Window Replacement and Glazing Costs for London Homes

The original vertical sliding sash windows in London Victorian terraces are often in various states of disrepair after 100-150 years of service — draughty, difficult to open, with failed putty, rotten timber sections, and single glazing. The main options are: Repair and restoration: retaining the original sash window frames, repairing or splicing rotten sections of timber, replacing broken weights and cords, draught-stripping the sashes, and potentially fitting secondary glazing or slim-profile double glazed units within the original frame. This approach: preserves the original character and materials (preferred by conservation officers and required for listed buildings); is often viable even where the windows appear in poor condition — a good joiner can repair sections that a glazing company would recommend replacing; can be thermal-upgraded through draught-stripping and slim double-glazed units (manufactured to fit the original thickness of the single-glazed rebate); costs: £300-£800 per window for repair and draught-stripping; £500-£1,200 per window for full restoration including slim double glazed unit. Replacement with timber sash: new purpose-made timber sash windows matching the original profile, with modern double glazing. The best visual match to the original — using the same materials and section profiles. Cost: £800-£2,000 per window (supply and fit), depending on size and specification. Replacement with UPVC or aluminium sash: modern UPVC or powder-coated aluminium sliding sash windows that replicate the appearance of timber sash. Better thermal performance than single-glazed originals; lower maintenance than timber. Cost: £400-£1,200 per window. However, UPVC sash is often not acceptable in conservation areas due to the visual difference in profile and reflectivity from traditional timber.

Window replacement in London is one of the most planning-sensitive alterations a homeowner can make — particularly in conservation areas, where the majority of the most desirable Victorian and Edwardian residential areas sit. In conservation areas: replacing original timber sash windows with UPVC windows is almost always unacceptable and requires planning permission (if not explicitly prohibited by an Article 4 Direction, it requires a Householder planning application which planning authorities will almost always refuse in conservation areas where timber sash is the established character). Article 4 Directions: most London inner conservation areas have Article 4 Directions that specifically remove permitted development rights for the replacement of windows or doors — meaning planning permission is required even for like-for-like replacement of windows in a conservation area in London. The practical effect: in most London conservation areas, you must obtain planning permission to replace original timber sash windows with ANYTHING — including replacement timber sash windows, even if the new windows look identical. Planning applications for window replacement in conservation areas are determined against the council's design guidance, which typically requires replacement windows to match the original profile, proportions, and materials exactly. Secondary glazing: fitting secondary glazing (an internal second pane, independent of the outer window) is typically permitted development and does not require planning permission in most conservation areas — it is often the preferred thermal solution for listed buildings and conservation area windows where replacement is not acceptable or practical. Outside conservation areas and for non-listed buildings: window replacement is permitted development for most properties — no planning permission is required as long as the replacement is of like-for-like appearance. Building Regulations Part L: replacement windows must meet the current energy efficiency standards under Part L (Uw value of 1.4 W/m²K or better for replacement units in most circumstances).

For properties outside conservation areas or where planning permission for replacement has been obtained, the main double glazing options and their costs in London: UPVC casement windows (standard replacement double glazing): the most cost-effective option for properties where appearance is not constrained. Cost: £300-£700 per window supply and fit, depending on size. UPVC sash windows (vertical sliding, matching the aesthetic of Victorian sash): £400-£1,000 per window. Aluminium casement or tilt-turn windows (more durable than UPVC, better sightlines, suitable for contemporary extensions): £600-£1,500 per window. Aluminium sash windows (replicating the sash profile in powder-coated aluminium, popular in high-end London renovations in conservation areas where approved or on non-conservation properties): £800-£2,000 per window. Timber casement or sash with modern double glazing (the premium option): £1,000-£2,500 per window. Slim double-glazed units for existing timber frames: £200-£600 per window, depending on size (the slim unit is manufactured to fit the existing frame without replacing it — the preferred solution in conservation areas where the frame is in acceptable condition). The total cost of re-glazing a typical 3-bedroom London Victorian terrace (8-12 windows): £5,000-£20,000 depending on window type, specification, and whether the existing frames are retained or replaced.

Secondary glazing — fitting an additional pane of glass on the room side of the existing window, independent of the original outer window — is the most widely accepted thermal upgrade solution for listed buildings and conservation area properties in London where replacement of the original window is not acceptable. How secondary glazing works: a separate inner frame, typically aluminium (in various powder-coat colours) or timber, is fixed to the inner face of the reveal. The secondary frame contains one or more panes of glass that can be slid or hinged to provide ventilation and cleaning access. The air gap between the secondary and the original single-glazed outer window acts as thermal insulation (similar to a double-glazed unit but with a larger air gap, which can reduce sound transmission more effectively than standard double glazing). Thermal performance: well-fitted secondary glazing can reduce heat loss through a single-glazed window by 50-65% — significantly improving the comfort of a Victorian London room without altering the external appearance. Sound reduction: secondary glazing with a large air gap (100-150mm) is particularly effective at reducing sound transmission — popular in London properties on busy roads or under flight paths. The asymmetric glazing (different thicknesses of the outer and secondary panes) enhances the sound reduction effect. Cost: secondary glazing in London: £200-£600 per window for a standard system (aluminium frame, single thickness glass). Acoustic secondary glazing with thicker glass: £300-£800 per window. Supply and installation, typically by a specialist secondary glazing company. Planning permission: secondary glazing is typically permitted development and does not require planning permission in conservation areas — it must not affect the external appearance of the building, which it does not (being fitted entirely internally).

The London glazing market includes thousands of installers ranging from national UPVC double glazing companies to small specialist timber joinery firms who only work on original period windows. How to choose the right type of installer for your London window project: For like-for-like UPVC replacement outside conservation areas: most established UPVC glazing companies are adequate. Get 2-3 quotes, check they offer a minimum 5-year guarantee, and verify they are registered with FENSA (the industry registration body that provides Competent Person Scheme certification for window installation, required instead of a Building Regulations application for straightforward replacements). For timber sash restoration and repair: look for joinery companies or specialist sash window restoration companies who have completed similar projects on London Victorian or Edwardian properties. The Guild of Master Craftsmen and the TTAC (The Traditional Timber Architecture) bodies list specialist restorers. Ask specifically for references from conservation area projects. For secondary glazing: specialist secondary glazing companies in London (Selectaglaze, Sash UK, and others) have specific expertise in conservation area and listed building secondary glazing — they are familiar with the planning considerations and the fixing methods that do not damage historic fabric. FENSA and Building Regulations: FENSA-registered installers self-certify window replacement compliance with Building Regulations Part L (thermal performance) — you do not need a separate Building Regulations application. Non-FENSA installers require a Building Regulations application before installation. For conservation area and listed buildings: the installer's knowledge of planning requirements is as important as their installation quality.