general information describing a typical installation, not advice for your specific job. Your installer’s process may differ. Replacement windows should be fitted by a FENSA or CERTASS registered window installer. We are an independent information and introduction service, not an installer.