Secondary Glazing Cost Per Window: A Real Breakdown

The frame system

The single biggest variable is which system the opening needs. A lift out panel is the simplest thing we make: one frame, one panel, turn catches, no moving parts. A two panel horizontal slider adds tracks, rollers, interlocking stiles and a lock. A balanced vertical slider adds concealed spring balances that have to be tuned to the exact weight of the glass in that sash, which is skilled work and is reflected in the price. A hinged unit sits between the two. If you are comparing quotes, check you are comparing the same system before anything else, because a slider against a lift out is not a like for like comparison.

• Lift out panel: the most affordable per square metre
• Horizontal slider: the common mid range choice
• Hinged unit: full access, moderate cost
• Vertical slider: balances and tuning add cost

The glass

Glass is priced by square metre and by specification. Standard 4mm toughened float is the baseline. Low E soft coat glass, which reflects room heat back inside, adds a modest amount and usually pays for itself if warmth is your goal. Acoustic laminated glass in 6.4mm or 6.8mm costs noticeably more because it is two panes bonded around a damping interlayer, and it is heavier, which in turn can push a slider up a hardware grade. On a large window the glass can be the majority of the panel cost, which is why we would rather specify it accurately than sell everyone the thickest option.

Size and the awkwardness factor

Price does not rise in a straight line with size. A window that is slightly over a standard sash size can force a three panel configuration or a heavier section, and a very tall opening may need an intermediate transom. Arched heads, splayed reveals, out of square openings and stone mullions all need templating rather than simple measuring, and templating is an extra visit and an extra manufacturing step. None of that is padding, it is genuinely more work.

The cavity and the subframe

Where the existing reveal is shallow, or where the original window furniture projects into the room, we may need to build the new frame out on a timber or aluminium subframe so the cavity is deep enough to be worth having and the new sashes clear the old handles. A subframe adds material and fitting time. It is also frequently the thing that separates a quote that will actually perform acoustically from one that will not, so it is worth understanding rather than stripping out.

Finish and colour

White is the standard polyester powder coat. Any other RAL shade, including the heritage greys and blacks that suit period joinery, is a separate coating run and carries a surcharge. Dual colour, where the room side and the cavity side differ, costs more again. On a whole house order the colour premium is spread across many frames and becomes proportionally smaller, which is one of the few genuine economies of scale in this trade.

Survey, fitting and making good

A quotation should include the technical survey, delivery, fitting, sealing to the reveal, removal of packaging and a working demonstration. Fitting time per window is usually measured in hours rather than days, but access changes that: a first floor bay over a conservatory roof, a stairwell window or anything needing a tower rather than a step ladder takes longer. Ask whether scaffolding or internal access equipment is included rather than assuming.

• Technical survey and templating
• Manufacture to your exact sizes
• Delivery and protection of the room
• Fitting, sealing and demonstration

What a fair quote looks like

A fair quote is itemised per window, names the system, states the glass specification in millimetres, states the frame colour by RAL number, gives a cavity depth, and puts a total with VAT shown separately. If a quote is a single lump sum with no specification, you cannot compare it to anything and you cannot hold anyone to it later. Ours are written per opening for exactly that reason.