How Much Does Sash Window Restoration Cost Compared to Replacement in 2026?
If your sash windows are draughty, sticking, or showing signs of rot, you will need to decide whether to restore them or replace them entirely. The cost difference between the two options is significant.
In 2026, restoring a pair of box sash windows costs £800–£1,200 including VAT, while a like-for-like replacement costs £1,800–£3,200 for the same pair. That is a saving of £600–£2,000 per pair by choosing restoration (Energy Saving Trust, 2026; Federation of Master Builders, 2026).
Restoration work typically takes 2–4 days per window. Replacement can be completed in 1–2 days but often requires scaffolding and waste removal, which adds to the total cost.
How Much Energy Can Restored Sash Windows Realistically Save?
A restored sash window with new draught-proofing, parting beads, and staff beads achieves a U-value of 2.0–2.8 W/m²K. A U-value measures how quickly heat passes through a window; lower numbers mean better insulation. Modern double-glazed replacement sash windows achieve 1.6–2.0 W/m²K (DESNZ, 2026; Energy Saving Trust, 2026).
For a typical 3-bed semi-detached house with gas central heating and an EPC band D, restoring a pair of windows reduces your annual heating bill by £40–£80 compared to leaving them unrestored. However, restored windows still cost roughly £20 more per year to heat than modern double-glazed replacements (Energy Saving Trust, 2026).
The energy saving comes mainly from draught-proofing rather than improved glazing. If your existing windows are single-glazed with no draught seals, restoration can cut heat loss by up to 50%.
Quick Numbers Cost, U-Value, and Payback Comparison
| Item | Restoration (per pair) | Replacement (per pair) | Difference |
|---|---|---|---|
| Average cost (inc. VAT) | £1,000 | £2,500 | £1,500 saving |
| Typical U-value achieved | 2.4 W/m²K | 1.8 W/m²K | 0.6 W/m²K worse |
| Annual heating cost impact | +£20 vs modern DG | Baseline | Restoration costs £20/year more to heat |
| Payback period (vs replacement) | Immediate (lower upfront) | Never pays back vs restoration | Restoration wins on cost |
Source: Energy Saving Trust, 2026; DESNZ, 2026; MCS, 2026.
When Restoration Is the Cheaper Choice (and When It Isn’t)
Restoration is cheapest when the sashes, frames, and cills are structurally sound. Historic England defines this as rot affecting less than 10% of the timber volume (Historic England, 2026). In that case, you pay £800–£1,200 per pair for restoration, and the work lasts 20–30 years.
Replacement becomes cheaper per square metre if more than 40% of the timber is rotten. At that level of decay, restoration can cost £1,500 or more per pair because the joiner must rebuild large sections of the frame. If your window is single-glazed with no historic value and more than 40% rot, replacement is often the more cost-effective option (Federation of Master Builders, 2026).
If you are unsure about the condition of your timber, ask a TrustMark-registered window specialist to inspect the cills and bottom rails first. how to check for rot in sash windows
The Direct Answer How to Decide Between Restoration and Replacement
For a listed building or conservation area property in England, restoration is almost always required by law unless the window is beyond economic repair. You should check your local authority’s guidance before ordering any work (GOV.UK, 2026).
If your window is structurally sound and you want to save money, choose restoration. If you need the best thermal performance and have the budget for replacement, choose modern double-glazed sash windows. Restoration saves you £600–£2,000 per pair upfront but costs roughly £20 more per year to heat than a replacement. Over 20 years, that is a total of £400 in extra heating costs, which is still less than the upfront saving.
How to Verify Your Installer Is Certified for Sash Window Work
For restoration work, TrustMark registration is sufficient for most jobs. If you are adding draught-proofing as an energy efficiency measure, the installer should hold MCS (Microgeneration Certification Scheme) certification (MCS, 2026; TrustMark, 2026).
For replacement windows, the installer must hold FENSA or CERTASS certification to comply with building regulations Part L (2026). You can check the register at fensa.org.uk or certass.co.uk (GOV.UK, 2026).
Always ask for written confirmation of the installer’s certification before work begins. If they cannot provide it, find another contractor.
What Grants or Funding Are Available for Sash Window Work in 2026
The Great British Insulation Scheme (GBIS) covers draught-proofing of existing windows for eligible low-income households. You can check eligibility at gov.uk/gbis (DESNZ, 2026).
The Home Upgrade Grant (HUG2) includes window draught-proofing for off-gas-grid homes in England. You apply through your local authority (DESNZ, 2026).
No government grant covers full sash window replacement unless the property is off-gas-grid and the household is on a low income. In that case, HUG2 may fund replacement windows, but this is limited to homes with no mains gas supply (GOV.UK, 2026).
If you live in a conservation area or listed building, check with your local authority for any local grant schemes that support heritage window repair. These are rare but may exist in some areas. energy grants for listed buildings in 2026
How Long Restored Sash Windows Last Compared to New Ones
Professional restoration with treated timber, new cords, and draught-proofing lasts 20–30 years before needing re-cording or re-painting (Energy Saving Trust, 2026).
Modern double-glazed replacement sash windows have a 15–25 year lifespan. After that, the sealed units may fog up and need replacing. The timber frames themselves can last longer if maintained, but the glazing units are the limiting factor (Federation of Master Builders, 2026).
Restored windows require repainting every 5–8 years and re-cording every 20–30 years. Replacement windows also need repainting every 5–8 years, plus sealed unit replacement after 15–25 years. Over a 30-year period, restoration typically costs less in total maintenance than replacement.
Frequently Asked Questions
In 2026, restoring a pair of box sash windows costs £800–£1,200 including VAT, according to the Federation of Master Builders. Costs vary by size, condition, and location.
Restoring is cheaper: £800–£1,200 per pair versus £1,800–£3,200 for like-for-like replacement, per Energy Saving Trust data. You save £600–£2,000 per pair by restoring.
Yes, restored windows with new draught-proofing reduce heating bills by £40–£80 per year compared to unrestored windows, based on Energy Saving Trust figures. They still cost £20 more per year to heat than modern double-glazed replacements.
Restored sash windows with draught-proofing achieve a U-value of 2.0–2.8 W/m²K, as stated by DESNZ. Modern double-glazed replacements achieve 1.6–2.0 W/m²K.
Restoration typically takes 2–4 days per window, while replacement can be done in 1–2 days but often requires scaffolding and waste removal, according to the Federation of Master Builders.
