Sash windows: repair, secondary glazing or replace?

White uPVC double glazed window in a brick wall

If your sash windows are original timber, repair them first. Most “rotten” sashes can be refurbished for well under half the cost of new timber ones. Adding draught-proofing and secondary glazing then gets you close to the comfort of double glazing while keeping the original windows.

Replace only when the frames are beyond economic repair, or when the windows are later and poor quality. If you do replace, timber that matches the original is the safe choice for period homes in Shropshire.

The options and what they cost

Option	Typical cost (2026 estimate)	Keeps original windows?
Draught-proofing	about £200 to £400 per window	Yes
Sash cord replacement	about £150 to £300 per window	Yes
Full refurbishment	about £500 to £1,200 per window	Yes
Secondary glazing, sliding	about £200 to £400 per window	Yes
Secondary glazing, bespoke sash-matching	about £400 to £900 per window	Yes
New uPVC sash	about £800 to £1,000 per window	No
New timber sash, double glazed	about £1,500 to £3,000 per window	No

Refurbishment, draught-proofing and cord figures come from Trade2Base (May 2026). Secondary glazing figures come from Less.co.uk (April 2026) and Trade2Base. Replacement prices come from the FMB (October 2026), which puts a uPVC sash at about £805 to £915 and a timber sash at about £2,450 to £2,775 for common sizes.

For a Victorian terrace with eight to ten sashes, Trade2Base puts refurbishment plus secondary glazing at about £8,000 to £14,000. Whole house restoration on its own runs about £4,000 to £12,000.

Option 1: repair and refurbish

A full refurbishment usually means freeing the sashes, cutting out and splicing in new timber where it has rotted, renewing cords and pulleys, reputtying, and repainting. Bottom rails and cills are where rot usually starts.

Heritage guidance favours this. Cotswold District Council says “the first thing you should do is make sure you overhaul and repair your windows”. City of York Council notes that “often windows look in worse condition than they actually are”.

On a listed building, small like-for-like repairs such as a new cill in matching timber do not normally need permission. That is according to York’s December 2025 guidance, but check with Shropshire Council if you are unsure.

Option 2: draught-proofing

Draught-proofing fits brush or rubber seals into the sash and staff beads so the sashes still slide but the gaps close up. It deals with most of the rattle and cold air that make old sashes feel uncomfortable.

At about £200 to £400 per window, it is the cheapest upgrade. Installing draught stripping in a home is zero rated for VAT until 31 March 2027, according to HMRC. After that, the rate reverts to the 5% reduced rate. New windows and secondary glazing are charged at the standard 20%.

York’s guidance says draught-proofing does not usually need permission, even on a listed building.

Option 3: secondary glazing

Secondary glazing is a slim second window fitted inside the original one. For sash windows, the panels normally slide vertically so they line up with the meeting rails.

City of York Council’s guidance calls it “usually a preferred method” of improving single glazed windows. It says that, with the right design and low-E glass, it “can also achieve comparable performance to replacement new double glazed timber windows”.

Two practical points from Cotswold District Council:

If the reveal is deep enough, the secondary panel itself can be double glazed.
When you fit secondary glazing, do not also draught-proof the outer single glazed sashes. Leaving them unsealed reduces “the chances of moisture and mould” building up between the two.

Internal secondary glazing usually needs no planning permission, according to the Planning Portal. In a listed building it may need consent, for example where it would affect internal shutters or panelling.

Option 4: slim double glazing in the existing frames

Slim double glazed units (Cotswold District Council quotes 6 to 14mm) can sometimes replace the single glass in existing sashes. The most effective have a vacuum rather than a gas fill.

There are limits. York’s guidance says the frames must be able to take the deeper rebate and heavier glass “in the same outer face position” without weakening them. On a listed building this needs listed building consent, and it is usually acceptable only where the windows are not leaded and contain no historic glass.

Option 5: new slim double glazed timber sashes

If the windows are too far gone, new timber sashes made to copy the originals are the replacement most councils accept in conservation areas. At about £1,500 to £3,000 per window, this is the most expensive route. The FMB puts triple glazing in timber sashes at about £365 to £415 more per window.

Shropshire Council’s guidance for Article 4 areas says “timber windows that match the original design are generally preferred”, and “double-glazed timber windows … may be acceptable” where they preserve historic character. Ask the joiner to match the glazing bar pattern, horns and meeting rail details of your existing windows.

Option 6: uPVC sashes

uPVC sashes cost roughly a third of the price of new timber and need little upkeep. On a modern or heavily altered house outside a conservation area, they are a reasonable choice. Our uPVC vs timber vs aluminium guide compares them in detail.

On period homes the picture changes. Shropshire Council says “uPVC windows are unlikely to be supported” in Article 4 areas where they would harm the area’s character. York’s guidance says uPVC is not expected to be acceptable on listed buildings and “is usually unable to replicate the design detail and glazing bar proportion of traditional timber windows”.

Sash windows in Shropshire towns

Sashes are a defining feature of Shropshire’s historic towns. The Shrewsbury Town Centre Conservation Area Appraisal describes Georgian brick frontages with “evenly spaced sash and blind windows”.

The Bridgnorth appraisal says “painted timber vertically-sliding sash windows are equally characteristic of the town”. It names uPVC replacements as harmful. Both towns have Article 4 Directions on parts of their conservation areas; check your address in our conservation area guide.

Building Regulations

The government’s guide to building work says most repairs do not need to be notified to building control, but replacing windows, doors or glazing units does. New windows must normally reach a U-value of 1.4 W/m²K or band B.

Approved Document L relaxes this for listed buildings and homes in conservation areas where full compliance would spoil the character. In that case the glass should reach a centre pane U-value of 1.2 W/m²K, or single glazing should be supplemented with low-emissivity secondary glazing. Make sure your installer can issue a certificate; our FENSA guide explains what to ask for.

How to decide

Get the windows surveyed. Ask for a window-by-window condition report, not a whole house replacement quote.
Check if you are listed or in an Article 4 area before ordering anything.
Price the repair route in full, including draught-proofing and secondary glazing, against replacement.
Replace only the windows that are beyond repair. Mixed approaches are common on older houses.

For more prices, see our window and door cost guide.

Got tired sash windows? Get a quote from a checked local installer.

Common questions

Is it cheaper to repair or replace sash windows?

Repair is usually cheaper. Full refurbishment typically costs about £500 to £1,200 per window, while a new timber sash is about £1,500 to £3,000. A uPVC sash is about £800 to £1,000 but may not be allowed on period or listed homes.

Can I put double glazing in my existing sash windows?

Sometimes. Slim or vacuum double glazed units can be fitted into existing frames if the frames can take the deeper rebate and extra weight. On a listed building this will need listed building consent, and it is not usually accepted where the windows have historic glass or leaded lights.

Does secondary glazing need planning permission?

Internal secondary glazing usually does not need planning permission. In a listed building it may need listed building consent, for example where it would affect shutters or panelling, so ask the council first.

How long do timber sash windows last?

With maintenance, a very long time. Cotswold District Council notes that many historic windows are over 100 or even 200 years old. Regular painting, putty repairs and attention to the bottom rails and cills keep them going.