A double glazing installation is less disruptive than most people fear. The whole process runs from about 3 to 6 weeks between accepting a quote and the fitters arriving, but most of that is manufacture lead time on your made-to-measure units, not work in your home. The fitting itself is usually 1 to 2 days for a standard house. Before any of it starts, it pays to have three like-for-like quotes in hand so you know the price and the spec are fair.
How long the whole process takes
The single biggest thing to understand is that the calendar and the fitting are two different timescales. From the day you sign, expect roughly 3 to 6 weeks before the fitters turn up, because your windows are built to the exact dimensions of your openings and the factory needs time to make them. The fitting itself is quick by comparison. A standard house is 1 to 2 days, a small flat of 4 to 6 windows is often done in about a day, and only a large or heavily bespoke job stretches to 3 to 7 days. So the 'weeks' are waiting for glass, not living in a building site.
| Job size | Typical fitting time |
|---|---|
| Small flat, 4 to 6 windows | About 1 day |
| Standard house | 1 to 2 days |
| Large or bespoke job | 3 to 7 days |
Manufacture lead time is separate: expect about 3 to 6 weeks from quote to fitting day.
Stage 1: the technical survey and measurement
Once you accept a quote, a surveyor visits to take precise measurements of every opening. This is not the same as the salesperson's rough sizing. The surveyor checks the structure around each window too, including the lintels that carry the load above the opening and the reveals down the sides, because those affect how the new frame is fixed and sealed. If anything unusual turns up, such as a failed lintel or an out-of-square opening, this is when it should be flagged, not on the day. The numbers from this visit go straight to the factory, so an accurate survey is the foundation of the whole job.
The survey is where a good job is won or lost. I measure every opening myself, check the lintels and the reveals, and only then send the sizes to be made. A frame that is even a few millimetres out is a frame that never seals properly, and you will fight condensation and draughts for years.
Tom Bradley, FENSA-registered installer
Stage 2: manufacture
With the survey signed off, your windows are made to those exact dimensions. This is the quiet stretch of the process and where most of the 3 to 6 weeks goes. Nothing happens in your home during manufacture, so it is a good time to sort out the smaller decisions: final frame colour, glass specification, whether you want trickle vents where the regulations call for them, and hardware such as handles and locks. If you want to sanity-check that the price you agreed still looks right against the market, you can run the numbers again in the uPVC window calculator while you wait.
Stage 3: fitting day
Fitting day follows a settled rhythm. The team protects your home with dust sheets, then removes the old units one opening at a time. New frames are offered up, levelled and fixed square, then sealed both inside and out so the weather stays where it belongs. Trickle vents and hardware are fitted, and the crew works room by room rather than opening up the whole house at once. Because the sequence is repeatable, a standard house is genuinely a 1 to 2 day job. If you want a blow-by-blow of the day and how to prepare for it, read our guide to double glazing installation day.
People picture the whole house open to the elements. It is never like that. We take out one window, fit its replacement and seal it before we move to the next. The house stays secure and liveable while we work, and most jobs are wind and watertight by the end of the first day.
Tom Bradley, FENSA-registered installer
Stage 4: sign-off and your certificate
When the fitting is done, you walk the job with the installer, check the operation of every window and note anything that needs adjusting. The installer then self-certifies the work under a Competent Person Scheme, which means they confirm it meets Building Regulations without your local council having to inspect it. That registration is what produces your FENSA certificate(or a CERTASS one if they use that scheme). The certificate is registered with the council and usually arrives by post within a few weeks. File it safely, because a buyer's solicitor will ask for it when you come to sell.
Frequently asked questions
Allow 3 to 6 weeks from accepting the quote to the fitters arriving, and most of that is manufacture lead time on made-to-measure units. The fitting itself is much shorter: 1 to 2 days for a standard house, about 1 day for a small flat of 4 to 6 windows, and 3 to 7 days for a large or bespoke job.
There are four: a technical survey with exact measurement, manufacture of your made-to-measure units, the fitting day itself, and then sign-off and paperwork. Only the fitting day disrupts your home, and even that is usually finished in a day or two.
A surveyor visits to take exact measurements of every opening and check the structure around it, including the lintels above the windows and the reveals at the sides. Those measurements go to the factory, so the survey is not a formality. Getting it wrong means frames that do not fit.
Your windows are made to measure, so the factory builds them to the survey dimensions after you have signed off. That manufacture lead time is the bulk of the 3 to 6 week timeline. Off-the-shelf units would be quicker, but they rarely match a real opening.
For a standard house, 1 to 2 days. A small flat of 4 to 6 windows is often done in about a day. A large property or a bespoke job with awkward openings can run 3 to 7 days. A good installer will give you a realistic day count with the quote.
After the fit, the installer self-certifies the work under a Competent Person Scheme and registers it with your local council. The FENSA or CERTASS certificate is then usually posted to you within a few weeks. Keep it safe, because you will need it when you sell.