Double Glazed Sash Windows Cost: 2026 London Prices

The quote says £1,400 a window and explains nothing. You are told you are buying double glazing, so you assume most of that is glass. The glass in it is worth about £200. Everything else is joinery, paint, labour, access, making good and VAT, and once you know roughly how those split you can read a quote you would otherwise have to take on trust.

Taking a £1,400 window apart

These are working figures for a middle-of-the-road job: new double glazed sashes made to match and hung in your existing box frame, which is the commonest sash upgrade in London. They are not a price list and they will not match your quote line for line. They are the right order of magnitude, rounded.

Glass is about a seventh of the bill. That single fact explains most of the confusion around sash pricing. A firm buying a cheaper unit has not made you a cheaper window, and a firm charging twice as much is almost never charging twice as much for glass. The money sits in joinery hours, in how many times somebody has to come back, and in what has quietly been left off the page. Every double glazing price we put on a sash window is built from those five lines plus VAT, and any quote worth comparing can be taken apart the same way.

The four jobs hiding behind one phrase

"Double glazed sash windows" covers four different pieces of work with well over £2,000 a window between them. Firms rarely say which one they have priced.

Supplied and fitted, VAT included, London. Access, size, condition and borough move all four. A multi-pane Georgian sash sits at the top of every band, because six or eight small units and the beads to hold them take far longer to make and fit than two large sheets.

Reglazing is the cheapest route and, on a sound window, usually the right one. The catch is rebate depth. Most Victorian and Edwardian sashes have a glazing rebate between 7mm and 12mm, cut for 3mm or 4mm glass. A 14mm slim unit will not go in without the joiner deepening the rebate and losing some of the original section; a vacuum unit at around 8mm usually will. The trade-offs by unit depth are set out in the piece on slimline units and sightlines.

New sashes into the old box is where most London specialists land when the sashes are past saving and the frame is not. What pushes a sash up that £820 to £2,000 band is joinery detail: run-through horns rather than stuck-on ones, a deep bottom rail, a putty-line glazing profile instead of a chunky modern bead, a slim meeting rail. Each of those costs the joiner time. Each is the difference between a window that reads as original from the pavement and one that does not.

Complete replacement takes the box out of the reveal, which means damage to plaster inside and pointing outside, and making good is a separate cost that quotes routinely bury. Ten to twelve windows done this way is £20,000 to £40,000 before decoration and scaffold. That is why it is the wrong answer for most period houses and the default answer from most national window companies. Our fuller breakdown of replacement costs works the sums through house by house.

The realistic whole-house number for the middle option is worth carrying in your head instead. Ten windows at a typical £1,400 is £14,000, and once decoration, access and making good are in, most London terraces land somewhere around £18,000 to £20,000 delivered.

What actually moves the number in London

Access. A ground-floor sash you can work from the pavement and a fourth-floor front on a red route are not the same job with a different postcode. Towers, scaffold, highway licences and suspended parking bays are genuine four-figure items on a tall terrace. Get access priced as its own line rather than assumed into the window rate.

Consent. Many London boroughs have taken away permitted development rights for window changes with an Article 4 direction, and coverage varies not just between boroughs but between conservation areas inside one borough. Check your council's Article 4 map before you assume anything. Flats do not have the permitted development rights houses do in the first place; a genuinely like-for-like replacement in a flat may not count as development at all, but anything that visibly alters the window normally needs an application, and councils read that more or less strictly. Whether you need permission is the first thing to settle, because it sets the timetable for everything after it.

What consent costs. A householder planning application in England is £548 from April 2026. Flats are excluded from the householder route, so an application for a flat goes in as a full application, at the same £548. Listed building consent carries no fee at all, which surprises people, but the drawings, window schedule and heritage statement that go with it are chargeable work. Add six to ten weeks either way. A firm that prices the application honestly will look dearer than one assuming you will never need it.

Joinery. London sashes are big and the sections are often heavier than catalogue stock. Matching them properly means bespoke work, and bespoke work is priced in hours.

The costs that arrive after you have signed

VAT. Glazing is standard-rated at 20% and there is no way round it. The zero rate for energy-saving materials, which runs to 31 March 2027 before dropping back to a 5% reduced rate, covers draught stripping and insulation. HMRC's list puts double glazing, secondary glazing and low-emissivity glass outside it. There is a practical consequence: draught-proofing carried out as its own job can be zero-rated, where the same work folded into a glazing contract cannot. Ask before the paperwork is drawn up, not after.

Building Regulations. A replacement window in an existing home has to reach a limiting U-value of 1.4 W/m²K, or Window Energy Rating band B. Period buildings get a concession. Where meeting that figure would damage the character of the building, a centre-pane value of 1.2 W/m²K is accepted instead, or single glazing may be kept and backed with low-emissivity secondary glazing. That clause is what makes slim and vacuum units viable in a conservation area at all, and it is worth knowing it exists before an installer tells you a new window is your only compliant option. Replacing the glass alone in an existing sash is generally not a notifiable replacement. Our guide to building regulations for windows works through it, and your borough's building control will confirm your own case.

FENSA and Certass. Competent person schemes that let an installer self-certify a replacement window against Building Regulations rather than pay building control to inspect it. They cover replacement, not repair, so a certificate produced for a reglaze or an overhaul is decoration. They also say nothing whatever about planning permission or listed building consent, which are separate approvals from a different department, and a certificate is not evidence you obtained them. FENSA's own guidance treats listed buildings and conservation areas as cases to work through with your installer rather than work its installers cannot touch. If that is your situation, ask the firm directly how the job will be certified and which consents it has assumed you already hold.

The late extras. Making good plaster and pointing. Redecorating the reveals. Taking off and refitting shutters, blinds and curtain poles. Waste disposal. Lead paint, which pre-1960s sashes very often carry and which is slower and dearer to remove safely than sanding. Get each of them named in the quote, or agreed in writing as excluded.

It will not pay for itself on the gas bill

Rough working for a typical London terrace with about 12 m² of glazing. Going from single glass at roughly 5.4 W/m²K to a slim unit at roughly 1.8 saves about 3.6 W per square metre per degree, so around 43 W per degree across the whole house. London runs somewhere near 2,000 heating degree days a year, which works out at about 2,000 kWh of heat, or around 2,300 kWh of gas once boiler efficiency is allowed for. At recent domestic prices that is £150 to £200 a year, and the true figure will be lower, because the arithmetic ignores the frame and treats the house as though it were sealed.

Set £160 a year against £18,000 of new sashes and the payback is not measured in decades. It is measured in lifetimes. That is not an argument against double glazing. It is an argument for buying it for the reasons it actually delivers on: comfort, quiet, an end to condensation running down the pane, an end to rattling. Those are real and worth paying for. The gas saving is a rounding error, and a firm leading with it is selling.

Draught-proofing does not pay back quickly on gas either. What it does is buy more comfort per pound than anything else on the list, because what makes a sash-windowed room feel cold is mostly air moving through the gaps rather than heat crossing the glass. A proper draught proofing job runs £300 to £600 a window and is the first place to put money if your sashes are structurally sound. Where the complaint is noise rather than cold, secondary glazing at £300 to £700 a window beats double glazing on both price and decibels, and the full comparison of secondary and double glazing explains why.

Eight questions that expose the gap between quotes

Ask every firm the same eight. Whichever one goes unanswered is where the price difference is hiding.

1. Which of the four jobs above is being priced, window by window.
2. Timber species and grade. "Softwood" is not a specification: Baltic redwood, engineered laminated pine and accoya are three prices and three lifespans.
3. The glazing unit: overall thickness, make-up, gas fill, spacer type, warranty term and who honours it. Treat any slim unit warranted for under ten years with suspicion; the edge seal is organic and it is the part that fails.
4. Whether sash cords and weights are being renewed and rebalanced.
5. Draught-proofing: included or not, and which system.
6. Ironmongery: makes and finishes, not "period style".
7. Finish: number of coats, primer system, site or factory applied.
8. Access, making good, waste and VAT, each on its own line.

A quote that answers all eight and comes in higher than one answering three is usually the cheaper job by the time the last van leaves.

Three checks to make before you spend anything

1. Push each bottom sash up and let go. If it drops back, the balance is already out, and cords and weights are in your future whatever you decide about glass.
2. Press a bradawl into the bottom rail and the cill, at the corners and under the putty line. Soft timber means repair costs that no glazing quote covers.
3. Take the staff bead off one sash and measure the rebate depth. That one measurement decides whether reglazing is even possible, and it is the difference between a £1,000 window and a £3,000 one.

Do those three things and you will know which of the four jobs you are shopping for before anybody crosses your threshold. If you would rather have the same checks done for you, with every window measured and priced separately rather than averaged across the house, that is the job of a proper sash window survey.