Window Replacement in London: Planning, Building Regs & Costs

Window replacement is one of the most common home improvement works in London — and one of the most frequently misunderstood in terms of what requires permission and what does not.

Permitted Development for Window Replacement

Standard residential properties outside conservation areas: Replacing like-for-like windows (same opening size and position, similar appearance) is generally permitted development. Planning permission is not normally required.

Conservation areas: Replacing windows in a conservation area requires more care. Article 4 directions in many London conservation areas remove PD rights for window replacements that alter the character of the property. Replacing original timber sash windows with UPVC in a conservation area may require planning permission and is likely to be refused.

Listed buildings: Any alteration to windows in a listed building requires Listed Building Consent. Original windows in listed buildings are almost never replaceable with modern UPVC or aluminium alternatives — restoration of the originals or high-quality slim-profile timber replacements are the expected approach.

Building Regulations for Window Replacement

Even where planning permission is not required, Building Regulations apply to window replacements in most cases:

• Thermal performance (Part L): Replacement windows must achieve a minimum thermal performance — currently WER Band C or U-value of 1.4 W/m²K for the whole window unit
• Ventilation (Part F): Where existing windows provided ventilation (trickle vents, opening lights), replacement windows must maintain adequate ventilation
• Safety glazing (Part K): Windows in critical locations (low-level, next to doors) must use safety glass

Most FENSA or CERTASS-registered window installers self-certify compliance with Building Regulations — you receive a compliance certificate without needing a local authority Building Regulations application.

Window Types and Materials

UPVC: Low cost, low maintenance, good thermal performance. Not appropriate in conservation areas or for listed buildings. Modern UPVC can be slim-profile but rarely matches the finesse of timber or aluminium.

Aluminium: Slim profiles, good thermal performance in thermally broken systems, long lifespan, premium cost. Suitable for contemporary extensions and renovations where a modern aesthetic is desired.

Timber: Authentic for period properties, paintable, repairable. Higher maintenance than UPVC or aluminium. Essential in conservation areas and listed buildings.

Timber-aluminium composite: Timber internally (authentic appearance), aluminium externally (low maintenance). Premium cost — best of both for period properties requiring low maintenance.

Typical Costs in London

Window replacement costs vary enormously by specification:

• UPVC casement window (supply and fit): £400–£800 per window
• Aluminium sliding sash (supply and fit): £800–£2,000 per window
• Timber sliding sash (supply and fit): £900–£2,500 per window
• Crittal-style slim steel frame (supply and fit): £1,500–£4,000 per window

RCB Design & Build

RCB Design & Build includes window replacement as part of full refurbishment packages — advising on specification, planning obligations, and Building Regulations compliance.