Replacing Windows in a London Home in 2025: Costs, Planning, Building Regulations, and What to Avoid

Planning permission for window replacement in a London home — when you need it and when you don't: GENERAL RULE — NO PLANNING PERMISSION REQUIRED FOR REPLACING LIKE-FOR-LIKE: in England, the replacement of windows in a DWELLING (a house) with similar windows of similar appearance is within Permitted Development rights (Class A — Schedule 2, Part 1, GPDO 2015) and does NOT require planning permission; this means: replacing old single-glazed sash windows with new double-glazed sash windows of similar appearance is PD; replacing old PVCu casement windows with new PVCu casement windows is PD; replacing old aluminium windows with new aluminium windows is PD; THE CRITICAL EXCEPTION — CONSERVATION AREAS: approximately 60% of London's inner-London Victorian and Edwardian terraced housing stock sits within a designated Conservation Area; within a Conservation Area, the PD rights for window replacement are MUCH MORE RESTRICTED; for a property within a Conservation Area in London: REPLACING WINDOWS IN A WAY THAT ALTERS THE APPEARANCE OF THE FRONT ELEVATION OF THE PROPERTY — specifically, replacing windows VISIBLE FROM A HIGHWAY (the public street or road) — is NOT within Permitted Development rights and REQUIRES PLANNING PERMISSION (or a Certificate of Lawful Development — CLD — confirming PD applies where the change is not visible from the highway); the specific PD restriction in a Conservation Area is set out in Article 3(1) of the GPDO — the Class A PD right for extensions (and, by extension, alterations to the principal elevation) is curtailed in Conservation Areas for works that affect the appearance of the property's principal elevation; COMMON SCENARIOS IN LONDON CONSERVATION AREAS: SCENARIO 1 — REPLACING THE EXISTING DOUBLE-GLAZED SASH WINDOWS WITH NEW DOUBLE-GLAZED SASH WINDOWS OF IDENTICAL APPEARANCE: this is likely to be PERMITTED in most London Conservation Areas — it is a LIKE-FOR-LIKE replacement that does not alter the appearance of the principal elevation; HOWEVER: some London LPA Conservation Officers take a strict view and require planning permission for ANY window replacement in a Conservation Area, even like-for-like; check with the LPA before proceeding; SCENARIO 2 — REPLACING SINGLE-GLAZED TIMBER SASH WINDOWS WITH PVCu SASH WINDOWS: this is VERY UNLIKELY to be permitted in a London Conservation Area; PVCu sash windows do not replicate the appearance, profile, or detailing of original Victorian timber sash windows; the glazing bar widths and depths, the sash rail thickness, the ovolo moulding, and the proportion of the glazing units are all different; the LPA will typically refuse planning permission for PVCu sash windows in a Conservation Area and require the homeowner to install timber or aluminium slimline double-glazed sash windows instead; SCENARIO 3 — REPLACING SINGLE-GLAZED TIMBER SASH WINDOWS WITH SLIM-PROFILE DOUBLE-GLAZED TIMBER SASH WINDOWS: this is typically ACCEPTABLE in most London Conservation Areas — the timber material, the slim-profile glazing bars, and the overall proportions of a high-quality double-glazed timber sash window can replicate the appearance of the original single-glazed window to an acceptable degree; the LPA may require the glazing bar profile, paint colour, and sash rail depth to match the originals as conditions of planning consent; SCENARIO 4 — REPLACING SINGLE-GLAZED TIMBER SASH WINDOWS WITH SLIM-PROFILE ALUMINIUM DOUBLE-GLAZED SASH WINDOWS: increasingly acceptable in many London Conservation Areas where the aluminium sash windows have been specifically designed to replicate the profile, depth, and proportions of original Victorian timber sash windows (e.g. Crittal Heritage, Selectaglaze, or other slim-profile aluminium sash systems); however, some London Conservation Officers remain resistant to aluminium in Conservation Areas on the grounds of material inauthenticity; check with the LPA before committing to an aluminium product; LISTED BUILDINGS: where a London property is a Listed Building (Grade I, II*, or II), ALL window replacements — regardless of Conservation Area status and regardless of whether they are visible from a highway — require LISTED BUILDING CONSENT from the LPA; listed building consent for window replacement must be applied for to the same LPA as a planning application; the standard for listed building consent is significantly more stringent than for a standard conservation area planning application: the LPA will typically require the replacement windows to match the originals in material, profile, glazing bar width, glass thickness, and finish; single glazing is often required to be retained in the most sensitive listed buildings (where the original single-glazed windows are part of the listed building's character); secondary glazing (see Section 2) may be the only acceptable thermal upgrade for listed buildings where single glazing must be retained; ARTICLE 4 DIRECTIONS: in addition to Conservation Area controls, some London Boroughs have applied ARTICLE 4 DIRECTIONS specifically removing the PD rights for window replacement across particular streets or areas — meaning that even a like-for-like replacement in a non-Conservation Area may require planning permission where an Article 4 Direction has been made; check the LPA's planning policy map for Article 4 Directions before assuming PD applies.

Building Regulations for window replacement in London — Part L energy efficiency requirements, the FENSA self-certification scheme, and what the U-value requirements mean in practice: BUILDING REGULATIONS AND WINDOW REPLACEMENT: the replacement of windows in a dwelling is NOTIFIABLE BUILDING WORK under the Building Regulations 2010 (Schedule 3, Part 3) — it must comply with Part L (energy efficiency) and Part Q (security) of the Building Regulations; the compliance route for window replacement is TYPICALLY via the FENSA COMPETENT PERSON SCHEME (see below) rather than a formal LABC Building Regulations application; FENSA (FENESTRATION SELF-ASSESSMENT SCHEME): FENSA is the Government-approved Competent Person Scheme for window and door replacement in England; a FENSA-REGISTERED INSTALLER can install replacement windows and self-certify that they comply with Part L (thermal performance) and Part Q (security) without the homeowner needing to make a separate Building Regulations application to the LABC; after installation, the FENSA-registered installer notifies FENSA, who register the installation and send the homeowner a FENSA CERTIFICATE; the FENSA CERTIFICATE is the Building Regulations compliance document for the replacement windows; it must be kept safely and produced at point of sale (the Property Information Form — TA6 — asks specifically about FENSA certificates for replacement windows); IF THE INSTALLER IS NOT FENSA-REGISTERED: the homeowner must apply for Building Regulations approval from the LABC before the replacement windows are installed (Full Plans application or Building Notice); the LABC will inspect the installed windows to confirm Part L and Part Q compliance; the LABC issues a Building Regulations Completion Certificate upon confirmation of compliance; the LABC route is significantly more time-consuming and expensive than the FENSA route — choose a FENSA-registered installer where possible; PART L 2021 REQUIREMENTS FOR REPLACEMENT WINDOWS: the 2021 update to Part L substantially tightened the minimum thermal performance requirements for replacement windows and doors; current requirements (from June 2022): WINDOWS: the replacement window must achieve a WHOLE WINDOW U-VALUE (including frame and glazing) of 1.4 W/m²K OR BETTER; (note: the whole-window U-value of 1.4 W/m²K is the MINIMUM LEGAL STANDARD — many modern double-glazed windows achieve 1.0-1.2 W/m²K whole-window U-value, and triple-glazed windows typically achieve 0.7-0.9 W/m²K); ROOFLIGHTS: 1.6 W/m²K or better; ROOF WINDOWS: 1.6 W/m²K or better; DOORS: 1.4 W/m²K or better (where more than 60% of the door area is glazed); EXTERNAL DOORS (where less than 60% is glazed): 1.4 W/m²K or better; CONSERVATORY ROOFS: 1.8 W/m²K or better (polycarbonate roofing ≥0.6m² must be insulated or replaced to achieve this standard); PRACTICAL U-VALUE PERFORMANCE OF COMMON WINDOW TYPES: SINGLE GLAZING (original Victorian sash): approximately 5.0-5.8 W/m²K (far exceeds the maximum — extremely poor thermal performance); STANDARD DOUBLE GLAZING (4mm glass — 16mm argon-filled cavity — 4mm glass, PVCu frame): typically 1.0-1.4 W/m²K whole-window (meets the legal minimum; some standard double-glazed windows achieve close to 1.4 W/m²K depending on frame); SLIM-PROFILE TIMBER SASH DOUBLE GLAZING: typically 1.4-1.8 W/m²K whole-window for standard slim-profile units (6mm glass — 6mm argon cavity — 6mm glass in a slim-profile frame); to achieve U ≤ 1.4 W/m²K with a slim-profile sash window, the glazing unit must be higher performance (warmer edge spacers; low-E glass coating); SLIM-PROFILE ALUMINIUM SASH DOUBLE GLAZING: typically 1.2-1.6 W/m²K whole-window (highly thermally-broken aluminium frames can achieve close to 1.0 W/m²K); TRIPLE GLAZING: typically 0.7-0.9 W/m²K (well above the minimum legal standard; typically used in high-specification passivhaus-approach projects or in north-facing or exposed locations where the additional thermal performance justifies the cost premium); THE PART Q SECURITY REQUIREMENT: replacement windows must also comply with Part Q (Security — Dwellings); specifically, ground floor windows (and first-floor windows accessible from a flat roof or other means) must meet BS PAS 24:2016 (or STS 204 Issue 3 for timber windows in conservation areas); FENSA-registered installers certify compliance with both Part L and Part Q simultaneously; SECONDARY GLAZING as an alternative to window replacement: where a London homeowner in a Conservation Area or a Listed Building cannot replace the windows (because planning consent would be refused for a change in window type, or because Listed Building Consent is not granted for double glazing), SECONDARY GLAZING is the preferred thermal upgrade: secondary glazing involves fitting an ADDITIONAL INNER FRAME of slimline glazing behind the existing single-glazed window; the inner frame creates an air gap of typically 50-200mm between the original single-glazed window and the secondary glazed inner frame; a 100-200mm air gap secondary glazing system can achieve an overall U-value of approximately 1.0-1.8 W/m²K (depending on the width of the air gap); secondary glazing also provides significant acoustic attenuation (noise reduction) — a particularly valuable benefit in London where road, railway, and neighbour noise is a significant issue; secondary glazing does NOT require planning permission (because the outer face of the window — visible from the street — is unchanged); secondary glazing is NOT notifiable under Building Regulations if the scope of work is limited to the secondary glazing installation only (the existing primary window is not replaced); COST OF SECONDARY GLAZING: approximately £250-£600 per opening (supply and install) — significantly cheaper than full window replacement; DOWNSIDE: secondary glazing is less effective than replacement double glazing; the opening and closing mechanism is less convenient; condensation can form on the inner face of the original single-glazed window.

Window replacement costs in London in 2025 and how to choose a window company: WINDOW REPLACEMENT COSTS IN LONDON IN 2025 (supply and install, fully installed and decorated): the cost of replacement windows in London in 2025 varies widely depending on: the window type (PVCu / aluminium / timber); the window size and configuration (fixed/casement/sash); the glass specification (standard double / enhanced low-E double / triple); the site (access, height, Conservation Area requirements); PVCU CASEMENT OR TILT-AND-TURN WINDOWS (standard non-Conservation Area): cost per window (supply and install): £350-£700 for a standard single-casement window in a typical London terrace; £500-£1,000 for a larger double-casement window; PVCu windows have the lowest upfront cost and the best maintenance-free performance; however, they are NOT acceptable in Conservation Areas or Listed Buildings (see above); life expectancy: 20-30 years; ALUMINIUM CASEMENT OR TILT-AND-TURN WINDOWS (modern profile — standard double glazing): cost per window (supply and install): £600-£1,200 for a standard single-casement; £800-£1,800 for a larger double-casement; aluminium windows have a slimmer sight line than PVCu (thinner frame relative to the overall window size) and a more architectural appearance; they are increasingly acceptable in London Conservation Areas depending on the LPA's attitude; life expectancy: 30-50 years; SLIM-PROFILE TIMBER SASH DOUBLE GLAZED WINDOWS (Conservation Area replacement for Victorian sash): cost per window (supply and install): £1,200-£2,200 for a standard two-over-two sash (500 × 900mm glass); £1,500-£2,800 for a larger two-over-two sash (600 × 1,100mm glass); the cost of slim-profile timber sash double-glazed windows is significantly higher than PVCu or standard aluminium — driven by the cost of the slimline double-glazed unit (a 6-4-6 or 6-6-6 slim-profile unit is substantially more expensive to manufacture than a standard 4-16-4 unit), the skilled labour required for proper sash installation and sealing, and the specification of the timber (engineered softwood, accoya, or hardwood); the thermal performance at 1.4-1.8 W/m²K (whole window) is lower than a PVCu or modern aluminium window, but acceptable for Part L compliance with higher-performance glazing units; SLIM-PROFILE ALUMINIUM SASH DOUBLE GLAZED WINDOWS (Conservation Area alternative): cost per window (supply and install): £1,400-£2,500 for a standard two-over-two sash; suppliers include Crittal Heritage, Slimsline, Ecoease, and other specialist manufacturers; these systems typically achieve 1.0-1.4 W/m²K whole-window — better thermal performance than timber sash equivalents; life expectancy: 40-60 years with minimal maintenance; HOW TO CHOOSE A WINDOW COMPANY IN LONDON IN 2025: the London window replacement market contains a wide range of contractors — from national installation companies to local independent glaziers and specialist Conservation Area window companies; key checks to carry out before appointing a window company: CHECK FENSA REGISTRATION: confirm the company is currently registered with FENSA at fensa.org.uk (FENSA's online register is publicly searchable); a company that is not FENSA-registered cannot self-certify the installation — you will need a LABC Building Regulations application if you use a non-FENSA-registered installer; CHECK INSURANCE: public liability insurance (minimum £2,000,000) and employer's liability insurance; CHECK REFERENCES: ask for references from recent London Conservation Area projects where the installer has replaced Victorian sash windows; request examples of the specific slim-profile timber or aluminium sash product they propose to install; CONSERVATISM AREA — CHECK WITH THE LPA FIRST: before appointing a window company for a property in a Conservation Area, confirm with the LPA whether the proposed product requires planning permission and whether the LPA's Conservation Officer has approved the product for use in the Conservation Area; do not order windows before planning consent is confirmed; AVOID: companies that do not provide a FENSA certificate; companies that request more than 30-40% deposit before installation; companies that claim planning permission is not required without checking with the LPA; window companies that propose PVCu sash windows as a like-for-like replacement for timber sash windows in a Conservation Area — PVCu sash windows are NOT acceptable in most London Conservation Areas and will result in a planning enforcement notice.