Heritage Windows in London: Sash, Crittal, Timber and Conservation Replacement 2025

THE BOX SASH (VERTICAL SLIDING SASH) WINDOW IS THE DEFINING WINDOW TYPE OF VICTORIAN AND EDWARDIAN LONDON TERRACES. THERE ARE TWO APPROACHES: RESTORATION — RESTORING ORIGINAL BOX SASH WINDOWS; REPLACEMENT — REPLACING WITH NEW BOX SASH WINDOWS. RESTORATION CONSIDERATIONS: IN MANY LONDON CONSERVATION AREAS AND FOR ALL LISTED LONDON BUILDINGS, RESTORATION OF ORIGINAL BOX SASH WINDOWS IS PREFERRED OR REQUIRED OVER REPLACEMENT; ORIGINAL LONDON SASH WINDOWS ARE OFTEN IN BETTER CONDITION THAN THEY APPEAR — THEY CAN BE DRAUGHT-PROOFED, REGLAZED, REPAIRED AND REPAINTED TO PERFORM ADEQUATELY; DRAUGHT-PROOFING OF ORIGINAL LONDON BOX SASH WINDOWS (USING PILE SEALS IN THE MEETING RAIL AND BRUSH SEALS IN THE SIDE CHANNELS) CAN DRAMATICALLY IMPROVE THEIR THERMAL AND ACOUSTIC PERFORMANCE AT LOW COST (APPROXIMATELY £150-£250 PER WINDOW BY A SPECIALIST). REPLACEMENT WITH NEW BOX SASH WINDOWS: WHERE ORIGINALS ARE BEYOND ECONOMIC REPAIR OR WHERE MODERN DOUBLE GLAZING IS REQUIRED, REPLACEMENT BOX SASH WINDOWS ARE AVAILABLE FROM SPECIALIST LONDON MANUFACTURERS; THE KEY SPECIFICATION CONSIDERATIONS ARE: GLAZING — STANDARD REPLACEMENT BOX SASH WINDOWS USE 4MM GLASS (MATCHING ORIGINAL THIN GLASS); MODERN REPLACEMENTS CAN INCORPORATE SLIM DOUBLE GLAZING (SLIMLINE INSULATING GLASS UNITS — TYPICALLY 10-14MM OVERALL UNITS IN A 4-2-4 OR 4-4-4 CONFIGURATION) WHICH GIVES MODERN THERMAL PERFORMANCE WHILE RETAINING THE THIN PROFILE REQUIRED BY LONDON PLANNING; PUTTY GLAZING PROFILE — IN LONDON CONSERVATION AREAS, PLANNING OFTEN REQUIRES WINDOWS TO BE PUTTY-GLAZED (FLUSH GLAZING BEAD IN PUTTY) RATHER THAN BEAD-GLAZED (BEADS PINNED OR SCREWED OVER THE GLASS); THE PUTTY GLAZING PROFILE IS CHARACTERISTIC OF LONDON PERIOD SASH WINDOWS.

THE MOST SPECIFIED HERITAGE WINDOW BRANDS FOR LONDON CONSERVATION AREA AND LISTED BUILDING PROJECTS IN 2025: MUMFORD & WOOD — THE MOST PRESTIGIOUS UK TIMBER WINDOW BRAND FOR LONDON HERITAGE AND CONSERVATION PROJECTS; SPECIALISE IN BOX SASH AND CASEMENT WINDOWS IN ACCOYA MODIFIED WOOD (HIGHLY DURABLE, DIMENSIONALLY STABLE TIMBER); THEIR TOWN HOUSE RANGE IS THE MOST SPECIFIED FOR LONDON VICTORIAN TERRACE REPLACEMENTS; MUMFORD & WOOD WINDOWS ARE FREQUENTLY SPECIFIED AND ACCEPTED BY LONDON CONSERVATION OFFICERS; SASH WINDOW WORKSHOP — A LONDON-BASED SPECIALIST IN BOX SASH WINDOW RESTORATION AND REPLACEMENT; ONE OF THE LEADING SASH WINDOW COMPANIES SERVING LONDON; RESTORATION AND REPLACEMENT BOTH OFFERED; WEIGHTS & PULLEYS RESTORATION FOR AUTHENTIC BOX FRAME CONSTRUCTION; THE LONDON SASH WINDOW COMPANY — ANOTHER LONDON-SPECIALIST; REPAIR, DRAUGHT-PROOFING AND REPLACEMENT IN TIMBER AND OCCASIONALLY uPVC SASH PROFILES; RATIONEL ALDAS PLUS — A DANISH TIMBER/ALUMINIUM-CLAD WINDOW OFTEN SPECIFIED IN LONDON HIGH-PERFORMANCE RENOVATION PROJECTS WHERE HIGH THERMAL PERFORMANCE IS REQUIRED ALONGSIDE AN AUTHENTIC SLIM TIMBER PROFILE; THE ALDAS PLUS HAS A 78MM OUTER FRAME IN ACCOYA AND CAN ACHIEVE A U-VALUE OF 0.79 W/M²K WITH TRIPLE GLAZING; USED IN LONDON EnerPHit CERTIFIED RENOVATIONS AND HIGH-PERFORMANCE PERIOD PROPERTY REFURBISHMENTS.

CRITTALL WINDOWS (SLENDER STEEL-FRAME CASEMENT WINDOWS) ARE A DISTINCTIVE FEATURE OF INTER-WAR AND POST-WAR LONDON BUILDINGS — FROM MODERNIST HOUSES OF THE 1930s THROUGH TO LOCAL AUTHORITY HOUSING OF THE 1950s AND 1960s. CRITTALL RESTORATION: ORIGINAL CRITTALL WINDOWS IN LONDON ARE FREQUENTLY BEYOND ECONOMIC REPAIR DUE TO CORROSION, BUT CAN SOMETIMES BE RESTORED BY: WIRE BRUSHING AND PRIMING THE STEEL FRAME (FINNAREN & HALEY RUST INHIBITING PRIMER, RUSTOLEUM RUSTY METAL PRIMER OR SIMILAR); REPLACING THE GLASS IF CRACKED OR BROKEN; SEALING THE GLAZING WITH LINSEED OIL PUTTY; APPLYING A DURABLE TOPCOAT (METAL PAINT); CRITTALL REPLACEMENT WINDOWS IN LONDON: CRITTALL WINDOWS (THE ORIGINAL CRITTALL BRAND, NOW OWNED BY CRITTALL WINDOWS LTD, BASED IN BRAINTREE, ESSEX) CONTINUES TO MANUFACTURE REPLACEMENT STEEL WINDOWS; THE CRITTALL W20 SYSTEM IS THE MOST SPECIFIED FOR LONDON HERITAGE AND CONSERVATION REPLACEMENT PROJECTS; MODERN CRITTALL W20 WINDOWS CAN INCORPORATE THERMAL BREAK TECHNOLOGY AND DOUBLE GLAZING TO ACHIEVE IMPROVED THERMAL PERFORMANCE WHILE RETAINING THE AUTHENTIC SLIM SIGHT LINES; ALTERNATIVE SLIM-PROFILE ALUMINIUM WINDOWS IN LONDON: FOR NEW CONTEMPORARY LONDON EXTENSIONS AND WHERE A CRITTALL-LOOK IS DESIRED, ALUMINIUM MANUFACTURERS OFFER VERY SLIM THERMALLY BROKEN STEEL-LOOK WINDOWS: STEEL-LOOK ALUMINIUM FROM SCHUCO (FW 50+SG AND AW 70 BS), INTERNORM (SLIM-LINE ALUMINIUM HF310) AND SLIM-FRAME ALUMINIUM FROM WISON/KONNECT ARE WIDELY USED ON LONDON NEW-BUILD AND CONTEMPORARY EXTENSION PROJECTS.

PLANNING RULES GOVERNING WINDOW REPLACEMENT IN LONDON PERIOD AND CONSERVATION PROPERTIES: LISTED BUILDINGS — ANY WINDOW REPLACEMENT OR RESTORATION IN A LISTED LONDON BUILDING REQUIRES LISTED BUILDING CONSENT, REGARDLESS OF SIZE OR MATERIAL; REPLACEMENT OF ORIGINAL WINDOWS IN A LISTED LONDON BUILDING WITHOUT CONSENT IS A CRIMINAL OFFENCE; CONSERVATION AREAS — WINDOW REPLACEMENT IN A LONDON CONSERVATION AREA IS PERMITTED DEVELOPMENT FOR DWELLINGHOUSES UNLESS THE LOCAL AUTHORITY HAS ISSUED AN ARTICLE 4 DIRECTION REMOVING PERMITTED DEVELOPMENT RIGHTS; MANY INNER LONDON BOROUGHS (KENSINGTON AND CHELSEA, WESTMINSTER, CAMDEN, ISLINGTON, SOUTHWARK, HACKNEY, LAMBETH) HAVE ARTICLE 4 DIRECTIONS ON MANY CONSERVATION AREAS THAT REMOVE PERMITTED DEVELOPMENT RIGHTS FOR WINDOW REPLACEMENT — IN THESE CASES PLANNING PERMISSION IS REQUIRED; PLANNING POLICY IN LONDON CONSERVATION AREAS — THE GENERAL PRINCIPLE IS THAT REPLACEMENT WINDOWS IN LONDON CONSERVATION AREAS SHOULD: MATCH THE ORIGINAL IN APPEARANCE (SIZE OF OPENING, GLAZING BAR PATTERN, DEPTH OF REVEAL, PUTTY GLAZING PROFILE); BE IN TIMBER (UUPVC IS GENERALLY OPPOSED IN LONDON CONSERVATION AREAS, THOUGH SOME BOROUGHS ACCEPT IT FOR REAR ELEVATIONS); HAVE THIN SIGHT LINES CONSISTENT WITH THE ORIGINAL WINDOW SECTION; THERMAL PERFORMANCE — BUILDING REGULATIONS PART L REQUIRES MINIMUM WINDOW U-VALUES ON REPLACEMENT; WHEN RESTORING OR REPLACING WINDOWS IN A LISTED LONDON BUILDING OR CONSERVATION AREA WHERE PLANNING REQUIREMENTS CONFLICT WITH PART L, A RELAXATION (A DISPENSATION) CAN SOMETIMES BE AGREED WITH BUILDING CONTROL.

HERITAGE WINDOW COSTS FOR LONDON PERIOD PROPERTIES IN 2025 (SUPPLY AND INSTALL, EXCLUDING SCAFFOLDING): BOX SASH WINDOW DRAUGHT-PROOFING (EXISTING ORIGINAL TIMBER SASH, PER WINDOW) — £150-£250; BOX SASH WINDOW RESTORATION (STRIP, REPAIR ROT, REGLAZE, REPAINT, PER WINDOW) — £300-£600; NEW TIMBER BOX SASH REPLACEMENT WINDOW (STANDARD 1000X1500MM, MUMFORD & WOOD TOWN HOUSE IN ACCOYA WITH SLIMLINE DGU, SUPPLY AND FIT) — £1,800-£3,200; NEW CRITTALL W20 STEEL CASEMENT WINDOW (STANDARD LONDON WINDOW SIZE, SUPPLY AND FIT) — £1,500-£3,000; SLIM ALUMINIUM (STEEL-LOOK, SCHUCO AW 70 BS OR SIMILAR, STANDARD WINDOW, SUPPLY AND FIT) — £800-£2,000; RATIONEL ALDAS PLUS TIMBER-ALU (HIGH PERFORMANCE TRIPLE GLAZED, STANDARD SIZE, SUPPLY AND FIT) — £1,200-£2,500. TYPICAL LONDON VICTORIAN TERRACE (6 BOX SASH WINDOWS, MUMFORD & WOOD TOWN HOUSE, SUPPLY AND FIT, EXCLUDING SCAFFOLD) — £11,000-£19,000. NOTE: LONDON WINDOW REPLACEMENT COSTS INCLUDE DECORATION, MAKING GOOD THE REVEALS AND INTERNAL REDECORATION IN MOST QUALITY CONTRACTS — ALWAYS CONFIRM WHAT IS INCLUDED IN THE QUOTE.