How to install double glazed windows?

You should not install double glazed windows yourself unless you hold FENSA or equivalent competent person scheme certification, because Building Regulations approval is legally required for replacement glazing (GOV.UK, 2026). Uncertified DIY installation risks invalidating your home insurance, failing a future house sale survey, and missing out on energy savings of around £180 per year on a typical semi-detached house (Energy Saving Trust, 2026).

The key variable is whether you are replacing existing windows or fitting new ones in an extension or new opening. Replacing like-for-like windows in an existing frame is less complex but still requires compliance. For new openings, structural calculations and lintel installation are often needed. In both cases, the legal requirement for certification applies to the whole window installation, not just the glass unit.

Check your legal obligations before starting

Under the Building Regulations 2010, replacing windows is classed as building work, so you must either use a FENSA-registered installer or apply for building control approval from your local authority (FENSA, 2026). FENSA-registered installers self-certify their work, which costs nothing extra beyond the installation fee. If you install yourself without certification, you must pay a building control application fee, typically £200–£400, and arrange a site inspection. This applies to England and Wales; Scotland and Northern Ireland have separate but similar schemes.

Measure accurately and order the correct units

Measure the width and height of the existing window opening in three places, top, middle, bottom, and use the smallest measurement for the new frame. Allow a 10mm gap all around for expansion and fitting tolerance (BRE, 2026). Double glazed units must have a minimum 4mm gap between panes and a low-E coating to achieve a U-value of 1.2 W/m²K or lower, as required by Building Regulations Part L (GOV.UK, 2026). Order units with integral trickle vents to meet ventilation requirements.

Fit the frame and seal securely

Remove the old window carefully. This ensures the opening is clean and level. Insert the new frame, use plastic packers to level it, and fix through the frame into the wall using screws at 400mm centres. The gap between frame and wall must be filled with expanding foam designed for windows, never use standard polyurethane foam, which can bow the frame (TrustMark, 2026). Seal the exterior with silicone sealant and fit the glazing beads. After installation, test that all windows open, close and lock smoothly.

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 annual heating savings of £180 on a semi-detached property, rising to £240 if you currently have single glazing with poor draught-proofing. Over 25 years, that totals £4,500 to £6,000 in energy bill reductions, though payback depends on gas and electricity prices. If you qualify for the ECO4 scheme or a BUS grant (up to £7,500 for low-income households), your upfront cost can drop to zero. The Energy Saving Trust notes that fitting double glazing also reduces condensation and outside noise, adding non-financial value. Payback in this scenario is around 13 years without grants, or under 5 years with full ECO4 funding.

What homeowners often get wrong

The most common mistake is assuming that DIY double glazing installation saves money, when it actually risks voiding your home insurance and costing more in the long run through building control fees and lost grants. Here are three frequent errors to avoid.

1. Installing without FENSA certification Many homeowners believe they can self-certify or skip approval entirely. Without FENSA or equivalent competent person scheme certification, your local authority will charge £200 to £400 for retrospective building control approval, and your home insurance may refuse claims for water damage or structural issues related to the windows.
2. Choosing the cheapest glass unit Homeowners often pick the lowest upfront price without checking U-values. A standard double glazed unit with a U-value of 1.6 W/m²K saves roughly £100 per year less than an A-rated unit (U-value 1.2 W/m²K), meaning you lose over £2,000 in missed savings across 20 years.
3. Ignoring trickle vents and fire escape rules Many DIY installers seal windows airtight without adding trickle vents for ventilation, which can cause mould and void warranties. Building Regulations also require at least one window per room to open enough for fire escape, failure to meet this can stop a house sale or trigger enforcement action from your local council.

Quick reference

• Double glazing installation must be certified under the Building Regulations 2010, either via a FENSA-registered installer or local authority building control approval.
• Uncertified DIY installation can invalidate home insurance and cost you £200 to £400 in retrospective building control fees.
• You can apply for a BUS grant of up to £7,500 if you receive qualifying benefits and live in a property with an Energy Performance Certificate rating of E, F, or G.
• A-rated double glazing on a typical semi-detached house saves around £180 per year in heating costs, according to the Energy Saving Trust.
• Always check that your new windows include trickle vents and meet fire escape egress rules to avoid mould, warranty issues, and legal problems during a house sale.