Slimline Sliding Doors in
London.

What "slimline" actually means

Three different measurements get sold as "slim", and only one of them is the number you look at every day.

The interlock (also called the meeting stile or central junction) is the vertical bar where two sliding panes overlap. It sits floor to ceiling, roughly at eye level, right in the middle of the view. On a standard aluminium slider it is 50 to 60mm. On uPVC patio doors it is typically 100mm or more. On a true slimline system it is 20 to 25mm, about the width of a finger. The outer frame is a separate figure, and on a well-detailed installation most of it disappears into the reveal, floor and ceiling anyway. Frame depth is the front-to-back dimension, which affects structural capacity, not what you see.

There is a trick worth knowing about before you compare quotes. Most sliding doors described as "20mm" only measure 20mm head-on. Look along the glass from the side and the interlock steps out 45 to 65mm, and the slim sightline you paid for is gone at exactly the angle you see it from when you are sitting in the room. Our APX20 Ultra Slim uses an integrated flush interlock that sits level with the sash face, so it reads 20mm from every angle, with a glass-to-frame ratio up to 94%.

Slimline systems are also called slim-frame sliding glass doors, slim sightline sliding doors, ultra slim patio doors and super slim sliders. They are all the same product category: a thermally broken aluminium sliding door with a reduced meeting stile.

The four slimline sliding systems we fit

Every figure below comes from the system's own tested data, not from a brochure adjective. Pick by the constraint that matters on your opening: sightline, span, weight or budget.

Read that table as a trade-off, not a ranking. A 20mm interlock has less aluminium in section than a 25 or 50mm one, so it carries less glass. APX20 Ultra Slim wins on pure sightline. APX20 Plus wins on span, taking a single sash to 4000 × 4000mm and up to 14m² of glass, with manual operation to 400kg and motorised to 700kg. APX Evolution wins on configuration flexibility and thermal performance. APX47 is the sensible workhorse patio door where the sightline is not the point.

Track and configuration options

The number of tracks decides how much of the opening actually opens. A double track with one fixed and one sliding pane gives you half the opening clear. A triple track with two sliders gives you two thirds. Pocket systems give you effectively the whole thing.

A pocket door is the only configuration that genuinely rivals a bifold for clear opening width, and it does it without any frames stacked in the corner. It needs to be designed in early, because the wall has to contain a cavity the full width of one panel.

Thresholds and level access

The threshold is where slimline sliding doors are won or lost, and it is the detail most quotes are vague about. There are three ways to do it.

A flush threshold is not just a track choice. It needs a drainage channel outside the door, a fall away from the opening, and the floor build-up depth agreed before the doors are made. If underfloor heating runs near the threshold, the pipework layout has to be co-ordinated too. We survey the structural opening, confirm the build-up, and liaise with your builder so the slab or screed is right before the doors arrive. Getting this wrong is the single most common reason a slimline installation looks and performs badly.

On structure: a 2-panel opening in a standard domestic wall is usually a straightforward lintel. Once you are into 3 and 4-panel runs, corner junctions, or single panels carrying 300kg or more of glass, the lintel and any supporting steel should be sized by a structural engineer. We flag it at survey rather than after the wall is open.

Panel size, weight and what limits it

Maximum pane size is governed by three things at once: what the roller carriage can carry, what the interlock section can hold without deflection, and what the glass itself weighs. A 4mm-20mm-4mm double-glazed unit weighs roughly 20kg per square metre, so a 2.5m by 3m pane is about 150kg of glass before you add the sash.

Above roughly 200kg of panel weight on an inline system, a lift-and-slide mechanism is the right answer: the handle lifts the sash clear of its seals before it moves, so it glides instead of dragging and the weather seals last. Above 400kg, motorisation stops being a luxury and becomes the sensible specification.

Practical limits also come from access. A 3.5m by 3m sealed unit has to physically reach the opening, which in a London terrace can mean a crane over the roof or through-the-house glazing robots. We work out the delivery route at survey, before the glass is ordered.

Thermal, acoustic and weather performance

Slim does not have to mean cold. The polyamide thermal break runs the full depth of the frame and sash on every system we fit, which is what keeps the whole-door U-value competitive with a good casement window.

For Building Regulations, a replacement door with more than 60% of its internal face glazed must achieve a maximum U-value of 1.4 W/m²K, or Doorset Energy Rating Band C as a minimum, under Approved Document L Volume 1, 2021 edition, Table 4.2. Every system on this page clears that threshold at its standard specification. Apex Glazing is FENSA-registered (Reg 40373), so compliance is self-certified and notified to your local authority.

If the opening faces a main road or a flight path, specify the acoustic glazing package rather than a thicker frame. Laminated acoustic interlayers do far more for perceived noise than any amount of extra aluminium.

Slimline sliding vs bifold vs French

The honest comparison, because the right answer genuinely depends on how you will use the opening.

Rule of thumb from installs: if the doors will be closed for eight months of the year, buy the sightline. If they will be fully open every weekend from May to September, buy the bifold. Our bifold vs sliding doors comparison goes further into cost and threshold differences.

On uPVC versus aluminium for doors specifically: uPVC sliding patio doors carry an interlock of 100mm or more because the material needs bulk to be rigid, and they cannot approach these panel sizes. For anything where the sightline is the point, aluminium is the only realistic option. See our uPVC vs aluminium comparison for the wider trade-off.

Steel-look and period rear elevations

A slimline slider with a matt black powder-coat and a slender interlock reads convincingly against a Victorian or Edwardian rear elevation, which is why steel-look sliding doors have become the default for London side-return extensions. If you want the true steel-window language, with glazing bars and a stepped profile, that is a different product family; our guide to steel window replacement options covers it, and internal steel-look screens are covered on the internal steel-look doors page.

On planning: a rear-elevation slider on an unlisted house outside an Article 4 Direction normally needs no planning permission, though a new or enlarged opening does. Front elevations of period or listed properties are a different conversation entirely. Our planning permission guide sets out the actual rules and how to check your address.

Slimline sliding door prices

Indicative fitted price ranges for London, including VAT, frame, double glazing, fitting, removal and disposal of the old door, and FENSA self-certification. Triple glazing, acoustic units, motorisation and a fully flush threshold all sit outside these ranges. Your real figure comes from the survey, because span, panel weight, threshold type and access drive it more than anything else.

The 20mm sightline carries a premium over a 50mm system of broadly comparable size, and a flush threshold adds cost on top because of the drainage and floor build-up work. Where the sightline genuinely will not be noticed, on a modest 2-panel replacement in a suburban rear wall for instance, we will say so and quote the APX47 instead. See sliding door costs in London for a fuller breakdown.

What a fitted price includes

Frequently asked

Where we install across London

We survey, supply and fit sliding doors across west, south-west and central London and into Surrey, working directly rather than through subcontractors. Common locations include Richmond, Twickenham, Kingston, Chiswick, Putney, Wimbledon, Fulham, Hammersmith, Ealing and Teddington. The full list is on our areas we cover page.

Related reading: what is the slimmest sliding door in the UK, 20mm interlock sliding doors explained, sliding door costs in London, and aluminium windows in London if you are replacing windows at the same time.