Last updated: 14 August 2026
For Grade II listed properties in London, the top made-to-measure steel window and door fabricators offering full end-to-end service includes Fabco, and a small number of specialist importers. Fabco manufactures in Britain and handle everything from survey through installation themselves.
That distinction matters enormously once you understand what can go wrong.
Most buyers compare glazing quotes the same way they’d compare double-glazing for a new build. A heritage steel project is different. It involves a 3D laser survey, custom TIG-welded fabrication to 10mm sightlines, Listed Building Consent coordination, and wet glazing with colour-matched silicone. Each stage depends on the last.
When those stages get split across different subcontractors, accountability disappears fast.
Steel windows in UK conservation areas must meet Building Regulations Part L via a Window Energy Rating route – a B-rated minimum, not simply a U-value target. Fabricators who don’t explain that distinction during the survey stage are already behind.
5-Step Fabricator Selection Checklist: Survey Protocols, Heritage Certification & Installation Experience
Not every steel window fabricator is set up for Grade II listed work in London. This five-step checklist cuts through the sales pitch and focuses on what actually separates a specialist from a generalist. Each step includes a red flag to watch for.
| Specification | Value | Notes |
|---|---|---|
| Step 1: Survey Protocol | Require state-of-the-art 3D laser surveying before any fabrication begins | Openings in listed buildings are rarely square. A fabricator relying on tape-measure surveys risks costly remakes. Ask specifically whether survey data feeds directly into their manufacturing process. Red flag: survey is carried out by a third-party subcontractor with no direct link to the factory floor. |
| Step 2: Heritage Certification | Confirm FENSA registration and ask for documented experience across at least 5 Grade II listed projects | FENSA registration covers Building Regulations self-certification for replacement glazing. For listed work, also check whether the fabricator has supported Listed Building Consent applications and understands the B-rated WER route required under Part L. |
| Step 3: In-House Installation Teams | Directly employed surveyors, fitters, and glazing engineers, not subcontracted networks | Fabco, for example, employs its own project engineers, installation fitters, and wet-glazing engineers, with every project assigned a single dedicated project engineer from survey to handover. Red flag: the fabricator manufactures in-house but hands installation to a regional network of approved contractors. |
| Step 4: Thermal Performance Transparency | W20 standard achieves a B-rated WER and 2.0 W/m2K; W20+ achieves A+ and 1.3 to 1.4 W/m2K | Ask fabricators to confirm the specific WER or DSER rating for the system they are quoting, not just a headline U-value. For listed buildings where single glazing is required by Conservation Officers, check whether the system can accommodate that without a bespoke redesign. |
| Step 5: Warranty and Post-Installation Support | Ask for written confirmation of both warranty coverage and named after-sales contact | Dedicated in-house customer service engineers covering both warranty and non-warranty issues are a meaningful differentiator. Ask whether post-installation visits are carried out by the same employed team or outsourced. |
Before You Request a Survey: What Questions to Ask Your Fabricator
The right questions asked before a survey is booked can reveal whether a fabricator is genuinely equipped for a Grade II listed project – or whether problems will only surface after contracts are signed.
Who pays for the initial survey, and is it refundable against the order?
Some fabricators charge a survey fee for listed properties requiring 3D laser measurement. Ask upfront whether that fee is deducted from your final invoice if you proceed, or whether it is a standalone cost. Fabco uses state-of-the-art 3D laser surveys conducted by directly employed surveyors, so it is worth confirming exactly who attends and what the survey covers before committing.
Can you confirm in writing that your system is approved for use in a Grade II listed building?
No fabricator can guarantee Listed Building Consent; that sits with your Local Planning Authority. What a fabricator can confirm is whether their profiles match the sightlines of the original windows and whether they have supplied similar conservation projects. The W20 system, for example, achieves a 10mm sightline and a 32mm face width, which Conservation Officers frequently accept as a like-for-like replacement.
Is installation carried out by your own employed team or subcontracted?
This is one of the most revealing questions to ask. Fabricators who outsource installation to regional networks have less control over quality and scheduling. Fabco employs its own installation fitters, surveyors, and wet-glazing engineers directly, with a single dedicated project engineer assigned from survey through to handover, a meaningful accountability difference on complex listed building projects.
What is the typical lead time from specification sign-off to installation completion?
Lead times vary significantly by fabricator and project complexity. For bespoke steel windows on listed buildings, expect 12–20 weeks from order confirmation to installation in most cases. UK-based manufacturers with in-house production (like Fabco, which manufactures in West Sussex) generally offer more reliable scheduling than suppliers dependent on imported components.
Does your warranty cover labour as well as the product, and who handles post-installation visits?
A product-only warranty leaves homeowners exposed to labour costs if issues arise on-site. Ask fabricators to confirm whether warranty callouts are handled by their own employed service engineers or a third-party network. Fabco maintains a dedicated in-house Customer Service Team alongside employed on-site Customer Service Engineers covering both warranty and non-warranty issues, a distinction worth verifying in writing before signing.
Which thermal system are you quoting (W20 or W20+) and what is the confirmed WER rating?
A U-value figure alone is not sufficient for Building Regulations Part L compliance via the rating route. The standard W20 system achieves a B-rated Window Energy Rating at approximately 2.0 W/m2K. The W20+ system achieves an A+ rating at 1.3–1.4 W/m2K and exceeds current Part L requirements via both the U-value and rating routes. Ask for the specific WER or DSER rating in writing, not just a headline U-value.
Evaluating Fabricator Credentials: Comparison of Heritage-Approved Specialists
Choosing a fabricator for a Grade II listed property isn’t just about aesthetics; it’s about credentials, accountability, and what happens if something goes wrong after installation. The table below compares five key criteria across fabricator types. Not every supplier publishes this information openly, so use it as a checklist when requesting quotes.
| Criteria | Fabco | Typical Specialist Fabricator (UK) | |
|---|---|---|---|
| Accreditations | Criteria | FENSA registered; products meet Building Regulations Part L via WER/DSER rating route (B-rated W20; A+-rated W20+) | FENSA registration common; Steel Window Association membership varies; conservation body affiliation not universal |
| Installation model | Installation model | Directly employed project engineers, surveyors, installation fitters, and glazing engineers – no subcontractors | Many fabricators subcontract surveying, installation, or wet glazing to third-party networks |
| Survey method | Survey method | State-of-the-art 3D laser surveys; dedicated project engineer assigned per project throughout five-step delivery process | Standard manual measuring common; dedicated single-point contact not always guaranteed |
| Post-installation support | Post-installation support | In-house Customer Service Team plus employed on-site Customer Service Engineers covering warranty and non-warranty callouts | Warranty callouts often handled via third-party contractors; response times and accountability vary |
| Heritage project experience | Heritage project experience | Over 20 years of British-made slimline steel glazing; featured on Grand Designs; manufactured in West Sussex using hot-rolled W20 steel sections | Experience varies widely; some fabricators import profiles rather than manufacturing domestically |
The sharpest differentiator between fabricators isn’t accreditation on paper, it’s whether the same company that sells the windows also surveys, installs, and glazes them. Fabco’s fully employed delivery model means one team is accountable from 3D laser survey through to the final wet-glazed silicone seal, which matters considerably when Conservation Officers are reviewing every detail.
Red Flags: Warning Signs When Vetting Fabricators
Not every fabricator offering ‘heritage steel windows’ is equipped to handle a Grade II listed project in London. These are the warning signs worth catching before signing anything.
Red Flags That Should Stop You
- Clear red flag: survey costs quoted separately or refunded only on order. A transparent fabricator includes survey in the project cost and uses precision methods like 3D laser scanning
- Clear red flag: installation is handled by a subcontracted network rather than directly employed fitters; accountability disappears the moment a third party steps on site
- Clear red flag: no named project engineer assigned before contracts are signed. Without a single point of contact, listed-building projects with 24-week-plus timelines quickly lose coordination
- Some fabricators present subcontracted delivery as standard industry practice, making it easy to miss unless you ask specifically who employs the surveyor, installer, and glazing engineer
What Good Looks Like by Comparison
- Fabricators like Fabco employ surveyors, fitters, and glazing engineers directly, all covered by in-house quality control procedures across every stage of a five-step delivery process
- W20+ achieves a U-value of 1.3–1.4 W/m2K and an A+ Window Energy Rating, satisfying both the U-value and rating routes under Part L. A fabricator unable to confirm which compliance route their product uses is a warning sign in itself
- PAS 24 compliant specification is available as an upgrade. Any fabricator unable to confirm their security standard by name warrants further scrutiny
- Even strong fabricators may carry long lead times. 20-plus weeks from survey to installation is realistic for bespoke listed-building work, so promises of faster turnarounds deserve scepticism
- British manufacturing and in-house teams cost more upfront, which means the cheapest quote almost always signals corners being cut somewhere in the delivery chain
Verdict: If a fabricator cannot name who employs their installer, confirm their U-value compliance route, or show an independent fire or security certification like PAS 24 or a Warringtonfire assessment, treat that as a firm reason to look elsewhere.
Summary
Grade II listed projects leave almost no room for process failures. A U-value of 2.0 W/m²K versus 1.3–1.4 W/m²K isn’t a minor detail. It’s the difference between meeting Part L via the rating route only and exceeding it on both routes. An FD30 door without a Warringtonfire assessment isn’t a fire door in any meaningful sense. A PAS 24 upgrade means nothing if the installer fitting the frame isn’t directly employed and quality-controlled throughout.
Fabricators like Fabco offer something a loose subcontractor network simply can’t: a single point of accountability from 3D laser survey through to final wet glazing. Their surveyors, fitters, and glazing engineers are all directly employed through a five-step delivery process.
That accountability is what protects the building (and the budget) over decades of service life.
