Key Takeaways
- Double-glazed timber sash windows cost £1,500 to £2,000 per window in London (GreenMatch / London Sash, 2026).
- A full-house replacement runs £4,000 to £12,000+ depending on number and material.
- Replacing sashes into existing box frames starts from around £1,250 + VAT per window.
- In conservation areas, timber sash style is usually required, not uPVC.
- Repair and draught-proofing is often a cheaper, conservation-friendly alternative.
Sash windows are one of the defining features of London's period homes, and keeping them right is essential to both comfort and character. This guide covers what sash windows cost in London in 2026, when to repair rather than replace, and how the conservation rules affect your options.
How much do sash windows cost in London?
Double-glazed timber sash windows cost around £1,500 to £2,000 per window in London, according to GreenMatch and London sash specialists. Replacement sashes fitted into existing, sound box frames are more economical, starting from around £1,250 + VAT per window. A full-house replacement typically runs £4,000 to £12,000 or more, depending on the number of windows and the material.
| Option | Typical London cost |
|---|---|
| Replacement sashes into existing frames | from £1,250 + VAT per window |
| New double-glazed timber sash (full) | £1,500 to £2,000 per window |
| uPVC sash window | £520 to £800 per window |
| Full-house replacement | £4,000 to £12,000+ |
Repair or replace?
The first question is whether you need to replace at all. Where the box frames are sound, repairing and draught-proofing the existing sashes is usually cheaper than full replacement, preserves the original character, and is often the only conservation-friendly option. Full replacement makes sense where frames are rotten beyond repair, or where you want the thermal performance of new double-glazed units throughout the house.
Conservation areas and listed buildings
This is the crucial constraint for London period homes. In conservation areas and on listed buildings, the council usually requires timber sash windows that match the original profiles and slim sightlines, and bulky uPVC is generally refused. Slim double-glazed units, replacement sashes into existing frames, and secondary glazing are typically the acceptable routes. Where an Article 4 direction applies, you will need consent even to replace windows, so always check first.
Timber vs uPVC
Timber offers the authentic look, slim profiles and repairability that period homes need, and can be repainted and maintained for decades. uPVC is cheaper and low-maintenance but its heavier frames and modern appearance are usually unsuitable for period and conservation properties. For most London period homes, timber is both the required and the right choice.
Energy efficiency
Original single-glazed sashes are draughty, but you do not have to lose the character to fix that. Modern double-glazed sashes, draught-proofing, and secondary glazing dramatically improve comfort and cut heat loss while keeping the period look. Secondary glazing is often the most effective and conservation-friendly upgrade of all.
GS Renovation restores and replaces sash windows sympathetically as part of period property renovations across London. For advice on the right approach for your home, contact us or call 07472 424 226. See also renovating a period property.