Last updated: 21 July 2026
If you’re renovating a period property in the UK, picking the right steel window manufacturer can make or break your project. Companies like Fabco how to handle the tricky balancing act these buildings demand. You’ve got two sets of rules to satisfy at once: local planning authorities want the windows to look right (e.g. Listing or
Conservation Area), while Building Regulations want them to perform efficiently. That’s a lot of pressure on one window frame.
And the gap between old and new standards? It’s significant.
Traditional single-glazed steel windows have U-values (a measure of how much heat escapes through a material – lower is better) of 5.0 W/m²K or worse. Current Building Regulations Part L require replacement windows to hit either a B-rated Window Energy Rating (WER) or a U-value of 1.4 W/m²K. Fabco’s W20 system achieves a whole-window U-value of 2.0 W/m²K and a B-rated WER, which means it clears Part L through the rating route. For listed buildings where planning conditions require those slim, elegant steel sightlines, that matters a great deal. If you need to go further, the W20+ reaches 1.3–1.4 W/m²K, actually exceeding Part L requirements.
There are in fact two separate routes to compliance, the U-value route and the WER/DSER rating route. They’re not the same thing, and which one you use determines whether your window / door is legally compliant or not. That distinction is worth understanding before you buy anything.
Renovating a period property in the UK means working through separate parts of Building Regulations. Mixing them up is a common (and expensive) mistake.
Part L: Thermal Performance
Part L is all about energy efficiency. When you’re replacing windows, there are two ways to show you’re compliant: the U-value route (1.4 W/m²K or better) or the Window or Doorset Energy Rating (WER/DSER) route (B rating or above). A U-value measures how much heat escapes through a material. The lower the number, the better the insulation. These two routes aren’t always interchangeable depending on the product you choose.
Listed buildings add another layer of complication:
- Local planning authorities can allow single glazing where double glazing would harm the building’s character
- But that permission doesn’t automatically take the work outside Building Regulations
- You’d typically need to prove compliance isn’t reasonably achievable, or get a formal exemption from Building Control
Planning Permissions & Conservation Requirements: What Period Properties Need
Period property owners face a two-tier approval process that many renovators conflate: planning permission (or listed building consent) and Building Regulations are separate legal requirements, and passing one does not satisfy the other. Understanding what each demands (and why material choice is central to both) is the critical starting point for any heritage property project.
| Specification | Value | Notes |
|---|---|---|
| Listed Building Consent | Required for any alteration affecting the character of a Grade I, Grade II*, or Grade II listed building, including window and door replacement | Consent is granted by the Local Planning Authority, not Building Control. Where existing steel is present, like-for-like or similar steel is routinely approved because its slim sightlines (as narrow as 10mm in systems such as Fabco’s W20) replicate original profiles that other materials cannot authentically match. |
| Conservation Area Approval | Some conservation areas are covered by Article 4 Directions, which remove permitted development rights, making planning applications less straightforward even for like-for-like window replacement | Local authority design guides typically specify that replacement windows must match the original material, profile, and sightline dimensions of the existing fenestration. Where steel windows are already insitu, hot-rolled steel sections meet this standard; most alternative materials do not. |
| Building Regulations Part L — Energy Efficiency | Minimum B-rated WER or 1.4 W/m²K U-value for replacement windows. | Building Control may accept single glazing only where an applicant formally demonstrates that double glazing would cause unacceptable harm to character. Therefore, there is no certified thermal performance for single glazing but Part L requirements are overruled by Building Control/ Conservation Officer. |
| Building Regulations Part K — Safety Glazing | Toughened glass to BS EN 12150 or laminated glass to BS EN 14449 required in critical locations: glazing in doors up to 1,500mm from floor level; glazing within 300mm of either side of a door up to 1,500mm from floor level; and glazing in windows or walls below 800mm from floor level. | Heritage status does not exempt a property from Part K. Any new or replacement glazing in these zones must meet the standard regardless of the building’s listed or conservation area status. At Fabco, ALL glass is toughened as standard, wherever it is being installed. |
| Pre-Application Consultation | Most Local Planning Authorities offer formal pre-application advice, typically charged at £100–£600 depending on authority and project scale | Engaging this process before specifying materials is strongly advisable. Conservation officers will often indicate in writing whether steel, aluminium, or timber is acceptable, providing a basis for detailed specification before fabrication begins. Engage with Fabco early in the process and we can provide cross-sectional details to support your application. |
| Material Authenticity as an Approval Factor | Steel windows were the dominant commercial and domestic glazing material in the UK from approximately the 1920s through the 1960s, making steel the historically correct replacement material for the majority of inter-war and mid-century listed buildings | Planning officers assess replacement windows against the original specification. |
Fire Regulations For Listed Buildings & Conservation Areas
Fire safety rules apply to listed buildings and conservation areas just as much as any other property, but you wouldn’t know it from most steel glazing articles, which rarely cover this. When a period property needs fire compartmentation (that means dividing a building into sections to slow the spread of fire), it’s usually triggered by Building Regulations Part B. The door has to stop fire from passing through for at least 30 minutes, all while keeping the building’s character intact.
Fabco’s W40 steel profile system is built for exactly this situation. It’s been assessed by Warringtonfire, and the W40 doorset and screen configuration meets FD30 integrity-only performance – meaning it holds back fire and hot gases for that critical 30-minute window.
So how does it manage to look the part too? The W40’s larger profile holds mechanically fixed steel glazing beads, which keep the fire-rated glass in place during a fire. You can spot fire-rated glass by the visible etching on each pane. Because of this, the system keeps the slim steel look that heritage glazing is known for.
That makes it a genuinely practical option for:
- Residential projects where Building Control requires fire compartmentation
- Commercial settings that need both safety performance and a sympathetic appearance
Summary
Dealing with planning permission and Building Regulations all at once is the defining challenge of any period property glazing project. Whether you get approval or refusal often comes down to one thing; whether your chosen supplier can prove they know the regulations before they start making anything.
Some things simply aren’t negotiable here. Thermal performance (how well a window holds heat in) must be backed up with a WER/DSER rating; that’s the official energy rating system for windows, and you need at least a B rating for replacement windows. Security specs must meet PAS 24 where it’s required, and any fire-rated setups must be independently tested to meet compartmentation (fire-separation) obligations.
So what does a supplier who ticks all these boxes actually look like?
Fabco’s directly employed team covers every stage of the job (surveyors, project engineers, installation fitters, and glazing engineers) which means real accountability from start to finish. The process runs from 3D laser survey all the way through to final wet glazing with RAL colour-matched silicone.
A few things worth knowing about Fabco:
- Over two decades of British manufacturing from their West Sussex facility
- Products achieving U-values between 1.3-1.4 and 2.0 W/m²K depending on the system
- Technical evidence built into the specification from day one
That last point matters more than it might seem. When Building Control officers and conservation officers come asking questions, you want the answers already documented, not scrambled together after the fact.
