When to Replace vs Repair Windows: A Triage Matrix

The average UK window replacement costs £500-£1,200 per unit fitted. A professional repair costs £80-£300. The wrong decision — replacing a window that could be repaired, or repairing one that will fail again in two years — wastes thousands. This guide gives you a decision framework, not a guess.

The triage principle

Before you look at any specific window, establish the frame material. This single fact determines the repair economics:

Frame material	Realistic repair lifespan	Replacement cost (per unit)	Repair cost (per unit)	Repair almost always wins if…
Timber (pre-1970)	10-25 years with proper joinery	£1,500-£3,000	£150-£500	Frame is sound — rot is localised
Timber (post-1970, softwood)	5-10 years	£1,200-£2,500	£200-£600	Rot has not reached the joint
uPVC	Limited — hardware only	£400-£900	£80-£250	Problem is hardware, not the frame
Aluminium	Limited — hardware only	£800-£1,500	£80-£250	Problem is hardware, not the frame

Timber windows can be repaired indefinitely if the frame is structurally sound. uPVC and aluminium windows cannot be repaired at the frame level — once the profile degrades or discolours, replacement is the only option.

Step 1: Start with the glazing

The first question is not about the frame — it is about the glass. If the sealed unit has failed (condensation between the panes), the decision tree forks:

Sealed unit has failed (misty/condensation between panes)

A failed double-glazed unit has lost its argon or krypton gas fill. The U-value has risen from approximately 1.0 to approximately 2.8 W/m²K — effectively the same as old double glazing. The window is still weathertight (the outer seal keeps rain out), but it is no longer energy efficient.

Can the unit be replaced without replacing the frame?

• uPVC with glazing beads: Yes. The beads are removed, the failed unit is swapped, new beads are fitted. Cost: £120-£250 per unit including labour. This is a repair, not a replacement.
• Timber with glazing beads or putty: Yes, but it requires a joiner. Timber beads are more fragile than uPVC and putty needs to be replaced. Cost: £150-£300 per unit.
• Aluminium with pressure plates: Yes. The pressure plate screws are removed, the unit is swapped, and new seals are fitted. Cost: £150-£300 per unit.
• Aluminium with structural glazing (bonded): No. The glass is bonded to the frame with structural silicone and cannot be removed without destroying the frame. The entire frame must be replaced.

Decision: If the frame is sound and the unit can be re-glazed (which it can in 95% of residential windows), replace the sealed unit — do not replace the whole window.

Multiple sealed units have failed

If three or more units have failed in a single elevation, the cause is usually systemic — the original units were poorly manufactured or installed. In this case, replacing individual units is a temporary fix. The remaining units will fail within a few years.

Decision: If more than 50% of the units in a single window run have failed, replace the windows rather than re-glazing. You will pay more now but avoid two rounds of scaffolding and disruption.

Step 2: Assess the frame

If the glazing is sound (or has been addressed), examine the frame.

Timber frames: the wet-finger test

Press your thumb firmly into the frame at the bottom rail and the sill junction. If the timber gives under pressure — even slightly — it has internal rot. Surface paint flaking is cosmetic; soft timber is structural.

Condition	Action	Cost
Surface paint flaking, no softness	Repaint — this is maintenance, not a repair	£200-£400 per window (if doing the whole house)
Localised soft patch (< 5cm)	Timber repair with two-part epoxy filler or splice	£150-£300 per repair
Soft bottom rail, sound jambs	Replace bottom rail only (joiner required)	£250-£500 per window
Soft jambs and sill	Replace the frame — repair is not economic	£1,500-£3,000 per window
Fungal growth (white mycelium)	Replace the frame — the rot has spread internally beyond what is visible	£1,500-£3,000 per window

uPVC frames: the alignment test

Open the window. Does the sash sit squarely in the frame, or does it drop at one corner? uPVC windows that are out of alignment have either:

1. Failed hinges (the sash drops at the hinge side) — repairable at £80-£150 per hinge pair
2. Warped frame (the sash drops at the lock side) — not repairable; replacement required

Other uPVC failure modes:

Condition	Repairable?	Cost
Yellowing or discolouration	No — the polymer has degraded	Replacement only
Cracked weld at the corner	No — the structural integrity is gone	Replacement only
Failed gasket (draughts)	Yes — gasket can be replaced	£80-£150 per window
Failed locking mechanism	Yes — replace the gearbox or handle	£100-£200 per window
Blown sealed unit	Yes — re-glaze (see above)	£120-£250 per unit

Aluminium frames: the corrosion test

Aluminium is the most durable frame material — but it is not indestructible. Check for:

Condition	Repairable?	Cost
Powder-coat fading	No — re-coating is specialist and costly	Live with it or replace
Surface corrosion (pitting)	Yes — if minor, a protective wax coating slows it	£20-£50 per window (DIY)
Structural corrosion at joints	No — the frame is compromised	Replacement only
Failed locking mechanism	Yes	£100-£200 per window
Failed hinges	Yes	£80-£150 per hinge pair

Step 3: Consider the regulation trigger

Replacing a window triggers Building Regulations compliance — the new window must meet Part L (U-value 1.6 for replacements), Part K (safety glass in critical locations), and Part Q (PAS 24 security). Repairing a window does not trigger any of these.

This matters because a window that you could repair for £200 might cost £800 to replace — not because the window itself is more expensive, but because the replacement must meet 2022 standards. If your existing window is pre-2002 and you are in a conservation area, the replacement must also have planning permission, which adds 8-12 weeks and potentially £300-£500 in fees.

Decision: If the repair cost is less than 40% of the replacement cost, repair — unless you plan to sell the property within 2 years, in which case replacement provides a cleaner EPC and certificate trail.

Step 4: Factor in the heritage dimension

In a listed building or conservation area, the rules change fundamentally. Replacing windows almost always requires consent. Repairing windows almost never does.

The Society for the Protection of Ancient Buildings (SPAB) advocates a “repair not replace” philosophy that aligns with conservation law. Their position is clear: every time you replace an original window, you permanently lose part of the building’s historic fabric. A replaced sash window is a replica — it is not original.

For listed buildings:

• Repair is always the first option — even at higher cost than replacement
• Secondary glazing is the thermal upgrade — not replacement double glazing
• Re-glazing the existing frame with slim-profile double glazing is acceptable in some Grade II properties with conservation officer approval

See our secondary glazing for listed buildings guide and sash window repair vs replace guide for the heritage-specific decision framework.

The triage matrix: your quick reference

For each window, answer four questions and follow the matrix:

Question	Answer A	Answer B	Answer C
1. Is the sealed unit failed?	No → skip	Yes, one unit	Yes, multiple units
2. Is the frame sound?	Yes	Localised rot	Frame-wide rot or warp
3. Is the property listed/in conservation area?	No	Yes	—
4. Repair cost vs replacement cost?	< 40% of replacement	40-70% of replacement	> 70% of replacement

Results

1	2	3	4	Decision
No	Yes	No	< 40%	Repair (hardware)
No	Yes	Yes	Any	Repair (heritage)
No	Localised rot	No	< 40%	Repair (joinery)
No	Localised rot	No	40-70%	Repair (borderline — consider replacement if budget allows)
No	Frame-wide	Any	Any	Replace
Yes (one)	Yes	Any	Any	Re-glaze the failed unit
Yes (one)	Localised rot	No	< 70%	Re-glaze + repair frame
Yes (one)	Frame-wide	Any	Any	Replace
Yes (multiple)	Yes	No	Any	Replace (systemic failure — remaining units will follow)
Yes (multiple)	Yes	Yes	Any	Re-glaze (heritage — replacement requires consent)
Yes (multiple)	Localised/frame-wide	Any	Any	Replace

When replacement is the clear winner

• uPVC windows older than 20 years: The polymer is degrading. Repairs will become more frequent. Replace the full set.
• Softwood timber windows with frame-wide rot: The cost of a joiner to splice every rail and stile will exceed replacement. Replace.
• Any window where the frame is distorted: A warped frame cannot be made weathertight. Replace.
• Multiple sealed unit failures across the house: The original units were poor quality. Replace the windows with a 10-year guarantee on the sealed units.

When repair is the clear winner

• Failed hinges or handles on a sound uPVC or aluminium frame: £100-£200 versus £500-£900 for replacement. No contest.
• Single blown sealed unit in a sound frame: £150-£250 versus £500-£1,200 for replacement. Re-glaze.
• Localised rot in a quality timber frame: £250-£500 for a splice versus £1,500-£3,000 for replacement. The original frame, once repaired, may outlast a replacement.
• Any window in a listed building: Repair unless the frame is structurally unsound. The law favours repair, and the conservation officer will too.

Summary

• Start with the glazing: a failed unit does not mean a failed window
• Timber frames are repairable; uPVC and aluminium frames are mostly not (at the frame level)
• Repair is preferred when it costs less than 40% of replacement, unless you are selling within 2 years
• In listed buildings and conservation areas, repair is almost always the correct first option
• Multiple sealed unit failures indicate systemic issues — replace rather than re-glaze piecemeal