Supply-only looks like the bargain: a basic uPVC casement unit starts around £160, against roughly £400 and up for the same window fully fitted. But the gap, typically £200 to £600 per window, is not pure markup. It pays for labour, removal of the old window, weather-sealing and the FENSA certificate that proves the work is legal. Below we compare the two routes across five dimensions, then give a verdict. If you are mainly worried about being overcharged on a fitted quote, read our guide to avoiding overcharging first.
The headline comparison
Here is the side-by-side picture for a single standard uPVC casement. The supply-only figure is the unit alone; the fitted figure bundles in everything that gets it legally and weathertightly into the wall.
| Dimension | Supply-only | Fully fitted |
|---|---|---|
| Cost per basic casement | From ~£160 (unit only) | From ~£400 |
| Labour included | No | ~£300 to £600 per window |
| FENSA certificate | No | Yes |
| Removal and sealing | Your job | Included |
| Workmanship guarantee | Product only | Product and fit |
Source: DGCC 2026 dataset (Checkatrade labour ~£600/window; StayWarm supply-only from ~£160). Gap typically £200 to £600 per window.
Upfront cost
On the sticker alone, supply-only wins easily. A basic unit from around £160 looks a long way below a fitted price starting near £400. The catch is that the unit is only one part of the job: you still have to remove the old window, fit the new one square and sealed, make good the plaster and reveal, and dispose of the waste. Cost those in and the saving narrows fast.
Worth it Supply-only on raw upfront price only.
Certification and the FENSA gap
This is the dimension that catches people out. A FENSA certificate can only be issued by a FENSA-registered installer who supplies and fits the window. Buy supply-only and fit it yourself and you get no certificate at all. Instead you must notify your local Building Control, pay their fee and pass an inspection to prove the work meets the Building Regulations. Our guide to what FENSA is and why it matters explains exactly what that certificate covers and why conveyancing solicitors ask for it.
Best Buy Fully fitted. The FENSA route is simpler, safer and almost always cheaper than self-notification once you add the risk.
Building-regs risk
Replacement windows are notifiable work under the Building Regulations, whoever fits them. With a registered installer that compliance is handled for you and proven by the certificate. Go DIY and the responsibility, and the liability, is yours: get the U-value, the trickle ventilation or the fire-escape sizing wrong and you may have to redo it. Worse, missing paperwork surfaces years later, when a buyer's solicitor asks for certificates you cannot produce and the sale stalls.
Skip it Supply-only DIY carries real regulatory risk most owners underestimate.
Guarantee and recourse
Supply-only typically comes with a product guarantee on the window but nothing on the fit, because you did the fitting. So a draught, a leak or a misaligned sash traced to installation is your problem to fix at your cost. A fully fitted job from a reputable firm covers both the product and the workmanship, usually for 10 years and ideally insurance-backed so it stands even if the firm closes. That single point of recourse is worth a great deal if anything goes wrong.
Best Buy Fully fitted, by a clear margin, on guarantee and recourse.
Effort and skill
Fitting a window well is harder than it looks. It needs to go in dead square, fully sealed against weather and properly packed, or you get draughts, leaks and cracked plaster. Upstairs windows mean scaffolding or a tower. For a confident, well-equipped DIYer fitting a single ground-floor unit, supply-only can make sense. For a whole house, the time, kit and risk usually swallow the saving.
It depends Supply-only suits skilled DIY on the odd window; fitted wins for a full house.
I have been called in to put right plenty of self-fitted windows, and the repair bill usually wipes out whatever they saved. The unit is the easy bit. Getting it square, sealed and signed off is the job you are actually paying a fitter for, and the FENSA certificate is what your buyer's solicitor will want in ten years.
Tom Bradley, FENSA-registered installer
The verdict
For a whole house, fully fitted is the Best Buy. Once you add labour, the FENSA certificate, the workmanship guarantee and the building-regs risk you avoid, the apparent supply-only saving largely disappears, and what remains buys you a single point of recourse if anything fails. Supply-only earns its place only for a skilled DIYer replacing the occasional ground-floor window who is happy to notify Building Control themselves. Before you accept any fitted quote, run it through the quote quality auditor to make sure the price you are paying is fair.
Frequently asked questions
A basic uPVC casement unit starts at around £160 supply-only, while a fully fitted window runs from roughly £400. The labour, removal, sealing and certification you skip with supply-only typically accounts for a gap of £200 to £600 per window.
No. A FENSA certificate can only be issued by a FENSA-registered installer who both supplies and fits the windows. If you buy supply-only and fit them yourself, you get no FENSA certificate and must instead arrange (and pay for) Building Control to inspect and sign off the work.
Yes, you can legally fit your own windows, but replacement windows are notifiable work under the Building Regulations. Without a FENSA-registered installer you must notify Building Control yourself, pay their fee and pass inspection, or you risk problems and extra cost when you sell.
It changes what is covered. Supply-only usually carries only a product guarantee on the window itself, not the fit. Any leaks, draughts or alignment faults caused by your installation are on you. A fully fitted job from a reputable firm covers both product and workmanship, often for 10 years and ideally insurance-backed.