What are sliding doors for extensions?
Sliding doors for extensions and garden rooms are doors designed into a new building from the start, rather than fitted into an opening that already exists. That gives you the chance to get the details right: an opening sized for the door, a beam stiff enough not to sag onto it, floor levels set for a low or flush threshold, and drainage planned outside. Slimline aluminium systems, with centre interlocks typically around 20–25 mm, are a common choice because they put as much glass as possible into a new rear wall.
We work alongside your builder, architect or structural engineer. Early on we can quote from drawings and give the structural opening size and tolerances the builder should form, the finished floor level the threshold needs, and any fixing or drainage details. Once the opening is built we carry out a check survey, measure it accurately and confirm the specification with you in writing before manufacture begins.
Timing is the part that most often goes wrong on extensions, because slimline doors typically take 6–8 weeks to make after sign-off. We agree a target fitting date with your builder, order once the check survey is done, and fit to the programme so the extension can be closed in without waiting on the glazing. Garden rooms follow the same process at a smaller scale.
Who are they for?
Best suited to
- Single-storey rear extensions
- Kitchen and family room extensions
- Garden rooms and home offices
- Self-builds with a glazing package
Worth considering
- Slimline doors typically take 6–8 weeks to make, so they must be ordered early enough to suit the build programme.
- Final sizes come from a check survey of the built opening, so the builder needs the opening formed before manufacture can start.
- Wide glass in a new extension depends on the steel being designed for it; a beam that deflects too much can stop the door sliding properly.
Why choose sliding doors for extensions?
- Being involved at drawing stage means the opening is formed to suit the door, rather than the door being compromised to suit an opening built a few centimetres out.
- We provide the structural opening size and fitting tolerances in writing, giving your builder a clear target and avoiding disputes about who measured what.
- Slimline aluminium systems with interlocks typically around 20–25 mm maximise the glass in a new rear wall, keeping the view as open as possible.
- Setting the finished floor level before the screed goes down allows a low threshold, typically 15–25 mm, or a flush one with a drainage channel outside.
- Telling the engineer the door type and span early lets them design the beam above it to limit deflection, protecting the smooth running of large sliding panels.
- A check survey once the opening is built means the door is made to the real dimensions, accounting for any small variation in the brickwork or blockwork.
- Agreeing a fitting date with your builder lets the extension be made weathertight on programme, so plastering and flooring can follow without waiting on the doors.
- Garden rooms benefit from the same approach, with a door sized to the frame and a header designed to carry the roof without pressing down on the door.
Typical specification
| System | Slimline aluminium as standard; other systems available |
|---|---|
| Interlocks | Typically around 20–25 mm on slimline systems |
| Sizing | Quoted from drawings; manufactured from a check survey of the built opening |
| Information for the builder | Structural opening size, tolerances, finished floor level and drainage details |
| Thresholds | Low thresholds typically 15–25 mm; flush with an external drainage channel |
| Typical lead time | 6–8 weeks after check survey sign-off for slimline systems |
| Energy (England) | Doors in existing homes with more than 60% glazing must achieve 1.4 W/m²K or better |
Figures are typical and vary by system and size; your quote confirms the exact specification.
How much do sliding doors for extensions cost?
Slimline sliding doors for an extension have a guide price of £7,795–£9,495 for a 3.6 m opening and £9,595–£11,695 for 4.8 m, fitted, inc VAT at standard 2.1 m height, with the final price fixed in a written quote after a check survey of the built opening.
| Overall width | Typical panels | Guide price (fitted, inc VAT) |
|---|---|---|
| 1.8 m | 2 panels | £5,095–£6,195 |
| 2.1 m | 2 panels | £5,495–£6,795 |
| 2.4 m | 2 panels | £5,995–£7,295 |
| 3.0 m | 2 panels | £6,895–£8,395 |
| 3.6 m | 2 panels | £7,795–£9,495 |
| 4.0 m | 3 panels | £8,395–£10,195 |
| 4.8 m | 3 panels | £9,595–£11,695 |
| 6.0 m | 4 panels | £11,395–£13,895 |
Guide prices for a standard 2.1 m high door, supplied and fitted into a prepared opening, including VAT. Your price is fixed in a written quote after survey.
Options and extras
- Two-panel slimline. One sliding and one fixed panel, suiting openings up to around 3.6 m depending on the system.
- Three or four panels. More panels for wider extension openings, with one or more sliding.
- Flush threshold. A level floor from inside to out, with a drainage channel set into the patio.
- Corner opening. Sliding panels meeting at a corner with no post, for extensions open on two sides.
- Solar-control glass. Coated glass for south- and west-facing extensions prone to summer overheating.
- Triple glazing. For low-energy extensions or north-facing walls, subject to panel size limits.
Lead times and fitting
Lead time: Typically 6–8 weeks for slimline systems after check survey sign-off.
Fitting: Typically 1–2 days, scheduled to the build programme.
Nothing is ordered until you confirm the surveyed specification in writing. See how it works.
Rules and approvals
England
In England, sliding doors in a new extension are covered by the extension's own Building Regulations application rather than a separate door notification. Approved Document L limits the glazed area: windows, roof windows and doors should not normally exceed 25% of the extension's floor area plus the area of any windows or doors lost to the extension, unless the extra glass is compensated for elsewhere. The doors must also meet the energy and safety glazing standards.
Wales
In Wales, sliding doors in an extension are approved as part of the extension's Building Regulations application, and the glazing must meet the Welsh energy and safety glazing requirements, including the limits on how much of an extension can be glazed.
Scotland
In Scotland, an extension needs a building warrant before work starts, and the doors are covered by it; the design must meet the Scottish building standards for energy and safety glazing.
Northern Ireland
In Northern Ireland, an extension needs building control approval from your district council, which covers the doors and glazing. Work in Northern Ireland is by arrangement.
Sliding doors for extensions: common questions
What structural opening size should my builder leave for sliding doors?
Your builder should leave the structural opening size we give in writing for your chosen door, which is the frame size plus a small fitting tolerance on each side and at the head. The exact figure depends on the system and the threshold detail. We confirm it from your drawings early, then check the built opening before the door is made.
When should I order sliding doors for an extension?
Order sliding doors for an extension as soon as the opening is built and a check survey can be done, and agree the target fitting date with your builder in advance. Slimline doors typically take 6–8 weeks to make after sign-off. Getting a quote from drawings early means the specification is settled and only the final measurements are needed.
Can you work with my builder and architect?
Yes, working with your builder, architect and structural engineer is the normal way we approach extension doors. We can provide technical details, opening sizes, threshold requirements and fixing information, and attend site for the check survey. Your builder forms the opening and prepares the floor, and our installers fit the doors to the agreed programme.
Can you measure from drawings rather than the built opening?
We can quote from drawings, but we manufacture from a check survey of the built opening, because real openings often differ slightly from the plans. Making a large made-to-measure door from drawings alone risks it not fitting. If a programme makes that unavoidable, any arrangement and who carries the risk would be agreed in writing first.
How do I get a level threshold in an extension?
A level or flush threshold in an extension needs the finished floor level inside to be set relative to the door sill and the outside paving before the screed goes down. Outside, a drainage channel is needed across the threshold so rain cannot run in. Our thresholds guide explains the options.
Why does the steel beam matter for sliding doors?
The steel beam above an extension's sliding door matters because every beam bends a little under load, and a large sliding door needs a straight, level head to run freely. If the engineer knows a wide slider will sit under the beam, they can design for low deflection. Too much movement can make panels stiff or bind.
How much do sliding doors for an extension cost?
Slimline sliding doors for an extension have a guide price of £7,795–£9,495 for a 3.6 m opening and £9,595–£11,695 for a 4.8 m opening, fitted, inc VAT at standard 2.1 m height. Taller doors, extra panels, special glass and flush thresholds move the price. Your written quote after the check survey fixes it.
What sliding doors suit a garden room?
A garden room usually suits a two-panel or three-panel sliding door sized to the front of the building, with a header strong enough to carry the roof without pressing on the frame. Aluminium systems are a common choice because they are stable and need little maintenance. Glass with good insulation helps if the room will be used all year.