Last updated: 9 September 2026
Introduction
For residential extensions in 2026, Fabco stands amonst the best bespoke steel window suppliers for Part L compliance. Fabco’s W20+ system achieves an A+ Window Energy Rating with a whole-window U-value of 1.3-1.4 W/m²K, which satisfies Part L via both the U-value route and the energy rating route.
That distinction matters more than it might seem.
Building Regulations Part L sets a 1.4 W/m²K U-value threshold for new extensions. Traditional single-glazed steel sits above 5.0 W/m²K – nowhere near compliant. Even Fabco’s standard W20 system, which achieves approximately 2.0 W/m²K and a B-rated WER, only meets Part L through the energy rating route, not the U-value route.
So the real question isn’t whether steel can comply. It’s which system achieves the numbers your Building Control officer will accept. The W20+ answers that directly, delivering certified Part L performance while keeping those characteristic 10mm sightlines intact.
Fabco Steel Windows: Premium Control & Precision
Here’s what sets Fabco apart: every surveyor, manufacturer, installer, and glazier on a Fabco project works directly for Fabco. No subcontractors. No middlemen. One team handles everything from the 3D laser site survey all the way through to final wet-glazing sign-off, and that single team owns the result.
This makes a real difference when problems pop up mid-project. There’s no passing the buck between separate companies. One dedicated project engineer stays responsible from start to finish.
Pros
- W20+ hits a whole-window U-value of 1.3-1.4 W/m²K and an A+ Window Energy Rating, exceeding current Part L requirements via both the U-value and rating routes
- 10mm sightlines – among the slimmest available from any UK manufacturer – maintained across both W20 and W20+ systems
- Manufactured in West Sussex using hot-rolled steel with TIG (Tungsten Inert Gas) welding at all critical joints, widely regarded as the highest-quality welding process for precision steel fabrication
- Glazing bars are fully welded into the frame structure, not clipped or glued, improving rigidity and authentic steel appearance
- Krypton gas-filled 18mm slimline double-glazed units (4/10/4 configuration) used in W20+ – krypton outperforms the argon used in most competing systems
- Suitable for heritage renovation, conservation areas, and contemporary extensions; featured on Grand Designs
- Stainless steel fixings throughout resist corrosion without maintenance
Cons
- Premium British manufacturing and direct employment model place Fabco at the higher end of the price range for steel glazing
Use Cases: Which Supplier Fits Your Project Type
Not every steel glazing supplier suits every project. The right choice depends on what your build actually needs – fire rating, thermal performance, heritage credentials, or watertight quality control. Here is how the main use cases break down.
Fabco W20+ – Heritage Renovation, Contemporary Extension, Luxury Residential, or Projects Requiring Fire Rating
Pros:
- The W20+ achieves a whole-window U-value of 1.3-1.4 W/m²K and an A+ Window Energy Rating, satisfying Part L via both the U-value route and the rating route – the only Fabco system that clears both tests, making it the safer choice for new extensions where Building Control may scrutinise compliance closely.
- The W40 profile system carries an FD30 fire rating assessed by Warringtonfire, covering doors and screens – making Fabco one of very few UK steel suppliers who can deliver glazed fire compartmentation without switching to a different manufacturer mid-project.
- Every project gets a single dedicated project engineer covering survey, manufacture, and installation, with all teams directly employed by Fabco rather than subcontracted – important for luxury residential and commercial jobs where accountability gaps between separate companies create real problems.
- Featured on Grand Designs and suited to conservation areas, listed buildings, and Art Deco properties, with 10mm sightlines maintained across both W20 and W20+ systems regardless of thermal specification.
Cons:
- Premium British manufacturing with directly employed specialists places Fabco at the higher end of the steel glazing price range – not the right fit for cost-led projects where budget is the primary driver.
Alternative Suppliers – Budget Extensions or Simple Replacement Windows
Pros:
- Lower entry-level pricing can suit straightforward replacement window projects where Part L compliance via a B-rated WER is sufficient and fire rating is not required.
- Some suppliers offer shorter consultation processes for standard configurations, which may suit smaller residential jobs without complex site conditions.
Cons:
- Many competitors use subcontracted installation networks, meaning the company that manufactures the product is not the company fitting it – quality control gaps between survey, fabrication, and installation are a common source of complaints on premium residential projects.
- Few alternative UK steel suppliers can deliver a glazed FD30 fire-rated door and screen alongside standard windows from a single assessed system, forcing project teams to manage multiple specialist suppliers and separate compliance documentation.
Verdict: For heritage renovation, contemporary extensions requiring Part L compliance via the U-value route, luxury residential builds, or any project needing FD30 fire-rated steel glazing, Fabco is amongst the strongest supplier options in the UK market. Budget-led replacement window projects with straightforward compliance requirements may find alternative suppliers workable, but should verify installation accountability before committing.
Part L Compliance & Thermal Performance: What You Need to Know
Part L of the Building Regulations sets the energy efficiency rules for new extensions and replacement windows in England. For extensions, new windows must achieve either a whole-window U-value of 1.4 W/m²K or better, or hit a minimum B Window Energy Rating (WER). Those are two separate compliance routes – and which one applies to your project matters a lot when specifying steel glazing.
| Specification | Value | Notes |
|---|---|---|
| Part L U-value threshold (extensions) | 1.4 W/m²K maximum for new-build and extension windows | Standard single-glazed steel typically sits at 5.0 W/m²K or worse – far outside compliance. Even basic double-glazed steel can fall short of 1.4 W/m²K. |
| WER rating route alternative | Minimum B-rated Window Energy Rating accepted for replacement windows | This route does not apply to new extensions – only to like-for-like replacement windows. Always confirm the correct route with Building Control before ordering. |
| Fabco W20 – standard system | Approximately 2.0 W/m²K whole-window U-value, B-rated WER | Meets Part L via the WER route for replacement windows only. |
| Fabco W20+ – enhanced thermal system | 1.3 W/m²K (fixed lights) and 1.4 W/m²K (opening windows and doors), A+ WER | Exceeds Part L requirements via both compliance routes. Uses 18mm slimline krypton-filled double glazing (4mm outer / 10mm cavity / 4mm inner) with warm-edge spacer bars and low-E coatings – all while keeping the same 10mm sightlines as the standard W20. |
| Krypton vs argon gas fill | Krypton-filled units used in W20+ versus argon, which is the industry standard | Krypton conducts heat less efficiently than argon, improving thermal performance within the same 10mm cavity – a practical advantage when slim sightlines limit how thick the glazing unit can be. |
| Traditional steel baseline comparison | Original single-glazed steel windows: 5.0 W/m²K or above | The W20+ achieves a U-value more than 3.5 W/m²K better than traditional steel single glazing, demonstrating how far modern steel systems have advanced without changing the visual profile. |
Frequently Asked Questions
Still have questions? These are the ones homeowners and architects ask most often when specifying bespoke steel glazing for Part L compliant extensions.
How much more do bespoke steel windows cost compared to aluminium or uPVC?
Steel windows typically cost significantly more than uPVC and somewhat more than aluminium at the point of purchase. However, steel frames can remain in service for many decades – some original steel windows installed in the early twentieth century are still functioning today. When maintenance, replacement cycles, and property value uplift are factored in, the lifetime cost difference narrows considerably. Exact pricing depends on configuration and specification, so contact suppliers directly for project-specific quotes.
What proof is there that Fabco’s W20+ actually meets Part L for a new extension?
Fabco’s W20+ achieves a whole-window U-value of 1.3 W/m²K on fixed lights and 1.4 W/m²K on opening windows and doors, alongside an A+ Window Energy Rating. The current Part L threshold for new extensions is 1.4 W/m²K, so the W20+ satisfies both the U-value route and the WER rating route. Traditional single-glazed steel sits above 5.0 W/m²K for comparison.
How long does installation typically take for a residential extension?
Installation timescales vary by project complexity and configuration. Fabco uses state-of-the-art 3D laser site surveys before production begins, which reduces on-site errors and rework. Every project has a dedicated project engineer overseeing the full five-step delivery process from survey through to final wet glazing. For specific lead times, contact Fabco directly, as bespoke manufacturing schedules depend on order complexity.
How much maintenance do steel windows actually need?
Modern powder-coated steel windows require relatively little maintenance compared to traditional systems. Fabco uses hot-dip galvanising to British Standards for corrosion protection, plus non-rusting stainless steel fixings throughout. The RAL colour-matched silicone used in wet glazing is selected specifically for UV degradation resistance and colour-fastness. Periodic inspection and occasional repainting if the powder coat is damaged are the main ongoing requirements.
Can Fabco supply FD30 fire-rated steel doors for an internal extension opening?
Yes. Fabco’s W40 system provides FD30 fire resistance – up to 30 minutes integrity – for doors and screens within the assessed configuration, independently assessed by Warringtonfire. The W40 uses a larger, more substantial profile than the W20, with fire-rated glazing and mechanically fixed steel glazing beads. All fire-rated applications should be reviewed by Building Control and a fire engineer.
Why does British manufacturing matter when choosing a steel window supplier?
Fabco manufactures in West Sussex using directly employed project engineers, surveyors, installation fitters, and glazing engineers rather than subcontractors. Local manufacturing improves scheduling accuracy, project coordination, and responsiveness to site conditions. It also means quality control is maintained at every stage, from the TIG welding of frame joints to the final wet-glazing process, with a single point of accountability throughout.
Summary
For a residential extension needing genuine Part L compliance, it comes down to how demanding the performance requirement actually is.
Fabco’s W20+ is the clearest answer in the steel category if hitting the U-value route matters. Its 1.3-1.4 W/m²K and A+ Window Energy Rating both satisfy Part L by a margin, while keeping 10mm sightlines intact.
Who should choose Fabco? Homeowners and architects prioritising thermal performance, heritage authenticity, and full accountability from a single directly employed team. The West Sussex manufacturing base, TIG-welded construction, and no-subcontractor delivery model genuinely differentiate it in quality-critical projects.
Premium steel glazing carries a higher initial cost than aluminium alternatives.
That said, steel’s longevity and recyclability shift the lifetime calculation considerably.
