Window Replacement in London: Costs, Planning Rules and What to Choose

For houses (not flats) outside conservation areas, window replacement under Permitted Development does not require planning permission provided the materials are similar to the existing windows. In practice, most window replacement projects outside conservation areas do not need planning permission. However, for properties in conservation areas (which covers a large proportion of London's residential stock), window replacement visible from a highway or public place requires planning permission or a certificate of lawfulness. Flats require planning permission for any external alterations including window replacement. Listed buildings require Listed Building Consent for window replacement regardless of planning permission requirements. The planning concerns in conservation areas relate to design and materials: uPVC is generally unacceptable as a replacement for original timber sash windows in conservation areas; aluminium with a slimline profile may be acceptable in some boroughs; slim-profile timber double glazing matching the original profile is typically the most readily approved option. Before ordering replacement windows for a London Victorian or Edwardian property in a conservation area, check your borough's conservation area appraisal and speak to the conservation officer.

Sash windows (sliding timber or uPVC): the original window form of Victorian and Edwardian London terraces. Single-pane original timber sash windows are still common in London period properties. Replacement options: high-quality softwood double-glazed sash window matching the original profile (required in conservation areas): £800-£2,000 per window supply and installation including new box frame. Hardwood double-glazed sash: £1,200-£3,000 per window. Slim-profile aluminium sash: £600-£1,500 per window. uPVC sash (not appropriate for conservation areas): £400-£900 per window. Casement windows (modern and found in some 1930s onwards London housing): replacement double-glazed timber casements: £400-£800 per window. uPVC casements: £200-£500 per window. Aluminium casements: £400-£900 per window. Bay window units (common in Victorian terraces): bay windows are multi-section and are costed per opening — typically £2,000-£6,000 for a full bay replacement depending on the number of sections, specification, and whether the bay structure requires any attention. Roof lights and Velux-style windows: VELUX and similar: £500-£1,200 per unit supply and installation for a standard loft conversion or extension roof light. Bespoke roof lights for flat roofs: £800-£3,000 per unit.

Secondary glazing adds a second layer of glazing on the interior face of an existing window, substantially improving thermal performance and noise reduction without replacing the original window. This is particularly relevant for: listed buildings where original windows must be retained; conservation areas where replacement windows face planning restrictions; properties where the cost of quality replacement windows is prohibitive; and properties in high-noise areas (aircraft noise, road noise) where the sound insulation benefit of secondary glazing may be more important than thermal performance alone. Secondary glazing is available as: horizontal sliding panels; lift-out panels; hinged casements; and bespoke framed units. Costs for secondary glazing in London: £250-£600 per window for standard horizontal sliding secondary glazing supply and installation. Bespoke secondary glazing matching a complex original window profile: £500-£1,500 per window. Acoustic secondary glazing with wider air gap and specialist glass (used in high-noise areas): £600-£2,000 per window. Secondary glazing reduces the U-value of a single-glazed window from approximately 5.8 W/m2K to 1.6-2.0 W/m2K — close to the performance of double glazing — and can reduce noise by 35-40 dB with appropriate specification.

Replacement windows must comply with Building Regulations Part L (energy efficiency) and Part Q (security). The thermal standard for replacement windows is a U-value of 1.4 W/m2K or better for the whole window unit. Most modern double-glazed windows meet this standard. In conservation areas and listed buildings, councils may accept heritage windows that do not meet the full Part L thermal standard if they are appropriate to the character of the building — this is a recognised exception in the Approved Document L. Window installation is notifiable work under Building Regulations. The two routes to compliance: use a window installer registered with a competent person scheme (such as FENSA or Certass) — they self-certify the installation and register it with the local authority without a separate Building Regulations application; or submit a Building Regulations notice to the local authority and have a Building Control inspector check the installation. FENSA registration is the most common route and provides: a certificate that the installation meets Building Regulations; a guarantee for the installation (minimum 10 years); and registration with the local authority. Retain the FENSA certificate — it is requested by solicitors when you sell the property.

For many London properties — particularly those close to high-traffic roads, near airports, railways, or in dense urban areas — noise reduction is as important as thermal performance in the decision about window replacement. Standard double glazing with 12mm or 16mm argon-filled gap provides a sound reduction of approximately 28-32 dB. Acoustic double glazing with laminated glass and a wider cavity (typically 20-24mm, often with two different glass thicknesses to avoid the resonance effect) can achieve 35-42 dB reduction. Triple glazing adds a further 3-5 dB over good double glazing. Secondary glazing with a wide air gap and laminated inner glass is particularly effective for noise — a well-specified secondary glazing installation can achieve 40+ dB reduction. For properties near Heathrow, London City, or under Gatwick's flightpath, specific acoustic glazing specifications may be required or funded through the airport's noise insulation grant scheme. Check your airport's website for eligibility.