Published 14 April 2026 · Sash Window Draught Proofing
The regulatory landscape for London's period properties shifted significantly on 24 March 2026, when the Ministry of Housing, Communities and Local Government published the updated Part L regulations. For homeowners of Victorian and Georgian terraces in areas like Chelsea, Kensington, and Westminster, these changes represent both a challenge and an opportunity to enhance the thermal efficiency of their properties.
Understanding the Part L 2026 Update
The primary objective of the Part L 2026 update is to accelerate the UK's transition toward net-zero housing by tightening thermal compliance across all residential dwellings. For standard modern homes, the new U-value target for replacement windows is now approximately 1.4 W/m²K. Heritage assets — listed buildings, Conservation Area properties and buildings of local historic interest — are covered by a "reasonable practicability" clause.
Heritage Exemptions
Standard double glazing is almost never approved by conservation officers for Grade II listed sash windows. Under Part L 2026 the focus shifts to high-performance, reversible upgrades: high-quality draught elimination, professional sash window refurbishment, and internal secondary glazing.
The Compliance Hierarchy
For most London period homes we recommend starting with conservation-grade draught proofing and layering internal secondary glazing where higher thermal or acoustic performance is required. See our detailed Victorian sash window guide, the secondary glazing buyer's guide and the secondary glazing vs draught proofing comparison.
| Measure | Cost per window (London, 2026) | Heat-loss reduction | Noise reduction | Typical payback | Part L 2026 status |
|---|---|---|---|---|---|
| Conservation-grade draught proofing | £150–£300 | Up to 30% | Removes rattle only | About 2 years | Reasonable measure, no consent required |
| Draught proofing + timber repair | £280–£550 | Up to 35% | Modest | 3–4 years | Reasonable measure, no consent required |
| Internal secondary glazing | £400–£800 | Up to 70% | Up to 50dB with acoustic glass | 5–8 years | Preferred heritage route, reversible |
| Replacement double glazing | £900–£1,600 | Up to 75% | 32–35dB | 10+ years | Rarely permitted on listed or Conservation Area sashes |
Specialist partners and further reading
We work alongside specialist glazing firms whose technical guidance is useful when specifying a compliant upgrade: Secondary Glazing Specialist on bespoke heritage units, London Secondary Glazing on conservation-compliant options, sash window secondary glazing systems and U-value performance, Soundproof My Window on acoustic specification, and secondary glazing installers on discreet detailing in period reveals.
Why Professional Restoration is Non-Negotiable
The 2026 regulations place a higher burden of proof on homeowners. Our window restoration and heritage window restoration services cover sash cord replacement, hardware refurbishment, timber stabilization and precision balancing — all backed by a 10-year workmanship guarantee.
Homeowners in Chelsea, Kensington and Westminster should audit their windows now. Our emergency services are available during peak winter months. Call 0207 060 1573 for a free survey.