Window Replacement in London: Costs, Materials, FENSA & Conservation Areas 2025

Window replacement in most London residential properties does NOT require planning permission — it is permitted development (PD) under Class C of Part 1 of Schedule 2 to the GPDO 2015 (windows, doors, and other openings at the same or similar position and of a similar size and appearance). However, there are important exceptions: Conservation areas: in a London conservation area, replacing windows on the front elevation or any elevation visible from the highway is NOT permitted development — it requires planning permission. This applies even to like-for-like replacement of uPVC windows with timber, or timber with timber, where the new windows are in a different style or the same style but technically a different unit. The LPA typically requires that window replacements in conservation areas match the original design — for a Victorian London terrace, this typically means timber sliding sash windows (double glazed heritage sash is a standard conservation area product). Installing uPVC windows on the front elevation of a conservation area Victorian terrace is a common planning breach — and local planning authorities do actively enforce this, including requiring retrospective removal. Article 4 Directions: some London LPAs have applied Article 4 Directions to specific residential streets outside conservation areas, removing the PD right for front window replacement — requiring planning permission even for a like-for-like replacement. Always check the LPA's planning portal or call the planning department before replacing front-elevation windows in any London street. Listed buildings: all window replacement in a listed building requires Listed Building Consent (in addition to planning permission where the works also constitute development). Even internal windows and roof windows in a listed building may require consent — always take specific listed building advice. Part Q (Security) of the Building Regulations: replacement windows and external doors in a dwelling must meet the security requirements of Part Q (enhanced security hardware — locking mechanisms, hinge security). Most window manufacturers supply Part Q compliant windows as standard — always confirm compliance before ordering. Exceptions: Conservation area windows, listed building windows, and windows specified by a heritage advisor may sometimes be installed with traditional hardware that does not meet Part Q standard — seek specific advice.

The three main material options for window replacement in London are uPVC (unplasticised PVC), timber, and aluminium. Each has specific strengths and appropriate uses for different London property types. uPVC (unplasticised polyvinyl chloride): the market-dominant window material for most UK domestic applications outside conservation areas and heritage contexts. Advantages: low maintenance (no painting required), good thermal performance (modern A-rated double or triple glazed uPVC casements achieve excellent U-values), competitive cost, wide choice of colours and woodgrain foil finishes. Disadvantages: inappropriate for Victorian and Edwardian terraces in conservation areas or on Article 4 Direction streets; the plastic appearance and thick sight lines of standard uPVC sash windows do not replicate the fine proportions of original timber sliding sash windows to the standard required by many LPAs and conservation officers. When uPVC is appropriate: for outer London and suburban properties outside conservation areas; for rear windows and windows not visible from the highway even in conservation areas; for modern and post-war properties where the frame material is not a planning or conservation concern. Timber (softwood or hardwood): the correct material for window replacement in a London conservation area Victorian terrace on the front elevation. Traditional sliding sash window boxes are typically made in softwood (Accoya, engineered softwood) with double-glazed heritage sash units. Modern timber heritage sash windows are available with factory-applied primer/paint finish, purpose-designed run-through weatherseals, and slim double-glazed units (4mm/6mm/4mm — a 14mm total glass unit that fits within the standard sash rebate without requiring modification to the box). Advantages: appropriate for conservation areas; more closely replicates the appearance of the original single-glazed timber sash than any other material; can be repaired and re-painted (unlike uPVC); lower environmental impact (embodied carbon) than aluminium. Disadvantages: higher maintenance than uPVC (repainting every 5-8 years recommended); higher supply cost than uPVC. Costs: timber heritage double-glazed sash window supply only (typically 1.0 x 1.5m unit): £400-£900 per window. Aluminium: slim-framed aluminium windows (powder-coated in any RAL or BS colour) are the premium contemporary architectural window material for London new-build and extension projects where a sleek modern appearance is required. Aluminium windows have very thin sight lines, high strength-to-weight ratios, and excellent long-term durability (no painting required; powder coat is UV stable for 25-40 years). Aluminium casement and sliding door systems are the standard specification for contemporary London extension glazing, bifold doors, and large-format corner glazing. Advantages: exceptional slim sight lines; wide range of powder coat finishes; suitable for large-format glazing; 25-40+ year life. Disadvantages: higher cost than uPVC; without a thermal break, aluminium conducts heat rapidly — all modern architectural aluminium windows for London residential use must be thermally broken (a plastic insulating section separating the inner and outer aluminium frames). Cost of aluminium casement windows: £300-£600 per sq m supply only (thermally broken, powder-coated); aluminium bifold or sliding doors: £800-£2,000 per linear metre supply only.

Window replacement in a dwelling in England is notifiable under Part L (Conservation of Fuel and Power) and Part Q (Security) of the Building Regulations. There are two routes to compliance: FENSA (or CERTASS) self-certification: replacement windows and doors installed by a FENSA-registered or CERTASS-registered installer are self-certified against the Building Regulations Part L and Part Q requirements. The registered installer notifies the installation to the relevant authority and issues the homeowner with a FENSA Certificate confirming compliance. FENSA is the standard route for the vast majority of window replacements in England. Building Control notification: where the installer is not FENSA or CERTASS registered, the homeowner must notify Building Control before the work commences and the installation is inspected. In practice, most professional window installers in London are FENSA or CERTASS registered. Part L thermal performance requirements for replacement windows (Approved Document L1B for existing dwellings, 2022 edition): replacement windows (the whole window unit, including frame) must achieve a minimum thermal performance of: Centre pane U-value: a whole-window U-value (Uw) of 1.4 W/m²K or better (the Energy Rating of the window unit must be C-rated or better under the BFRC Window Energy Rating scheme). Or a Window Energy Rating (WER) of Band C or better. Modern double-glazed uPVC windows with low-emissivity glass and argon-filled cavities typically achieve A or A+ Window Energy Ratings (WER) — significantly better than the minimum required. Slim double-glazed heritage sash units (14mm overall glass package) achieve B or C rated WER — meeting the minimum requirement. Triple glazing: triple-glazed windows achieve Uw values of 0.7-1.0 W/m²K — significantly better than double glazing, but at a higher cost (typically 25-40% more than equivalent double-glazed windows) and with additional weight (which may require reinforced frames and sash hardware). Triple glazing is increasingly popular in London properties undergoing high-specification renovation where the whole-house energy performance strategy includes triple glazing. FENSA Certificate: the FENSA Certificate is an important document confirming that window replacement has been carried out by a FENSA-registered installer in compliance with the Building Regulations. Keep it permanently with the property deeds — buyers' solicitors routinely request FENSA Certificates for recent window replacement as a condition of purchase.

For London Victorian terraces in conservation areas with original timber sliding sash windows that are still present (even if in poor condition), restoration of the original windows is frequently a better option than full replacement — both from a planning perspective (the conservation officer's preference is always for retention and repair over replacement) and from a value perspective (original single-glazed Georgian or Victorian sash windows in good condition are a genuine positive feature of a London property, contributing to authenticity and character). Sash window restoration options: Draught-stripping: new spring-loaded or brush pile draught seals are installed in the meeting rail, top and bottom rail, and side beads of the sash box, significantly reducing air infiltration without any modification to the appearance of the window. A draught-stripped single-glazed sash window achieves dramatically better thermal and acoustic performance than an un-stripped single-glazed sash. Cost: £100-£200 per window. Secondary glazing: a secondary glazing panel (typically in an aluminium frame, clear acrylic or glass) is installed on the internal face of the existing sash window reveal, creating a sealed air gap between the secondary glazing and the existing single-glazed sash. Secondary glazing achieves excellent acoustic performance (critical in London, where traffic noise is a major quality-of-life issue) and reasonable thermal performance — and is fully reversible. Secondary glazing panels can be supplied and fitted to the original sash window configuration and are acceptable (and sometimes preferred) by conservation officers over full sash replacement. Cost: £200-£500 per window (internal secondary glazing unit). Full restoration: the original sash is removed, stripped of paint, and repaired (new sash rope, new pulleys, repair of damaged or rotted sections of the sash stile or rail). The restored sash is re-glazed with either the original single glass or (more commonly for energy performance) a slim 14mm double-glazed unit that fits the existing sash rebate without modification to the box. Full restoration with slim double glazing: £400-£800 per window. Full replacement with new timber heritage double-glazed sash: appropriate where the original sash is beyond economic repair, or where the existing window boxes themselves have failed. Cost: £600-£1,500 per window supply and fit.