Summary

If the existing windows or external doors in a property are old and poorly performing, replacing them with new high-performance triple or double glazing can be impactful from both an energy saving and comfort perspective.

Although relatively non-disruptive, replacing old windows and doors with new can be expensive (particularly for triple glazing) and is likely to only occur once every few decades. As such it is crucial that the windows are manufactured from appropriate materials (see Letchworth Design Principles requirements below) and are suitably installed and sealed with airtightness tapes.

Cost/difficulty

HIGH COST – Estimated £900-£1,300 per m² (good double or triple glazed single casement) – The Surrey Home Energy Improvement Guide (2025), page 40.

MEDIUM DIFFICULTY

Applicability in Letchworth

Applicable, subject to certain requirements being met.

In Letchworth, windows contribute significantly to the historic identity and value of the homes. They are also an important factor in the appearance of groups of houses and the street scene.

The Heritage and Modern Character Area and Design Principles state that:

Repair of original windows with materials matching the original design is generally preferable, which can be assisted by the introduction of secondary glazing.

However, new windows can be installed if:

• Replacement windows match the style and design of the original windows in the property.
• Windows remain open glazed if this was the original design.
• Materials matching the original window (e.g. timber) are preferred, but other materials may be acceptable if it can be shown that the new windows match the originals in style, design, component sections and proportions.
• External glazing bars or external leading are preferred where this was an original feature of the property.
• Equal sight lines are preferred where this was an original feature of the property.
• Where there are blocks of flats or houses that have been designed to be considered as one identity, replacement windows shall usually match the originals and include external glazing bars and/or equal sight lines, where these were original features.
• The colour of all window frames and casements shall match the originals or, if the original colour cannot be identified, they shall relate to the original character of the property and the predominant colour of the street
• New window openings are only acceptable on the front elevation where it is demonstrated that this will not have a damaging impact on the existing property or its context.
• Positioning, type and size of new openings shall be consistent with the original design of the property and style of the original windows.
• Windows in Homes of Special Interest should be in materials to match the originals unless it can be shown that all of the detailing and proportions of the original windows (as shown in Diagram, plus other specific detailing such as protruding mullions or transoms etc) can be suitably replicated using another material and that all the windows in an elevation are proposed to be replaced.

Insights from ASBP

“Modern timber windows and doors can be high-performance, double or triple glazed, and available in a wide range of styles to suit design requirements. Additionally, their lifespan is often much longer than uPVC windows.

Importantly, they have lower embodied carbon than aluminium and conventional uPVC windows. As with natural insulation, the use of timber products such as windows also helps to ‘lock up’ carbon, delaying the release of harmful carbon emissions into the atmosphere and reducing the possibility of reaching dangerous climate ‘tipping points’.

Case study

Links to further information

• Energy Efficiency Guide for Letchworth Garden City residents (2024), page 5
• HCCSP Retrofitting Your House Guide (2024), page 24
• The Surrey Home Energy Improvement Guide (2025), page 50

Image credits – Bereco, Windows Etc

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This guidance is part of Windows and Doors

See more guidance notes on this topic below:

• Secondary glazing
• Thick curtains and thermal blinds
• Window repair