Retrofit Double Glazing Cost 2026: Keeping Your Existing Frames

Retrofit double glazing is for people who want to keep their windows, usually timber sashes or casements, often because the planners want them kept too. There are four routes, and the pages that rank for the term mostly sell one of them. This guide puts the routes side by side on price, with every figure's source and date, then asks the questions that decide the answer before any quote does: will your frames take it, what does each route achieve in heat terms, and which rules apply. It does not average the prices. Where sources disagree, both figures stay.

The four retrofit routes

One sash installer sums up the choice as "reglazing the original sashes ..., new double-glazed sashes hung in your original box, full replacement windows, or secondary glazing fitted inside the room". Historic England describes the first two in almost the same words. Add film and clip-on DIY panels at the cheap end, and the routes are:

• Reglaze the existing sashes or casements. The single pane comes out, the rebate is cut deeper where needed, and a sealed unit goes in. The unit can be a standard argon one, a slimline double glazing unit or a vacuum glazing unit, and the choice drives most of what follows.
• New double glazed sashes in the existing box frame. The frame, pulleys and linings stay; the sashes are made new to take the units. This is what installers do when the old sashes are beyond repair.
• Secondary glazing. A second, independent pane inside the room. The original window is untouched.
• Film and DIY kits. Heat-shrink film or magnetic plastic panels you fit yourself.

What retrofit double glazing costs, route by route

These are per-window figures from the sources named, each with its date and what it includes. Several are from London firms, and two vacuum installers set minimum orders, so read the notes.

Figures per window unless stated, as published by each source; not averaged. Research in lib/research/retrofit-double-glazing-2026.md.

Three things explain the spread. First, what the price wraps in: a reglaze-only job with a standard unit sits at £650 to £1,300, while the vacuum packages at £1,100 to £1,600 include rot repair, new cords and weights and draught-proofing. One of those installers prices restoration alone from £700, typically £820 to £960, and puts reglazing at around 30% on top. Second, glazing bars: London Sash Window Repairs says small sashes with little or no glazing bar land at the lower end, and Scott James prices Georgian sashes separately because the bar work takes longer. Third, VAT, which some figures include and some do not.

On timber for new sashes, London Sash Window Repairs adds 20% for hardwood; Wandsworth adds about 10% for Utile and about 20% for Accoya. The Wandsworth firm's own worked example, three pairs of simple one-over-one sashes in a Fulham basement bay with the frames repainted, came to about £5,000. For a whole new window rather than a retrofit, the sash window cost calculator prices your own openings; the site's range for a new timber heritage sash is £1,330 to £2,850. No source we found prices a casement reglaze on its own, or the rebate routing as a separate job.

Ask every quote the same three questions: which unit goes in, standard, slim or vacuum; does the price include new cords, weights and draught-proofing; and is VAT on top. Most of the gap between two retrofit quotes is in those answers, not in the glass.

Double Glazing Cost Calculator Team

Will your frames take it?

Retrofit double glazing lives or dies on the frame. Four checks decide it, and a surveyor or joiner can answer all of them in one visit.

Rebate depth and unit thickness

Single glazing is 4 to 6 mm thick. Historic England puts a conventional sealed unit at 22 to 28 mm, a slim one at 10 to 16 mm and a vacuum unit at about 6.5 mm. The rebate, the step in the timber that holds the glass, has to take the unit and cover its edge. Historic Environment Scotland describes the rebates being made about 6 mm deeper for slim units. One joiner says a standard unit needs a glazing bar of at least 40 mm, while a slim one fits within 22 mm; a vacuum installer says a bar 22 mm or wider will usually take a vacuum unit without cutting timber. No heritage or standards body publishes a minimum rebate for a standard unit in an old sash, so the answer comes from measuring yours. Historic England has warned that multi-paned windows often cannot take even slim units without altering the frames and bars, which makes a finely barred Georgian sash the hardest case.

Weight and balance

Glass weighs about 2.5 kg per square metre for each millimetre of thickness. A 4 mm single pane is therefore about 10 kg/m², and a standard unit with two 4 mm panes about 20 kg/m². One slim unit is listed at 17.5 kg/m², and a vacuum installer says its units weigh roughly twice the pane they replace. Historic England, Historic Environment Scotland and the installers agree the fix: new cords and heavier weights in the box. One vacuum installer calls skipping this "the most common failure in retrofit reglazing". No source we found covers casement hinges and stays under the extra weight, so ask the fitter how they will deal with it.

Timber condition

Sound timber first. Chameleon Restoration puts it plainly: retrofitting "will not give desired benefits if window frames are not structurally sound or if cold air still comes through the gaps between moving parts of the window." Where a sash is rotten or already lost, London Sash Window Repairs makes new sashes for the original frame, which is route two. Deferring the reglaze has a cost too: one installer notes the sash would have to come out again, be rebated deeper and be rebalanced. The timber windows guide covers what a sound frame looks like.

Putty, beads and drainage

A sealed unit cannot simply be puttied in like a single pane. Historic England warns that "standard linseed oil putty can damage the seals to the units", so a compatible glazing compound is needed, and Historic Scotland's 2013 follow-up said edge-seal deterioration may, in some instances, be caused by incompatibility with the putty. LABC Warranty's guidance asks for a 5 mm drainage path around the unit and says fully bedding units on sealant leads to failures. A unit whose seal goes will mist between the panes, which is a replacement-unit job covered in our blown double glazing guide. Leaded lights are a separate problem with their own routes, in stained glass double glazing.

Metal frames

We found no heritage-body or trade-body guidance on fitting sealed units into old steel or iron frames, only forum opinion. Treat any metal-frame reglaze as a specialist question and price secondary glazing alongside it.

Performance: the one test that measured every route in the same window

Most performance claims for retrofit glazing come from sellers. The exception is Historic Scotland's Technical Paper 1 (October 2008), in which Glasgow Caledonian University tested one sash and case window with each option in turn, finishing by reglazing it with Slimlite low-emissivity double glazed units in the existing sash.

Source: Historic Scotland Technical Paper 1, Improving the thermal performance of traditional windows (October 2008), Table 5. Estimated uncertainty 0.3 W/m²K. A new replacement window must reach 1.4 W/m²K or band B under Approved Document L.

The paper called secondary glazing "the most effective overall option", but the 0.2 gap is inside its own 0.3 uncertainty, so the fair reading is that the two came out about the same. Historic Environment Scotland's 2023 retrofit guide ranks them the other way, 1.3 for a double glazed pane in the existing sash against 1.7 for secondary glazing, without reconciling the two. Its 2010 tests of ten slim systems in Edinburgh sashes found most near 2.0. A vacuum unit does better on paper: Pilkington declares 1.1 for Spacia and 0.7 for its best product.

The same 2008 study measured draught-proofing on its own. It cut air leakage through the window by 86%, while the whole-window U-value moved from 4.5 to 4.2, a difference the authors found not statistically significant. That is the right way to think about it: draught-proofing stops cold air, glazing slows heat through the glass, and a draughty sash with new units in it still leaks. Our guide to draughty windows covers the causes, and draught-proofing a sash costs £260 to £360 from one installer (September 2026) and £328 to £588 for a small or medium sash from another (March 2025).

Regulations: what applies to a retrofit

Building Regulations in England

Approved Document L (2021 edition, with 2023 amendments) treats glass in an existing frame differently from a new window.

The term controlled fitting refers to the entire unit of a window, roof window, rooflight or door, including the frame. Replacing glazing, or a window or door in its existing frame is not providing a controlled fitting. Such work does not need to meet the energy efficiency requirements.

Approved Document L, Volume 1, paragraph 10.6

So a reglaze does not have to hit any U-value. Whether new sashes in an old box count as "a window ... in its existing frame" is a reading to confirm with building control rather than assume. A full replacement is a controlled fitting and must reach 1.4 W/m²K or Window Energy Rating band B, as set out in our Part L building regulations guide. FENSA's compliance guide says its scheme covers full replacement only, because "sash or glass only replacement" is repair work outside it, so there is no FENSA certificate for route one or two. What the scheme does certify is in our FENSA guide.

Planning and Article 4 directions

Under section 55 of the Town and Country Planning Act 1990, works that "do not materially affect the external appearance of the building" are not development. Whether a particular reglaze does is the council's judgement. Historic England's February 2026 advice note adds that houses in conservation areas keep permitted development rights unless an Article 4 direction removes them, while flats and Article 4 homes need permission for many external works. The street-level detail is in our conservation area window rules guide, and the effect on the budget is on the conservation area window costs page.

Listed buildings

Historic England says inserting double or triple glazing into historic windows "will usually require Listed Building Consent", and that secondary glazing "does not generally need" it. Its February 2026 advice note is more open than older guidance: slim-profile or vacuum units within historic frames "will generally be acceptable", except where historic glass of interest survives, the frames cannot support the units, or the windows have leaded lights. So the claim that listed buildings cannot have double glazing is out of date; they often can, with consent. SPAB takes the harder line that double glazing in old windows "is rarely cost-effective". The cost side is on our listed building window costs page.

Scotland

The Scottish Building Standards Technical Handbook (April 2026) lists "any work to a door, window or rooflight, including glazing which is not a complete replacement" among work that needs no building warrant, though it must still meet the standards. For secondary glazing, it says the altered window "should achieve a U-value of about 3.5 W/m2K". Historic Environment Scotland says retrofitting double glazing to a listed building "will be likely to require consent".

VAT

HMRC's VAT Notice 708/6 lists secondary and double glazing as standard-rated. Some pages claim double glazing has carried 0% VAT since 2024; that is wrong. Draught stripping is zero-rated until 31 March 2027, which is why a good quote shows it as its own line.

What DIY film and kits do, and do not do

Heat-shrink film is a thin plastic sheet taped to the frame and tightened with a hair dryer, trapping an air layer against the glass. A Stormguard 6 m² pack sells for £12.99, enough for four to eight average windows, with a one-year guarantee; the maker calls it "seasonal". The Energy Saving Trust says film "doesn't last long and can be hard to make aesthetically pleasing. It offers the least thermal benefits of the options available." No measured U-value for film on a window has been published. The 45% figure sometimes quoted comes from Technical Paper 1's roller blind with low-e film, a different product.

Magnetic panels are a step up: the Energy Saving Trust says they "can help reduce heat loss and can be easily removed or replaced", and the materials run to about £75 to £80 for a 1.2 by 1.0 m window. Neither film nor panels is double glazing. They leave the window's Part L status unchanged and do nothing for a draughty sash. Fitted systems and their prices are on our secondary glazing costs page.

Verdict: which retrofit route fits which home

Draught-proofing comes first on every sash, whatever else you do. It is the cheapest measure, and it is the one the only same-window test found cut air leakage by 86%.

• Sound sashes, few glazing bars, not listed: reglaze with a standard or slim unit. £650 to £1,300 on the published ranges, and outside Part L.
• Sound sashes with fine Georgian bars, or a listed house: a vacuum unit, which needs the least routing, in a package with rebalancing and draught-proofing at £1,100 to £1,600. Expect to need consent if listed.
• Sashes rotten or already lost, box frame sound: new double glazed sashes in the existing box, £820 to £2,000.
• Listed, historic glass, leaded lights or metal frames: secondary glazing, £300 to £500 fitted. It usually needs no consent and matched retrofit double glazing in the one same-window test. Which of the two suits you is weighed in secondary vs double glazing.
• Renting, or one cold winter to get through: film or a magnetic panel, knowing it is a stopgap.
• Frames beyond saving and no heritage constraint: full replacement, which must meet Part L. The Energy Saving Trust puts the saving from A-rated double glazing across a whole single-glazed semi at £85 a year in Great Britain; it publishes no figure for retrofit units or secondary glazing.

If you are still weighing sash styles and how they work, the sash windows guide is the place to start, and the rest of our window and glazing guides cover the other decisions.

Get retrofit or replacement quotes

Retrofit prices vary with the unit, the bars and the timber, so quotes on your own windows are the real evidence. Tell us what you have and Leads 2 Trade will arrange quotes from installers covering your postcode, for reglazing, new sashes or replacement windows.

Price-checkedLast reviewed 23 September 2026

Reviewed byDouble Glazing Cost Calculator TeamCross-checked against Checkatrade, the FMB & GreenMatch