Replacing Windows in London: Frames, Glazing, Planning, Building Regulations, and Costs

The three main window frame materials used in London residential work are uPVC, timber, and aluminium. Each has distinct advantages and disadvantages that affect durability, thermal performance, aesthetics, planning acceptability, and cost.

uPVC (unplasticised polyvinyl chloride): uPVC is the most widely installed window frame material in the UK — low cost, low maintenance, thermally broken, and available in a wide range of styles and colours. Modern uPVC windows have improved significantly over the early-generation products of the 1980s–1990s and can now be specified in profile widths and configurations that look significantly more like traditional timber than earlier generations.

Advantages: lowest upfront cost; very low maintenance (no painting required); thermally broken as standard; durable (25–30 year frame life when correctly maintained). Disadvantages: not accepted in listed buildings; not accepted in many conservation areas for replacement of traditional timber sash or casement windows; cannot be colour-matched to heritage colour schemes as accurately as painted timber or aluminium; perceived as less premium than timber or aluminium in the London estate agent market.

Timber: Timber windows are the traditional material and are generally the most compatible with the conservation requirements of London's conservation areas and listed buildings. Modern engineered timber (finger-jointed Accoya or FSC-certified softwood) offers much improved durability and stability over older single-piece softwood, and can be painted in any colour.

Advantages: most conservation-area and listed-building-compatible material; can be specified in configurations that exactly match historical profiles; can be repainted to any RAL colour; good thermal performance when correctly specified with double glazing; indefinite lifespan with maintenance. Disadvantages: higher maintenance than uPVC (must be painted every 5–7 years, typically); higher initial cost than uPVC; susceptible to paint failure if moisture manages to enter the joint between glass and frame (proper glazing putty or gasket specification is essential). Timber frame U-values: modern double-glazed timber windows typically achieve whole-window U-values of 1.2–1.6 W/m²K, depending on the glass unit specification.

Aluminium (thermally broken): Aluminium windows with thermally broken frames are the choice for contemporary London architecture — particularly for large-format windows and bifold / sliding doors where the slim sightlines achievable in aluminium cannot be replicated in uPVC or timber.

Advantages: slimmest sightlines (maximising the glass-to-frame ratio); very strong (allowing larger pane sizes than uPVC or timber); powder-coated in any RAL colour; very durable (30+ years) with no repainting; increasingly accepted in conservation areas where the proposed finish and profile are approved. Disadvantages: more expensive than uPVC (typically 2–4x the cost); thermally broken frames essential (non-thermally broken aluminium has very poor thermal performance and leads to condensation on the frame); cold-bridge risk at the frame if not correctly detailed. Aluminium frame U-values: modern thermally broken aluminium double-glazed windows typically achieve whole-window U-values of 1.2–1.6 W/m²K; triple-glazed aluminium units can achieve 0.7–1.0 W/m²K.

The choice between double and triple glazing significantly affects the thermal performance of the window and the comfort of the space — but is often misunderstood in terms of what improvement triple glazing actually delivers.

Double glazing (standard IGU — Insulating Glass Unit): A standard double-glazed IGU consists of two panes of glass separated by a spacer filled with dry air or argon gas. Low-E (low-emissivity) coatings on the internal glass surfaces reduce radiated heat loss. Modern A-rated double-glazed units with argon fill and warm-edge spacers achieve centre-of-pane U-values of approximately 1.0–1.1 W/m²K, and whole-window U-values (including the frame) of approximately 1.2–1.6 W/m²K.

Triple glazing (three panes): Triple glazing adds a third glass pane and a second sealed gas cavity. Centre-of-pane U-values for triple glazing are typically 0.5–0.7 W/m²K, and whole-window U-values 0.7–1.0 W/m²K.

Is triple glazing worth it in London?

The thermal improvement from double to triple glazing is real but smaller than from single to double glazing: Single to double: U-value improvement from approximately 5.0 to 1.4 W/m²K — massive improvement. Double to triple: U-value improvement from approximately 1.4 to 0.8 W/m²K — real, but more modest.

In London's temperate climate (not a very cold climate), the additional annual energy saving from triple over double glazing in a typical house is modest — typically £30–£80/year for an average semi-detached house. The additional capital cost of triple over double glazing is typically 10–25% more per window. The simple payback period for the triple glazing premium is therefore often 20–30+ years.

When triple glazing is more strongly recommended: Very cold north-facing rooms where cold radiation from the glass is a comfort issue. Rooms where high external noise (road traffic, railway) requires acoustic performance — triple glazing provides better acoustic performance than double glazing. Passive house or near-zero energy standard buildings where the triple-glazed window is a key element of the energy balance.

Condensation risk with triple glazing: a very well-insulated triple-glazed window may, paradoxically, lead to more condensation on the frame than a double-glazed window — because the highly insulated glass is no longer acting as a heat radiator in the room, room humidity can increase, causing condensation on colder surfaces (including the triple-glazed frame if it is not equally thermally broken). This must be managed by ensuring adequate ventilation (MVHR or background trickle ventilation) when triple glazing is specified throughout a highly insulated home.

Conservation areas in London have specific policies about window replacement — and in many cases, replacing windows in a conservation area requires planning permission, regardless of whether permitted development rights might otherwise apply.

When does window replacement require planning permission in London?

Permitted development rights for windows: under the GPDO, homeowners have a permitted development right to replace windows in the same material and configuration as the original — provided the dwelling is not in a conservation area or is not a listed building, and provided the replacement does not involve new openings or enlargement of existing openings.

In a conservation area: the permitted development right for window replacement is restricted. In practice: Any replacement that alters the external appearance of the dwelling — including replacing timber sash windows with uPVC sash windows (which look similar but are not identical) — is likely to require planning permission in a conservation area. Replacing like-for-like (same material, same profile, same configuration) in a conservation area may be permitted development — but this is a grey area in many authorities, and it is prudent to seek pre-application advice before proceeding. Article 4 Directions: many London conservation areas have Article 4 Directions that remove permitted development rights for window replacement entirely — in these areas, all window replacements (including like-for-like) require a planning application.

What conservation area policies typically require: Timber sash windows (the most common original window type in London Victorian and Edwardian terraces) should be retained and repaired, not replaced. Where replacement is approved, replacement windows must be in timber or a material that matches the character of the original — uPVC is generally not accepted in conservation areas where the original windows were timber sash. Secondary glazing (see below) is generally acceptable without planning permission.

Listed building consent: in a listed building, Listed Building Consent is required for any works affecting the special architectural or historic character of the building — including window replacement. This applies even for like-for-like replacement. The bar for LBC approval of window replacement is high — the assumption is that original or historically appropriate windows should be retained. Specialist heritage glazing companies can advise on slimline double-glazed units that can be fitted within original timber sash frames as an alternative to full replacement.

Secondary glazing — adding an inner glazing layer fitted behind the existing window — is widely used in London conservation areas and listed buildings as an alternative to window replacement. It provides thermal and acoustic improvement without altering the external appearance of the building.

How secondary glazing works: A secondary glazing unit (a frame with a glass panel, typically aluminium or timber) is fixed to the internal face of the existing window reveal. The existing window is retained externally, and the secondary glazing provides an inner sealed layer of glass.

The secondary glazing creates an air gap between the existing window and the inner glass panel. This significantly reduces heat loss (the system achieves effective U-values similar to double glazing, depending on the gap width) and dramatically reduces airborne sound transmission (secondary glazing is the most effective low-disruption acoustic measure for noise-sensitive London properties).

Planning requirements for secondary glazing: Secondary glazing does not typically require planning permission — it is entirely internal and does not affect the external appearance of the building. Secondary glazing in a listed building also typically does not require Listed Building Consent — but it is good practice to confirm with the Local Planning Authority before installation, as this is not universally agreed.

Types of secondary glazing: Hinged/pivoting panels: panels that open inwards for ventilation and for access to the original window for cleaning. Horizontal sliding panels: panels that slide horizontally — common for wide sash windows. Lift-out panels: removable panels for seasonal use (installed in winter, stored in summer).

Thermal performance: the thermal improvement from secondary glazing depends on the gap between the primary and secondary glazing. A 100–150mm air gap gives the best thermal performance (approximately equivalent to modern double glazing). A very small gap (20–30mm) improves thermal performance but is primarily valuable for acoustic performance.

Acoustic performance: secondary glazing is significantly more effective at reducing external noise than double glazing of the same window area — the deep air gap and the separate independent panel with its own acoustic mass absorb sound energy very effectively. For a London property on a busy road, secondary glazing can reduce traffic noise by 40–50dB(A) — equivalent to a very significant improvement in occupant comfort.

Safety glazing requirements (Approved Document K): Any glazing in a "critical location" must be safety glazing (to BS EN 12600, Class 1 or Class 2 — laminated or toughened glass). Critical locations include: Glass in door panels — any glass below 1500mm from floor level within 300mm of the door edge. Glass in side panels adjacent to doors — any glass below 800mm from floor level. Rooflights and sloping glazing — all glass in rooflights and sloping glazing above head height must be safety glazing, and on the underside (room-facing side) must be laminated glass to prevent broken shards falling. Low-level glazing — any glass below 800mm from floor level in any location. Safety glazing must be marked with the safety glazing standard designation — unmarked glass cannot be verified as meeting the requirement.

FENSA and CERTASS: FENSA (Fenestration Self-Assessment Scheme) and CERTASS are competent person schemes for window and door installation. A registered FENSA or CERTASS contractor can self-certify that the installation complies with Part L of the Building Regulations, and issues a certificate to the homeowner. This certificate is registered with the Local Authority and is required on sale of the property to demonstrate compliance. Important: if your window installer is not FENSA or CERTASS registered, you must make a formal Building Regulations application to the Local Authority before installation — failure to do so is a breach of the Building Regulations, which will need to be resolved before the property is sold.

Window replacement costs (London 2024, supply and install):

uPVC double glazed casement windows (standard sizes): Small (600 x 900mm): £350–£600 per window installed. Medium (900 x 1200mm): £500–£800 per window installed. Large (1200 x 1500mm): £700–£1,100 per window installed.

Timber double glazed sash windows (box sash, new manufacture): Small: £800–£1,400 per window installed. Medium: £1,100–£1,800 per window installed. Large: £1,400–£2,500 per window installed.

Thermally broken aluminium windows (contemporary style): Small to medium: £900–£1,600 per window installed. Large: £1,500–£2,500+ per window installed.

Triple glazing premium: approximately 15–25% above equivalent double-glazed unit. Secondary glazing (aluminium, horizontal sliding): £400–£800 per panel installed.