Wimbledon’s Victorian, Edwardian and inter-war houses take aluminium glazing well, but the frame has to suit the elevation it sits in. Steel-look profiles stand in for Crittall-era steel windows and internal screens; slim-frame sliding doors open up side-return and rear extensions. Direct Trade Windows has taken 16 orders at 11 distinct SW19 postcodes since 2022.
Wimbledon is one of the places where our search presence has never matched our van diary. We manufacture aluminium in Slough and we have been working across SW19 since 2022, mostly on the two project shapes the area produces over and over: a period frontage that has to stay recognisable, and a rear extension that wants as much glass as the structure will carry. Every figure below is either published manufacturer test data or a count from our own order book.
What glazing suits a Wimbledon side-return or rear extension?
Most SW19 extension projects use a slim-frame aluminium sliding door across the rear opening, a fixed picture window or rooflight where daylight has to reach deeper into the plan, and steel-look internal screens to zone the knocked-through ground floor without losing light.
The side-return extension is the dominant shape in Wimbledon’s terraced and semi-detached stock, and it creates a specific glazing problem: a long, narrow plan with the only significant daylight at one end. That is why our SW19 jobs so often combine three things — a wide slider at the garden end, rooflights or structural glazing over the deep middle, and internal glazed screens so the light that does arrive is not stopped by a solid wall.
On the sliding door, the constraint people underestimate is sash size rather than overall width. Cortizo’s published technical file for COR Vision gives a maximum sash of 2,500 mm wide by 3,000 mm high at up to 320 kg, with configurations of one, two, three, four or six sashes on one, two or three rails. That is what decides whether your 4.8 m opening is two big panels or three smaller ones, and it is worth settling before the steelwork is designed, not after.
The same technical file carries the performance data, which is worth quoting properly rather than in round figures:
- Thermal transmittance: Uw ≥ 1.3 W/m²K, subject to typology, dimensions and glass specification
- Acoustic insulation: up to Rw = 41 dB
- Glazing: 26 mm minimum, 30 mm maximum unit thickness
- Weather performance: air permeability Class 4 (BS EN 12207), watertightness Class 7A (BS EN 12208), wind resistance Class C5 (BS EN 12210)
- Sightlines: 37 mm sash, 116 mm frame (182 mm on three rails). Direct Trade Windows publishes a 20 mm central interlock for this system — that is the meeting stile between two sashes, not the frame.
Because we fabricate aluminium sliding doors ourselves, sash splits are a design conversation rather than a catalogue choice.
Which system goes where on an SW19 villa?
Street elevations in Wimbledon usually take heritage-range vertical sliders or steel-look windows, so the frontage still reads correctly. Rear elevations take slim-frame sliders, bifolds and fixed picture windows. Internal steel-look screens handle the open-plan ground floor. The table below is how we specify it in practice.
| Where | Typical Wimbledon situation | What usually goes in | Why |
|---|---|---|---|
| Street frontage, Victorian or Edwardian villa | Original or replaced sash windows in a bay | Aluminium vertical sliding sash windows | Keeps the sash proportion and the vertical opening action the elevation expects |
| Street frontage, 1920s–30s stock | Slim original steel casements, often Crittall-era | Steel-look aluminium windows | Reproduces the narrow bar pattern with a modern thermal break and sealed units |
| Rear opening, full width | Side-return or full-width rear extension | Cortizo COR Vision sliding doors | Largest glass area per opening; sash up to 2,500 × 3,000 mm |
| Rear opening, garden access both leaves | Narrower opening, want the whole aperture clear | Bifold doors | Folds the whole width away rather than stacking one panel behind another |
| Rear elevation, fixed light | Kitchen or utility outlook | Fixed picture window | No hinges, no handles, no opening hardware interrupting the view |
| Deep plan, flat roof over | Middle of a side-return with no external wall | Rooflights or structural glazing | Daylight where no window can reach |
| Internal, hall to reception | Knocked-through ground floor needing zoning | Steel-look internal doors and screens | Separates rooms acoustically while letting daylight run the length of the plan |
Five Wimbledon projects, and what went into them
Direct Trade Windows has taken 16 orders at 11 distinct SW19 postcodes since May 2022, spread across six of the district’s postcode sectors. The five below are described by sector and system; we do not publish customers’ addresses.
Internal steel-look screens, SW19 5
A period house near Wimbledon High Street, fitted in 2024 with Aluspace steel-look internal doors and screens. Aluspace is an internal-only system, which matters: the narrow-bar look is available inside the house with none of the thermal penalty of putting single-glazed steel on an external wall. The screens zone a hall and two reception rooms while keeping one run of daylight from the front door through to the garden.
Bifolds, heritage windows and a picture window, SW19 5
A 2026 rear-elevation job combining Cortizo bifold doors, heritage-range windows and a fixed aluminium picture window set into red brickwork under a brick arch. The picture window is a good illustration of the principle: where a window has no job to do except frame a view, taking the opening hardware out of the specification takes the frame out of the sightline too.
Slider, bifold and rooflights, SW19 8
A rear extension completed in 2026 using a three-panel COR Vision sliding door across the garden opening, a bifold, and rooflights over the deeper part of the plan. This is the standard SW19 extension package and the reason we quote sliders, bifolds and rooflights as one order rather than three.
A three-phase remodel, SW19 7
The largest single SW19 project in our book: four orders at one address between April 2025 and September 2026, covering aluminium casement windows, COR Vision glazing, structural glazing, rooflights and finally a front entrance door. Phased glazing is a programme problem more than a product one — each phase has to survey against what the previous phase actually built, not against the drawing.
Sliding doors, heritage windows and bifolds, SW19 7
A 2022 villa refit combining COR Vision, heritage-range windows and bifold doors across the same house — a front elevation that stays period-correct and a rear elevation that does not have to.
Can steel-look aluminium replace Crittall windows in a Wimbledon house?
Steel-look aluminium reproduces the narrow sightlines and grid of an original steel casement with a polyamide thermal break and a modern sealed unit. Historic England’s position is that historic windows should be repaired where they can be, and that replacement is a last resort — so the honest answer depends on whether your windows are of interest, and whether the frames are still sound.
Historic England’s guidance on traditional windows records that after the First World War firms such as W F Crittall revolutionised the use of the metal casement, developing the hot-rolled sections that became “the classic metal windows of the 1920s and 30s”, with residential windows made to a standard ‘F-range’ introduced around 1914. Merton’s own character study puts Wimbledon’s growth peak in the 1930s, which is why that window turns up across SW19.
Historic England records that steel windows were strong, slim and fire-resistant, and could open wider than a timber sash, which suited a period preoccupied with light and air. The appeal now is what it was then: narrow frames dividing the glass into a grid, so the window reads as glass with a line drawn across it rather than a frame with glass in it.
What goes wrong is corrosion and heat, in that order. Pre-1950s steel windows were generally not galvanised — the practice began around 1945 and became mandatory in 1956 — so they are prone to rust, which Historic England notes usually appears first on the horizontal glazing bars and the bottom members. Rust expands to as much as seven times the volume of the un-oxidised metal, and the resulting ‘rust jacking’ can distort the frame and crack the glass. Thermally, Historic England’s own climate-chamber research found the frames and leading “contribute strongly to the heat transfer through the window”, and that even the best sealed unit will not overcome thermal bridging through the frame and bars — something it flags as “particularly an issue when IGUs are added to steel windows”.
Here is the part a glazing company is not supposed to tell you. For genuinely historic windows, Historic England’s advice is that repair is the better option, that complete replacement should be a last resort, and that rust “does not mean replacement is necessary” because corrosion often looks far worse than it is. If your steel casements are original and repairable, get a metal-window specialist to look at them before you get a quotation from anyone, including us.
Steel-look aluminium is the right answer in the two cases Historic England itself accepts. The first is where corrosion loss has gone far enough to compromise the strength of the frame. The second is where the existing windows are not of historic or architectural interest — and its Advice Note 18 of February 2026 states that replacing such windows with double-glazed windows of appropriate material and pattern will generally be acceptable. On much of Wimbledon’s stock, what is actually in the opening today is a 1980s or 1990s replacement, and that is precisely the situation this product is for.
When it is the answer, what we make is thermally broken aluminium drawn to look like steel: slim face widths, bars set out in the same grid, carrying a modern sealed unit. Direct Trade Windows makes steel-look windows, doors and bifolds and manufactures in Slough rather than buying in finished frames, which is why we can match a bar layout to the elevation in front of us rather than offering the nearest standard grid.
A note on the word: Crittall is a trademark belonging to Crittall Windows Ltd. We describe our product as steel-look aluminium, or as a Crittall-style replacement. It is not Crittall product and we do not represent it as such.
Which parts of Wimbledon are in a conservation area, and what does that change?
Merton Council publishes 29 conservation areas, and a substantial group of them are in and around Wimbledon. Being in one does not by itself stop you replacing windows. On designated land the General Permitted Development Order withdraws cladding, side extensions and two-storey rear extensions — it does not withdraw like-for-like window replacement.
The Wimbledon-named conservation areas on Merton’s published list are Wimbledon Village, Wimbledon North, Wimbledon West, Wimbledon Hill Road, Wimbledon Broadway and Wimbledon Windmill. Several smaller designations sit in the same postcode district, including Bathgate Road, Leopold Road, Vineyard Hill Road, Kenilworth Avenue, South Park Gardens, Merton Hall Road, Bertram Cottages and Pelham Road, with the two John Innes areas a little further south at Merton Park and Wilton Crescent.
For a window replacement, the practical position under Schedule 2, Part 1, Class A of the General Permitted Development Order 2015 is this:
- Materials must be of a similar appearance to those used in the construction of the existing house (condition A.3(a)). That is the test a replacement window has to pass, and it is about appearance rather than an exact material match.
- Upper-floor windows in a side elevation must be obscure-glazed and either non-opening or openable only more than 1.7 m above the floor (condition A.3(b)).
- On designated land, including conservation areas, restriction A.2 removes permitted development for cladding, for extensions beyond a side elevation, and for rear extensions of more than one storey. Windows are not on that list.
- If the building is listed, none of the above helps: listed building consent is required under section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 for any alteration affecting its character, inside or out.
So in most of Wimbledon, a replacement window of similar appearance is still permitted development. What actually removes that right is an Article 4 direction, or the building being listed — which brings us to the one part of the borough where Merton has done exactly that.
Does Merton have an Article 4 direction covering windows?
Yes, but only in two conservation areas. Merton’s second Article 4(2) Direction of July 2007 covers John Innes Merton Park and John Innes Wilton Crescent. It withdraws permitted development for replacing doors or windows with ones of a different style or material where the work fronts a public highway or public space. No other Merton Article 4 direction affects windows.
The wording is worth reading properly, because it is narrower than people assume and it is also stricter than they expect. The direction withdraws:
The enlargement, improvement or other alteration of a dwellinghouse, including any extension, where any enlargement or improvement would front a public highway or other public space, including replacement of doors or windows (including attic or dormer windows) with doors or windows of a different style or material, or changes to the external finishes…
Note what that turns on: fronting a public highway, and a different style or material. A rear elevation is generally outside it. A front elevation replacement in a different material is squarely inside it, and needs an application.
We would rather say this plainly than win the enquiry: in those two John Innes areas, Merton’s own design guide states that replacement windows should be timber and painted, and that unfinished aluminium is not acceptable. If your property is in John Innes Merton Park or Wilton Crescent, get pre-application advice from Merton before you specify anything — including ours.
Elsewhere in Wimbledon there is no such direction. Merton’s Wimbledon Hill Road character appraisal of January 2007 recommended seeking one to control front-elevation window replacement there; no such direction is published today, so as things stand it was not enacted. It is still a fair indication of how the council views the issue, and a reason to confirm the current position rather than assume it.
What U-value does a replacement window in Wimbledon have to meet?
Approved Document L Volume 1 sets a maximum U-value of 1.4 W/m²K for a replacement window in an existing dwelling, or Window Energy Rating Band B. Doors more than 60% glazed must reach 1.4 or Doorset Energy Rating Band C; other doors 1.4 or Band B. Rooflights are allowed 2.2.
| Element replaced in an existing dwelling | Maximum U-value W/(m²·K) | Alternative route |
|---|---|---|
| Window, including roof windows | 1.4 | Window Energy Rating Band B minimum |
| Door with more than 60% of its internal face glazed | 1.4 | Doorset Energy Rating Band C minimum |
| Other doors | 1.4 | Doorset Energy Rating Band B minimum |
| Rooflight | 2.2 | — |
Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, Table 4.2. The 2026 edition carries the same figures and takes effect on 24 March 2027.
Two details in that document change what a Wimbledon project should actually do.
The first is paragraph 10.6. A controlled fitting is the entire unit including the frame, and replacing the glazing, or a window or door within its existing frame, is not providing a controlled fitting — so that work does not have to meet the energy efficiency requirements. If you are keeping original frames on a front elevation for planning reasons, that is the paragraph that makes it workable.
The second is paragraph 4.10, and it is written for exactly the situation a Wimbledon conservation area creates. Where windows or fully glazed external pedestrian doors cannot meet Table 4.2 because of the need to maintain the character of the building, they should instead either not exceed a centre-pane U-value of 1.2 W/m²K, or be single glazing supplemented with low-emissivity secondary glazing. In other words, the regulations already anticipate a heritage frontage; there is a defined route through, and it is not a matter of arguing with building control.
Planning glazing for a Wimbledon property?
Send us your drawings, or tell us the elevation you are trying to solve. We manufacture aluminium in Slough and install across SW19 and nationwide, and we will tell you what the structure and the planning position will actually allow before anyone quotes a panel layout.
