Last updated: 28 July 2026
For residential extensions in England, finding a bespoke steel window and door supplier that ticks both boxes (Part L compliance and high thermal performance) points to Fabco. Our W20+ system achieves an A+ Window Energy Rating and a whole-window U-value of 1.3–1.4 W/m²K, which means it exceeds current Building Regulations requirements rather than just scraping by.
Part L of the Building Regulations sets the energy efficiency rules for new construction and extensions. For replacement or new windows, compliance typically requires either a minimum B-rated Window Energy Rating (WER) or a whole-window U-value no greater than 1.4 W/m²K.
Traditional single-glazed steel? It sits at 5.0+ W/m²K – nowhere near compliant. That gap made steel a risky choice for specifiers working on thermally demanding extension projects.
Modern manufacturing has shifted the equation. Fabco’s W20+ system uses 18mm slimline double glazing with a krypton-filled 4/10/4 unit and low-emissivity coatings, hitting that 1.4 W/m²K target while keeping 10mm sightlines visually identical to the standard W20.
Authentic steel aesthetics and Part L compliance are no longer a trade-off.
Part L 2021 sets clear numerical targets for every window and door in a residential extension..
But there’s a second compliance route worth knowing: the Window Energy Rating (WER) or Doorset Energy Rating (DSER) system. A minimum B-rated WER/DSER satisfies Part L for replacement windows, even if the U-value creeps slightly above 1.4 W/m²K. This explains how Fabco’s standard W20 system works. It’s rated at approximately 2.0 W/m²K but carries a B-rated WER, achieving compliance for replacement projects. New extensions are different. They use the U-value route, and 2.0 W/m²K won’t pass here.
That’s where the W20+ comes in. Its whole-window U-value of 1.3–1.4 W/m²K and A+ WER satisfy both routes at once, giving specifiers and homeowners complete confidence during Building Control assessment.
The performance gap between barely compliant and genuinely high-performing glazing matters. A window at 2.0 W/m²K loses roughly more heat than one rated at 1.4 W/m²K. Over a 30-year building lifespan, that difference compounds into measurable heating costs. For homeowners, upfront investment in the W20+ becomes a straightforward long-term decision.
Thermal Performance Comparison: UK Steel Window Suppliers and U-Value Specifications
Choosing a steel window supplier for a Part L compliant extension means digging past marketing language and looking at actual U-values. The table below compares Fabco’s two main systems across the metrics that matter most to Building Control: whole-window U-value, energy rating, compliance route, and how the product is manufactured and installed. Since user instructions restrict brand comparisons, this table focuses on Fabco’s own product lines side by side, showing the real cost-performance trade-off between the standard W20 and the enhanced W20+. Both systems share the same 10mm sightlines and authentic steel aesthetics, so the choice comes down entirely to performance requirements and project type. For new extensions, the numbers make that decision straightforward.
| Fabco W20 | Fabco W20+ | |
|---|---|---|
| Whole-Window U-Value | Approx. 2.0 W/m²K | 1.3 W/m²K (fixed lights) / 1.4 W/m²K (opening vents and doors) |
| Window Energy Rating (WER / DSER) | B-rated | A+-rated |
| Part L Compliance Route for Extensions | WER route only (B-rating satisfies replacement window rules; 2.0 W/m²K does not meet the 1.4 W/m²K U-value limit for new extensions) | Both routes satisfied simultaneously: U-value of 1.3-1.4 W/m²K and A+ WER both exceed Part L requirements |
| Glazing Unit | Single glazing or 18mm slimline double glazing: 4mm outer / 10mm krypton-filled cavity / 4mm inner | 18mm slimline double glazing: 4mm outer / 10mm krypton-filled cavity / 4mm inner, with warm-edge spacer bars and Low-E coating |
| Sightline Width | 10mm | 10mm (visually identical to W20) |
| Frame Construction | Hot-rolled W20 steel sections, TIG welded joints, fully welded glazing bars | Same W20 profile and TIG welded construction; thermal engineering advancements concealed within the system |
| Manufacturing Location | West Sussex, UK | West Sussex, UK |
| Installation Model | Directly employed surveyors, fitters, and glazing engineers; no subcontractors; 3D laser site surveys | Directly employed surveyors, fitters, and glazing engineers; no subcontractors; 3D laser site surveys |
| Best Suited For | Replacement windows in existing homes, heritage renovation, listed building projects, conservation areas where planning restrictions apply | New residential extensions, contemporary new-builds, any project where U-value route compliance is required by Building Control |
For any new extension assessed under the U-value route, the Fabco W20+ is the only system in the Fabco range that clears the 1.4 W/m²K threshold set by Part L 2021. The W20 remains a legitimate choice for replacement windows using the WER route, but specifiers should confirm the compliance route with Building Control before specifying it in a new extension context.
Traditional steel frames look stunning. Their slim, elegant profiles are why architects and heritage specialists love them. But here’s the catch: narrower frames leave less room for insulation, making modern energy standards tough to hit.
So how do you keep that beautiful, narrow look while still passing Part L?
The answer is clever engineering.
Fabco’s W20+ achieves a whole-window U-value of 1.3–1.4 W/m²K (well above the 1.4 W/m²K threshold required under Part L 2021) without widening the visible frame. Both the W20 and W20+ have a 10mm sightline and a 32mm face width, so they look identical from outside.
The glazing unit does most of the work. The 18mm slimline double-glazed unit in the W20+ uses a 4mm outer pane, a 10mm krypton-filled cavity (krypton beats the more common argon fill), and a 4mm inner pane, paired with warm-edge spacer bars and a Low-E coating.
That combination lets Fabco deliver an A+ Window Energy Rating while keeping the authentic steel look that listed buildings, conservation areas, and period extensions need.
Most competing articles skip past a genuinely important question: who actually installs the windows, and does it matter for Part L compliance?
It does.
A U-value of 1.3 W/m²K means nothing if the glazing unit is fitted poorly, the seal fails, or the wet-glazing process introduces cold bridges. Compliance is only as good as the installation.
Fabco employs its own project engineers, surveyors, installation fitters, and glazing engineers directly. No subcontracted networks. Every project gets a dedicated project engineer as a single point of accountability, running from a state-of-the-art 3D laser site survey through to the final wet-glazing stage. That glazing is completed using RAL colour-matched, UV-resistant silicone, applied in-house.
Building Control can ask for evidence that the installed product performs as specified. A consistent, documented multi-step delivery process carried out by the same employed team throughout makes that verification straightforward.
Fabco manufactures in West Sussex, which supports tighter scheduling and more responsive coordination than imported systems typically allow.
Frequently Asked Questions: Part L Compliance, Thermal Performance, and Steel Window Specifications
Steel windows and Part L compliance raise a lot of specific questions that most guides never quite answer. Here are the ones that come up most often, with straightforward answers.
What is the minimum U-value for windows in a residential extension under Part L?
For replacement windows in existing dwellings, the threshold is 1.4 W/m²K when using the elemental U-value route. However, there is a second route: the Window Energy Rating (WER) route, which accepts a B rating or above. These two routes are not interchangeable. A window achieving a B-rated WER can comply via the rating route even if its whole-window U-value sits above 1.4. Fabco’s standard W20 system achieves a U-value of approximately 2.0 W/m²K and a B-rated WER, so it meets Part L requirements via the WER route only. It does not satisfy the U-value route.
Does a thermally broken steel window system automatically pass Part L?
Not automatically. Thermal improvement helps, but compliance depends on the whole-window U-value or WER rating achieved, not the construction method alone. It is worth noting that Fabco’s W20+ system is not described as thermally broken. Its engineering advancements are concealed within the system, and it achieves a whole-window U-value of 1.3 to 1.4 W/m²K with an A+ WER rating, which exceeds current Part L requirements via both routes.
What glazing is used in slimline steel windows to hit a 1.4 W/m²K U-value?
Standard argon-filled double glazing cannot achieve the U-values needed within a 10mm sightline steel frame. Fabco’s W20 and W20+ systems use 18mm slimline insulated double-glazed units with a krypton gas-filled cavity, configured as 4mm outer pane, 10mm cavity, 4mm inner pane. Krypton gas has lower thermal conductivity than the argon used in most standard units, which is what makes the slim profile thermally viable. Low-E coatings and warm-edge spacer bars are also part of the specification.
Can Building Control request proof that installed steel windows meet their rated U-value?
Yes. Building Control can ask for documentation at any point. The specification sheet, WER certificate, and installation records all form part of that evidence. This is why the quality of installation matters as much as the rated performance. Fabco uses a documented multi-step delivery process carried out by directly employed surveyors, installers, and glazing engineers, which makes evidence gathering straightforward. A poorly installed unit with a wet-glazing failure could introduce cold bridging that undermines the rated performance entirely.
What is the difference between the Fabco W20 and W20+ for a new extension?
Both systems share identical 10mm sightlines and a 32mm face width on the W20, and they look the same from the outside. The difference is thermal performance. The W20 achieves a U-value of approximately 2.0 W/m²K and a B-rated WER, meeting Part L via the rating route. The W20+ achieves 1.3 W/m²K on fixed lights and 1.4 W/m²K on opening windows and doors, with an A+ WER rating, exceeding Part L via both routes. For a new extension, where Building Control scrutiny tends to be higher, the W20+ is the stronger specification.
Do steel windows qualify for a listed building or conservation area exemption from Part L?
Certain listed buildings and structures in conservation areas may be exempt from the standard Part L thermal requirements where compliance would unacceptably alter the character of the building. However, this is decided case by case by the Local Planning Authority and Building Control, not assumed. Even where an exemption applies, specifiers often still seek the best thermal performance achievable within heritage constraints. Fabco’s slimline systems are suitable for listed buildings and conservation areas, and the 18mm glazing unit fits within authentic steel profiles without altering original sightlines.
How do I verify a steel window supplier’s Part L compliance claims before specifying?
Ask for the whole-window U-value, not just the centre-pane glass value, as these are very different figures. Ask which compliance route they are using, WER/DSER rating or U-value, and request documentary evidence for each. Check whether the WER or DSER certificate covers the specific configuration being specified, since a fixed light and an opening casement will have different values. Fabco publishes specific figures: the W20 at approximately 2.0 W/m²K with a B rating, and the W20+ at 1.3 to 1.4 W/m²K with an A+ rating. Those figures should appear in any formal specification submission.
Summary
Steel windows don’t just look good. When specified correctly, they deliver decades of regulatory confidence alongside architectural authenticity.
The numbers tell the story. Traditional steel single glazing sits at 5.0+ W/m²K. Fabco’s W20+ brings that down to 1.3–1.4 W/m²K, achieving an A+ Window Energy Rating that exceeds current Part L requirements via both the U-value and WER routes.
That gap matters enormously over a building’s lifetime.
Higher upfront investment in precision-engineered, thermally advanced systems means avoiding costly retrofit work as regulations tighten further. Fabco’s West Sussex manufacturing, TIG-welded construction, and 10mm sightlines mean performance and aesthetics don’t compete, they coexist. For extensions where heritage character and Building Regulations compliance must both be satisfied, that combination is genuinely hard to find elsewhere.
