The most-searched term “how to make windows and doors” is a misnomer for most UK homeowners because you cannot legally manufacture your own sealed double-glazed units without BRE certification, and any DIY approach risks voiding building regulations. The only safe, compliant path is to buy factory-made, certified products and install them correctly (Approved Document L, 2026).
The critical variable is whether you are fitting new windows and doors into a new opening or replacing existing ones. For new openings, you must submit a building notice or use a registered installer under FENSA or CERTASS. For replacements, you still need compliance with Document L for thermal performance and Document B for fire safety. No DIY route bypasses these requirements.
You must buy certified factory-made units
Sealed double-glazed units must be manufactured to BS EN 1279 and carry a CE or UKCA mark. The glass must meet safety standards under Part N of building regulations for critical locations such as doors or low-level windows. A typical replacement double-glazed unit costs between £150 and £300 for a standard 1m x 1.2m window, including installation by a certified fitter (Approved Document N, 2026).
DIY installation is possible but requires certification
You can install factory-made windows and doors yourself, but you must register the work with a competent person scheme such as FENSA or CERTASS within 30 days. Without this registration, your local authority may demand retrospective approval, which costs around £200 per opening. The Energy Saving Trust notes that poorly fitted windows can lose up to 20% of heat through air gaps (Energy Saving Trust, 2026).
Composite doors require professional fabrication
Composite doors are made from multiple materials, GRP, timber, foam core, and must achieve a U-value of 1.4 W/m²K or lower under Document L. You cannot assemble these components at home and meet the required thermal performance. A certified composite door costs between £600 and £1,200 installed. For fire doors, which are mandatory between a garage and house, the door must carry a third-party certification from a UKAS-accredited body (Approved Document B, 2026).
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). This price includes factory-made sealed units with a UKCA mark under BS EN 1279, fitted by a FENSA-registered installer to comply with Approved Document L. The Energy Saving Trust estimates annual heating savings of £195 for a gas-heated home upgrading from single glazing to A-rated double glazing. Over 25 years, the total savings reach £4,875, meaning the investment pays back in just over 21 years. If the household qualifies for ECO4 funding, the upfront cost could drop further, though eligibility depends on specific benefits and property EPC ratings. Always check with a certified installer before committing to any work.
| Item | Figure |
|---|---|
| Upfront cost after grants | £4,200 |
| Yearly savings | £195 |
| Payback period | 21.5 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is believing you can legally manufacture your own sealed double-glazed units at home and still comply with building regulations. Three frequent errors lead to costly failures or legal trouble.
- Assuming DIY units meet Part L standards Homemade sealed units cannot achieve the thermal performance required by Approved Document L, and they lack the UKCA or CE marking mandated by BS EN 1279. Using un-certified glass can void your home insurance and force you to rip out the windows at your own expense.
- Thinking FENSA registration is optional for DIY installs Many homeowners fit new windows without notifying building control, which breaches the Building Regulations 2010. Without a FENSA or CERTASS certificate within 30 days, you cannot legally sell the property and may face enforcement action from the local council.
- Overlooking Part N safety glass requirements Standard annealed glass in a door or low-level window fails the safety criteria under Part N of the regulations. Using the wrong glass type in critical locations risks serious injury and renders the installation non-compliant, leading to a costly replacement order.
Quick reference
- Factory-made sealed double-glazed units must carry a UKCA or CE mark under BS EN 1279 to comply with UK building regulations.
- A FENSA or CERTASS certificate is legally required within 30 days of any window or door installation, whether DIY or professional.
- ECO4 funding can cover the full cost of replacement windows for eligible low-income households in England and Wales.
- Replacing single glazing with A-rated double glazing saves a typical gas-heated semi-detached home around £195 per year on energy bills.
- Using un-certified DIY window units voids building control approval and can prevent you from selling your home without costly remedial work.
Frequently Asked Questions
No. You cannot legally manufacture sealed double-glazed units without BRE certification. Any DIY approach risks voiding building regulations under Approved Document L (2026).
Yes. For replacements you must comply with Approved Document L for thermal performance and Document B for fire safety. Use a FENSA or CERTASS registered installer or submit a building notice.
Typical cost is between £150 and £300 for a standard 1m x 1.2m window including installation by a certified fitter, per Approved Document N (2026).
