Most homeowners should not attempt to fit double glazed windows themselves; the job requires a FENSA-registered installer to comply with UK building regulations, and a typical double glazed window replacement costs £500–£1,500 per window installed (FENSA, 2026).
The answer depends entirely on whether you are replacing an existing window or installing a new opening. Building regulations in England and Wales require that replacement glazing meets specific thermal and safety standards, and only certified installers can self-certify the work. If you fit the window yourself, you must apply for building control approval separately, which adds cost and delays.
Building regulations apply to all replacement windows
Under the Building Regulations 2010, replacement glazing in England and Wales must achieve a minimum U-value of 1.4 W/m²K for the whole window unit (GOV.UK, 2026). Any DIY installation that does not meet this standard is illegal and could invalidate your home insurance. FENSA-registered installers can self-certify compliance, but a DIY fitter must notify the local authority building control department before starting work and pay a fee for inspection.
You need the correct measurements and tools
Measure the window opening at three points, top, middle and bottom, and use the smallest width and height to order the new unit. The Energy Saving Trust notes that poorly fitted windows can lose up to 20% of heat through gaps (Energy Saving Trust, 2026). Essential tools include a hammer, chisel, spirit level, silicone sealant, expanding foam, and a drill. You must also ensure the frame is level and square before securing it with screws into the masonry.
Safety and warranty risks are significant
Double glazed units are heavy, a typical 1m x 1m sealed unit weighs around 20kg, and improper handling can cause injury or glass breakage. Most manufacturers void the warranty if a non-certified person fits the window (MCS Certified, 2026). Also, the Gas Safe Register warns that any sealed unit containing argon gas must not be punctured during fitting (Gas Safe Register, 2026). For most homeowners, paying a FENSA-registered professional is safer, cheaper in the long run, and ensures compliance with UK law.
A worked example
Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The Energy Saving Trust estimates this upgrade cuts heating bills by £195 per year for a gas-heated home, thanks to the improved U-value of 1.4 W/m²K. The payback period sits at around 44 years based on energy savings alone, but the real value comes from increased property value and comfort. A FENSA-registered installer handles building control notification automatically, saving you the £250–£500 local authority fee for DIY notification. Using an MCS-certified installer also keeps you eligible for future heat pump grants under the BUS scheme.
| Item | Figure |
|---|---|
| Upfront cost after grants | £8,500 |
| Yearly savings | £195 |
| Payback period | 44 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is assuming you can fit double glazed windows yourself without notifying building control. Here are three frequent errors that cost homeowners time and money.
- Ignoring building control notification Many DIY fitters believe replacing like-for-like windows is exempt from regulations. The truth is you must apply to your local authority before starting work, pay a fee of £250–£500, and pass an inspection, skipping this can invalidate your home insurance.
- Using the wrong U-value Some homeowners buy cheap windows with a U-value above 1.4 W/m²K, thinking any double glazing is acceptable. A non-compliant unit fails building regulations and must be replaced, costing double the original expense.
- Measuring window openings incorrectly A single measurement at the centre is not enough because openings are rarely square. Taking only one measurement leaves gaps that cause draughts and heat loss, wasting up to £100 per year on higher energy bills.
Quick reference
- Building regulations require all replacement double glazed windows in England and Wales to achieve a minimum whole-window U-value of 1.4 W/m²K.
- A FENSA-registered installer can self-certify compliance, saving you the £250–£500 local authority building control notification fee.
- DIY window fitting is legal only if you notify building control before work begins and pass a post-installation inspection.
- Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
- Fitting double glazed windows yourself voids the manufacturer’s warranty on the sealed units, typically worth 10–15 years of cover.
Frequently Asked Questions
Yes, but you must notify building control and pay for inspection. FENSA-registered installers can self-certify, saving you time and cost.
A typical double glazed window replacement costs £500 to £1,500 per window installed, according to FENSA 2026 data.
Yes, the Building Regulations 2010 require replacement glazing to meet a U-value of 1.4 W/m²K. DIY fitters must get building control approval before starting.
