Do I need planning permission to replace timber windows in London?

Replacement windows in the same location and size are generally permitted development in most London houses outside conservation areas. In a conservation area, replacement windows visible from the public highway require planning permission and must match the original material and style. Listed buildings require Listed Building Consent for any window change. Always check conservation area status and Article 4 directions for your specific postcode before ordering.

What is Accoya timber and is it worth the premium for windows?

Accoya is radiata pine that has been acetylated — a chemical modification process making it dimensionally stable and resistant to rot and insect attack. Accoya has a 50-year above-ground rot-resistance guarantee (far exceeding standard treated softwood at 15–20 years), takes paint systems exceptionally well, and costs 20–30% more than untreated softwood. For London conservation area windows with regular painting requirements, Accoya is worth the premium — reduced maintenance costs and extended repaint interval make it cost-effective over a 30-year horizon.

Can I get double glazing in original timber sash windows?

Yes — slimline double-glazed units (typically 4mm–10mm–4mm, total 18mm) fit within the original glazing rebate of Victorian box sash windows without altering the profile. These achieve a U-value of approximately 1.8–2.0 W/m²K versus 4.8 W/m²K for original single glazing. Conservation officers increasingly accept slimline double glazing in timber sashes for conservation area and even some listed building applications. Specify low-emissivity (low-e) glass on the inner pane (Pilkington K Glass or equivalent) for the best thermal performance.

What is the difference between a box sash and a sliding sash window?

A box sash uses a built-in weight-and-pulley counterbalance mechanism within a hollow 'box' frame on each side — the sashes slide up and down balanced by lead or cast iron weights. A spring-balanced sash uses coil springs or spiral balances instead of weights and boxes, with a slimmer frame profile. For conservation area replacements, a box sash is typically required to match the original — spring-balanced alternatives are sometimes acceptable if the frame profile is identical to the original from the outside.

Timber Windows Cost London 2025 | Builderr

How Much Do Timber Windows Cost in London?

Timber windows in London cost £700–£2,500 per window installed, depending on species, style and glazing specification. A 3-bed Victorian terrace with 10 timber sash or casement windows costs £9,000–£22,000. Timber is the only frame material accepted in most Grade II listed properties and many conservation areas. Hardwood (Accoya, engineered oak) is more durable but costs 30–50% more than softwood.

Timber window cost by type and species

Timber window cost in London (supply and installation, double-glazed, painted softwood unless noted). Softwood casement (600mm × 900mm, factory-primed): £500–£800 supply; £700–£1,100 installed. Softwood sliding sash (2-over-2 pane, 750mm × 1,200mm): £650–£950 supply; £900–£1,400 installed. Softwood sliding sash (6-over-6 pane, box sash, 750mm × 1,500mm): £900–£1,400 supply; £1,200–£1,900 installed. Hardwood (Accoya or engineered oak) casement (600mm × 900mm): £800–£1,400 supply; £1,100–£1,800 installed. Hardwood box sash (750mm × 1,500mm): £1,300–£2,200 supply; £1,700–£2,800 installed. Full house replacement — 3-bed Victorian terrace (10 box sash windows, softwood, including scaffold): £12,000–£18,000 installed. Specialist heritage window manufacturers include: Mumford & Wood, Marvins, Reddiseals, Wood Window Alliance members — all offer thermally broken double-glazed timber windows meeting current Building Regulations.

Timber windows in conservation areas and listed buildings

Timber is the default requirement for original window openings in conservation areas and listed buildings in London. Conservation areas: planning policy in most London conservation areas requires that replacement windows in the original house envelope match the original material and style — timber sash windows in a Victorian terrace must be replaced like-for-like with timber sash windows, maintaining the same pane configuration. The planning authority will often specify the ovolo moulding profile, horn presence, and glass type. Slimline double glazing (4mm–10mm–4mm units, 6mm cavity) is the standard specification for conservation area timber sash replacement — Pilkington K Glass or Saint-Gobain Planitherm are common choices. Listed buildings: LBC is required for any window change in a listed property. LBC conditions routinely specify timber, matching original profile, matching glass size. Conservation officers are increasingly accepting slimline double glazing where the unit thickness is within the original glazing rebate.

Timber window repair versus replacement

Repairing original timber windows is almost always preferable to replacement — both for conservation reasons and for cost. A Victorian timber sash window that appears rotten and draughty can frequently be repaired, draught-proofed and redecorated for £250–£600 per window versus £1,000–£2,000 for full replacement. Repair options: Draught-proofing (Ventrolla or equivalent spring-seal system): £150–£300 per window; eliminates the major source of heat loss in original sash windows with no alteration to the original fabric. Epoxy resin repair (Repair Care, Osmo): rotted sections of rail or stile can be cut back to sound timber and rebuilt with a two-part epoxy filler — can be painted identically. Suitable for up to 30–40% rot in any given section. Secondary glazing: Selectaglaze or similar secondary glazing units fitted internally provide thermal performance equivalent to double glazing (effective U-value ~1.6 W/m²K) without altering the original window — the preferred option in listed buildings where double glazing LBC is refused.

Timber window maintenance and lifespan

Properly maintained timber windows last 60–100+ years — original Georgian and Victorian timber sashes are still performing in many London properties after 150 years. Key maintenance programme: Repainting: softwood timber windows require repainting every 5–7 years (exterior) using a quality flexible paint system — Sikkens, Dulux Trade, or Teknos Futura. Failure to repaint allows moisture penetration that leads to rot at joints and sill junctions. Hardware: sash window pulleys and cords should be oiled or replaced every 10–15 years. Sash cord is the most common maintenance item — replace as soon as a window drops or requires propping. Putty: the glazing putty in timber windows hardens, cracks and shrinks over time — cracked putty allows water ingress to the glazing rebate, the leading cause of sill and rail rot. Reputty all windows with cracked or missing putty before repainting. Accoya: significantly extended maintenance interval — Accoya has a 50-year rot-resistance guarantee and a repaint cycle of 8–12 years versus 5–7 years for untreated softwood.