Crittall-style steel windows — characterised by their narrow sight lines, black or dark-grey finish, and grid-pattern glazing — have become a fixture in industrial-aesthetic extensions, orangeries, and kitchen-diner openings. Genuine Crittall or equivalent steel windows are expensive: supply-only costs of £1,500–£3,500 per window are common, and installation adds further. There are now several cheaper alternatives that replicate the look using different frame materials, each with different cost, performance, and compliance profiles.
In 2026, a uPVC slim-profile window can be had for £350–£700 per unit; aluminium slim-profile systems run £600–£1,800; composite (aluminium-faced, thermally broken) systems sit at £700–£2,000. All must comply with Building Regulations in England — primarily Part L for thermal performance and Part Q for security in dwellings.
Why Steel Windows Are Expensive
Genuine Crittall (and similar heritage steel systems such as Jansen or Hope’s) are expensive because hot-rolled or cold-rolled steel is inherently hard to process into thermally broken sections. Early steel windows had no thermal break at all — the frame was a single piece of steel conducting cold straight through to the interior. Modern thermally broken steel systems involve precision manufacturing, are typically bespoke, and come from a small number of specialist fabricators.
That manufacturing complexity is exactly what the alternatives eliminate. By using aluminium, uPVC, or composite profiles shaped to mimic the narrow sightlines of steel, manufacturers can achieve a similar aesthetic at significantly lower cost.
Frame Material Comparison
| Material | Typical Sightline Width | Thermal Break | Typical U-Value (frame) | Supply Cost per Window | Notes |
|---|---|---|---|---|---|
| Genuine steel (Crittall-style) | 25–35 mm | Optional; varies by system | 1.8–5.0 W/m²K (non-broken) | £1,500 – £3,500 | Heritage, listed buildings; poorest thermal performance without break |
| Thermally broken steel | 25–40 mm | Yes | 0.9–1.4 W/m²K | £2,000 – £4,500 | Best aesthetic match; expensive |
| Slim-profile aluminium | 35–55 mm | Yes (polyamide) | 1.0–1.6 W/m²K | £600 – £1,800 | Best cost-performance balance |
| uPVC slim-profile | 50–75 mm | Inherent (multi-chamber) | 1.0–1.4 W/m²K | £350 – £700 | Widest sightlines; closest to standard windows |
| Composite (aluminium/timber or alu/uPVC) | 40–60 mm | Yes | 0.8–1.3 W/m²K | £700 – £2,000 | Good performance; varied aesthetics |
Sightline width is important aesthetically — it is the visible face width of the frame between panes of glass. Genuine Crittall achieves as little as 25 mm. uPVC cannot physically match this; most slim-profile uPVC systems achieve 50–70 mm, which is noticeably chunkier. Aluminium sits in the middle and is the most convincing visual substitute.
Aluminium Slim-Profile Windows
Aluminium slim-profile windows use extruded aluminium sections with a polyamide thermal break — a strip of low-conductivity material that separates the inner and outer aluminium faces, preventing the frame from acting as a cold bridge. This allows them to achieve whole-window U-values of 1.2–1.6 W/m²K when combined with appropriate double or triple glazing.
For the industrial aesthetic, aluminium systems are the most convincing alternative to steel. They are available in any RAL colour, powder-coated, and can be specified in anthracite grey (RAL 7016), black (RAL 9005), or a mid-grey (RAL 7015) — the finishes most associated with the industrial look. Sightlines of 35–50 mm are achievable, compared to 25–35 mm for steel, but in most installations the difference is not glaring.
Supply-only costs for aluminium slim-profile windows range from roughly £600 for a modest single casement to £1,800 or more for large fixed-pane or multi-pane units. Installation by a competent window fitter adds £150–£350 per window depending on size and access.
uPVC Slim-Profile Windows
uPVC slim-profile windows are the cheapest route to the aesthetic but make the most visual compromise. The best uPVC systems are reinforced with steel internally and can achieve good thermal performance, but the profiles are physically wider than aluminium. Black or anthracite-grey uPVC foils have improved in quality and are more convincing than they were five years ago.
For a budget extension or a secondary space where the aesthetic matters but cost is the overriding factor, uPVC slim-profile offers a reasonable result at £350–£700 per window. They are also the easiest to source quickly, as many local fabricators carry the profiles in stock.
Composite Systems
Composite windows — typically aluminium-clad timber or aluminium outer with a uPVC inner — bridge the gap between the two. They offer good thermal performance, a slim outer profile in aluminium, and the option to finish the interior in timber for warmth. Costs sit at £700–£2,000 per window, making them closer to aluminium than to uPVC.
Building Regulations Compliance
Part L — Energy Efficiency
In England, Part L of the Building Regulations (2021 edition, currently in force) sets a limiting U-value for windows in new dwellings and extensions of 1.4 W/m²K (whole window). For replacement windows in existing dwellings (controlled fittings), the same 1.4 W/m²K limit applies.
All the alternative materials listed above can comfortably meet this threshold with double glazing. To achieve a whole-window U-value of 1.4 or better, the glass unit itself typically needs to be a low-emissivity (low-e) double-glazed unit with an argon gas fill. This is standard specification from most window manufacturers.
Triple glazing is not required for compliance, but is increasingly specified where overheating risk assessments or enhanced fabric performance is sought. Triple-glazed units add approximately 15–25% to the glass cost.
Note: Scotland (Section 6), Wales (Part L), and Northern Ireland (Technical Booklet F) have separate standards with slightly different U-value thresholds. Always confirm the requirement with your local authority if in doubt.
Part Q — Security
Part Q applies to all new dwellings and to extensions where new windows are provided in vulnerable locations (ground floor, or accessible from a flat roof or adjacent structure). Windows must be specified to PAS 24:2022 or equivalent. PAS 24 sets minimum requirements for frame strength, glazing retention, and hardware resistance to attack.
Most reputable slim-profile aluminium and uPVC window manufacturers offer PAS 24-compliant products as standard or as an available option. It is worth confirming PAS 24 certification before ordering, particularly with budget uPVC suppliers. Genuine thermally broken steel systems from established manufacturers are also available in PAS 24-certified configurations.
Part Q does not apply to replacement windows in existing buildings (only to new builds and regulated alterations), but many homeowners specify PAS 24 regardless for insurance and security purposes.
Planning Considerations
In conservation areas or for listed buildings, permitted development rights for window replacement are restricted. You will normally need planning permission, and like-for-like or sympathetic replacement will be expected. Where the existing windows are thin-profile steel, the planning authority may resist replacement with uPVC on aesthetic grounds — aluminium or thermally broken steel is more likely to be approved.
Installation Costs
| Window Type | Supply per Window | Installation per Window | Total Installed per Window |
|---|---|---|---|
| uPVC slim-profile | £350 – £700 | £150 – £280 | £500 – £980 |
| Aluminium slim-profile | £600 – £1,800 | £180 – £350 | £780 – £2,150 |
| Composite | £700 – £2,000 | £200 – £380 | £900 – £2,380 |
| Thermally broken steel | £2,000 – £4,500 | £250 – £500 | £2,250 – £5,000 |
Installation costs assume standard openings in existing masonry or timber-frame walls, with no structural lintels required. Unusually large windows, openings that require structural assessment, or windows in roof structures (rooflights) are priced separately.
How to Choose
Choose aluminium if the look is the priority and budget allows. It is the closest visual match to steel and offers a good combination of slim sightlines, durability, and thermal performance without the cost of genuine steel.
Choose uPVC if cost is the primary driver and the window will be viewed mainly from inside, or in a secondary space such as a utility room or outbuilding.
Choose composite if you want the industrial exterior look combined with a natural timber interior finish, or if the thermal performance spec is ambitious.
Stay with steel only if the planning context demands it — listed buildings, conservation areas where aluminium is not accepted, or where the exact heritage sightline is non-negotiable.
For most residential extension projects in 2026, a slim-profile aluminium system in anthracite grey or black delivers a convincing industrial aesthetic, meets all Building Regulations requirements, and costs roughly half as much as an equivalent genuine steel installation.
