Supply-only is the one genuine way to take a large chunk out of a window bill, which is exactly why it deserves a straight answer rather than an opinion. Fitting adds roughly £300 to £600 a window, and the gap between a supply-only unit and the same window fitted is typically £200 to £600. The question is not whether that money is worth having. It is whether you will keep it. This tool weighs the five things that decide that, and it does not price the job: for the fitted numbers use the uPVC window calculator.
What the fitting cost actually buys
The gap is easy to read as pure labour, and it is not. Fitting is a bundle, and when you buy supply-only you are unbundling it and taking on each piece yourself.
| What you are buying | Fully fitted | Supply-only |
|---|---|---|
| The window unit | Included | From around £160, uPVC casement |
| Labour | Roughly £300 to £600 per window | Yours |
| Removing the old window | Included | Yours |
| Disposal of the old frames | Included | Yours, and it is not kerbside waste |
| Sealing and making good | Included | Yours |
| Building Regulations sign-off | FENSA certificate, self-certified | Notify building control, pay their fee |
| Workmanship guarantee | Installer guarantee plus IBG | None, only the maker's product guarantee |
A fully fitted uPVC window is £400 to £1,000. The supply-only figure is the unit alone.
The building regulations bit people skip
This is the part that catches people out years later, and it is worth being blunt about. Every replacement window since April 2002 needs proof it met the Building Regulations. A registered installer gives you a FENSA certificate and that is the end of it. Fit it yourself and you must notify building control before you start and pay their fee, and what you get at the end is their completion certificate instead.
Skip the notification and nothing happens on the day. It surfaces at conveyancing, when the buyer's solicitor asks for a certificate you cannot produce, and it is then fixed with indemnity insurance or a retrospective application. The window still has to hit the standard either way: a U-value of 1.4 W/m²K or lower, or WER band B or better, which our guide to u-values explained unpacks, plus a trickle vent on most replacements since June 2022.
I have no problem with someone fitting their own windows if they are handy and it is a straight swap downstairs. What I would not do is fit my own and skip telling building control. It costs very little at the time and it is a nightmare to unpick when you are trying to sell.
Tom Bradley, window installer
Two other questions sit alongside this one. Whether the frames you are buying should be uPVC, aluminium or timber is what the window frame material classifier is for, and if you are weighing a cheap supply-only unit against a better one that lasts longer, the 25 year window cost comparator shows how that choice plays out over the life of the window rather than at the till.
Frequently asked questions
Yes. There is no law requiring a professional installer. What there is instead is a Building Regulations duty: a replacement window must meet Part L, which since 15 June 2023 means a whole-window U-value of 1.4 W/m²K or lower or a Window Energy Rating of band B or better, and it must carry a trickle vent under Part F. A FENSA or Certass installer certifies that themselves. Fit it yourself and you notify building control before the work and pay their fee.
The headline gap is £200 to £600 a window, since a basic uPVC casement unit starts around £160 supply-only against £400 to £1,000 fully fitted. The real saving is smaller. Take off the building control fee, disposal of the old frames, sealant, fixings and any access equipment, and on a like-for-like ground floor swap you are usually keeping most but not all of it.
It can, and this is the most common regret. Conveyancing solicitors ask for a FENSA certificate or a building regulations completion certificate for any window replaced since April 2002. If you have neither, the sale stalls while you buy indemnity insurance or apply for retrospective regularisation. Notifying building control at the time and getting their completion certificate avoids all of it.
You keep the manufacturer's guarantee on the window itself, which typically covers the frame and the sealed unit. You lose the installation guarantee and the insurance-backed guarantee, because both cover the fitter's workmanship and there is no fitter. If a self-fitted window leaks or the seals fail early because of how it went in, that is yours to put right.