Choosing between full removal and installation on the existing frame is not a matter of preference. Keeping a frame means taking responsibility for it, which is why BS 8213-4 makes a survey of the existing opening the starting point of the job, and why a frame that carries a guarding function is not a frame you install over. The performance threshold, meanwhile, has nothing to do with the method: Approved Document L sets a replacement window at a Uw of 1.4 W/m²·K or better, whatever the frame underneath. The method is therefore decided on the condition of the retained frame and on how much rework you are prepared to take on, not on the energy rating of the unit.
Understanding the two window-removal methods: what you're really changing
Here is what each method actually changes on site.
| What changes | Full removal | Installation on the existing frame |
|---|---|---|
| What you take out | Frame, sashes, jamb linings and sometimes the trim | Nothing of the frame: it stays in place and takes a new frame on top, then cover strips |
| The opening | Reworked before installing: sealing, weatherproofing, sills | Kept |
| Glazed area | Gained | A bit of light passage lost |
| The job | Longer, more finishing rework | Faster and less invasive |
Full removal: taking out the existing frame and fully reworking the opening
It's the cleanest method for starting from a sound base and aiming for lasting airtightness.
Installation on the existing frame: keeping the frame and adding a new one
In exchange, the performance depends heavily on the condition of the substrate and the installation.
When the existing frame becomes a weak point (wood, PVC, aluminium)
The frame becomes a weak point if it's deformed, poorly anchored, or if the old sealing lets air and water through. In wood, watch for blackened or soft zones. In PVC, watch for deformation and poorly sealed cuts. In aluminium, watch for thermal bridges and corrosion of the fixings. In these cases, full removal avoids "building on top of a problem", related to the trade-offs between full replacement vs changing just the glazing.
Full removal: when a window renovation justifies the heavier intervention
Gaining airtightness and avoiding sealing defects
When the old frame is kept, the connection between the new window and the building often relies on "best effort" sealing. With full removal, you start from a bare substrate. You can address the clearances, install suitable compression tape and take care with sealing continuity all the way around. The result: fewer air infiltrations, fewer cold draughts and less risk of condensation around the perimeter.
Correcting plumb defects and damaged frames without a patch job
If the frame is warped, rotten or cracked, installing on top of it in renovation masks the problem without fixing it. Full removal lets you restore square, rework the reveals and fix the window on a sound support. You avoid endless shimming and closures that force. It's cleaner, more durable, more adjustable.
Preserving glazed area: avoiding a loss of light
Installing on the existing frame often reduces the glazing area. In a small room, that shows quickly. With full removal, you limit the loss of glazed surface and keep a good glazed area. You gain natural light, and sometimes solar gains, without changing the balance of the façade.
Installation on the existing frame: the right choice when the substrate is sound
Checking the frame: rigidity, condition of the wood, fixing and absence of moisture
This method involves keeping the existing frame and fixing the new window to it. Before starting, the substrate is checked point by point.
What you check on the retained frame
- The frame properly square, rigid and solidly anchored into the masonry.
- On wood, the sensitive zones (bottom, corners, transom) free of any trace of rot or swelling.
- With an awl or screwdriver, the wood should stay hard.
- No lingering moisture, or the installation won't hold.
Limiting interior work: trim, finishing and managing the sills
The advantage is reducing the mess. Most of the time, you keep the reveals and part of the finishes. Plan for clean trim to cover the old frame, and anticipate jamb extensions if interior insulation has thickened the wall. On the sill side, take care with water drainage: drip edge, slope, and continuity with the sill piece.
Watch out for performance: thermal bridges, seals and added thickness
You save time, but not at the cost of airtightness. Treat the perimeter with suitable seals, continuous and durable over time. Otherwise, air infiltration and thermal bridges come back quickly. Another point: the added thickness reduces the glazed area and can change the light comfort. Also check the adjustment of the sashes, to keep a clean, even closure.
Choosing the best window solution in 2026: technical criteria and site constraints
Depending on the wall and reveal type: stone, brick, concrete, timber frame
The substrate dictates the method. In stone, the reveals are rarely square. Plan for shimming and localized rework to guarantee airtightness. In brick, watch for chipping when drilling and the thermal bridge at the frame. In concrete, fixing is simpler but the adjustment must stay precise. In a timber frame, the window must respect the membranes. Take care with the rain barrier and vapour barrier connections.
Depending on the goal: insulation, acoustics, security, façade aesthetics
For thermal comfort, aim for a low Uw and glazing suited to the orientation. In a noisy area, favour acoustic glazing. For security, laminated glazing and reinforced hardware make the difference. On the façade, check the constraints on colour, glazing bars or alignment. Your window needs to fit in without degrading the envelope.
Depending on the schedule and budget: installation time, rework, overall cost
Installation time is only part of the equation. Cost out the overall price including trim, sills, touch-ups, and disposal. In 2026, check the subsidies at quote time, since the rules keep changing.
Thresholds and evidence in 2026: what goes on the quotation
The U-values that decide whether the work is compliant
Approved Document L does not care how you installed the unit, only what the installed unit achieves. The thresholds below are the ones that must appear on the quotation and the invoice, taken from the manufacturer's declaration of performance rather than from a brochure.
| Component | Maximum U-value (W/m²·K) |
|---|---|
| Replacement window | 1.4 |
| Replacement rooflight | 2.2 |
| Replacement external door | 1.4 |
What you remain liable for beyond the product
The U-value on the data sheet says nothing about what you hand over. What you are liable for is the continuity of the air seal around the perimeter, the treatment of sills and reveals, and the ventilation of the dwelling once the units are in. Record the existing background ventilators and their compatibility with the system in place before ordering, as Approved Document F requires: an airtight window fitted onto undersized ventilation produces condensation at the reveal, and you are the one going back to it.
The evidence pack to build during the job
A detailed quotation (number, dimensions, opening type, U-value and g-value per reference), a pre-works record of the condition of any retained frames, before, during and after photos, the installation instructions and the delivery note. The pre-works record is what protects you in a dispute over an installation on an existing frame: without it, the condition of the substrate before you arrived is no longer demonstrable. That window schedule need not be built by hand: Argile's AI picks out the openings on the façades and feeds them into the pricing.
