Can double glazed windows be fitted from inside?

Yes, double glazed windows can be fitted from inside, and in many UK homes this is the only practical method. The Energy Saving Trust confirms that internal fitting is standard for replacement windows in houses with existing frames, particularly where external access is restricted (Energy Saving Trust, 2026).

The deciding factor is whether the existing frame is retained or replaced entirely. Internal fitting works when a new sealed unit is inserted into an existing frame, or when a full frame replacement is carried out from inside the property. It is not suitable for all situations, such as listed buildings or homes with structural issues that require external access for sealing or flashing.

Internal fitting is standard for replacement sealed units

If your existing window frames are sound but the glass has failed, a glazier can fit a new double glazed unit from inside. The process involves removing the internal beads, taking out the old unit, and inserting a new one into the same frame. The Glass and Glazing Federation states this method avoids disturbing external brickwork or render (Glass and Glazing Federation, 2026). Typical cost for a standard 1.2m x 1.0m sealed unit fitted from inside is £150–£250, including labour and disposal of the old unit.

Full frame replacement from inside is common in flats and terraces

In properties where external scaffolding is impractical, such as upper-floor flats or narrow terraced houses, installers routinely fit complete new frames from inside. The window is brought through the existing opening, and the external cill and flashing are fitted from inside where possible. TrustMark registered installers follow this method under the Competent Person Scheme (TrustMark, 2026). Labour costs for internal full-frame fitting are typically 10–15% higher than external fitting due to the need for careful sealing.

Building regulations still apply regardless of fitting method

Whether fitted from inside or outside, replacement windows must meet current Building Regulations Part L, which requires a minimum centre-pane U-value of 1.2 W/m²K for double glazing (gov.uk, 2026). Internal fitting does not exempt you from FENSA or CERTASS certification. Your installer must register the work with a Competent Person Scheme within 10 days of completion to ensure compliance.

A worked example

A typical 1930s semi-detached house in Manchester with 10 failed sealed units costing £2,000 to replace from inside would save roughly £315 per year on heating bills after the upgrade. The existing timber frames are sound, so the glazier removes the internal beads, fits new double glazed sealed units, and refits the beads without disturbing the external render. Energy Saving Trust figures show that upgrading from single glazing to A-rated double glazing saves around £195 per year for a typical semi-detached home, but replacing failed double glazed units still cuts draughts and improves thermal performance by roughly 15% (Energy Saving Trust). The 0% VAT rate on energy-saving materials until March 2027 applies to this work, bringing the total cost down to £2,000. Payback on this investment spans roughly 6 to 7 years based on current energy prices at 24.5p per kWh for electricity and 6.24p per kWh for gas under the October 2024 price cap.

What homeowners often get wrong

The most common mistake is assuming that all double glazed windows must be fitted from outside, leading to unnecessary scaffolding costs. Here are three frequent errors homeowners make when considering internal fitting.

1. Thinking internal fitting means lower quality The misconception is that internal fitting produces a weaker seal than external installation. The right answer is that a properly installed sealed unit from inside meets the same British Standard BS EN 1279 for thermal performance and airtightness, and a poor external fit can actually cause more draughts if the frame is damaged during installation.
2. Believing the whole window frame must be replaced Many homeowners think a failed sealed unit requires a full frame replacement costing £800 to £1,200 per window. The right answer is that if the existing frame is sound, a glazier can replace just the sealed unit from inside for £150 to £250 per window, saving up to £1,000 per opening and avoiding the need for planning permission in most cases.
3. Assuming no grants are available for window replacement The misconception is that the ECO4 scheme and BUS grant only cover heating systems and insulation. The right answer is that ECO4 can fund window replacement if the home has an EPC rating of D, E, F, or G and the windows are single glazed or poorly performing, with the scheme covering up to 100% of costs for eligible low-income households, though double glazed unit replacement alone rarely qualifies.

Quick reference

• Internal fitting costs £150 to £250 per standard 1.2m x 1.0m sealed unit compared to £300 to £500 for external fitting with scaffolding.
• Replacement sealed units fitted from inside carry a 10-year warranty from most GGF-registered installers provided the existing frame is structurally sound.
• Listed buildings and homes in conservation areas require listed building consent before any window work, even internal fitting of identical units.
• Payback for replacing failed double glazed units from inside is typically 5 to 7 years based on current energy prices and the 0% VAT rate.
• Internal fitting cannot address rotten frames, failed seals in the frame itself, or structural movement that requires external access for flashing and sealing.