A great deal has been written about the Future Homes and Buildings Standards over the past few months, mostly about heat pumps, insulation, and solar panels. Very little has addressed glass, and almost none has made clear which projects the changes actually affect. For anyone working on structural and architectural glazing, that distinction is the most useful thing to understand.
The dates themselves are settled. The 2026 editions of Approved Document L take effect on 24 March 2027 for building work not connected with higher-risk building work, and on 24 September 2027 for work that is. More importantly, the headline reforms apply to new buildings. Extensions, alterations and work to existing buildings continue to be governed by their own route through Part L, and on that route very little has moved.
At Glasstec Systems most of our work sits on the existing-buildings side of that line, so it is worth setting out both halves of the picture.
Where the real change lands
For new dwellings, the notional window U-value remains at 1.20 W/m²K, unchanged from Part L 2021, and the substantive change is in how that figure is evidenced. Window U-values have historically been declared against a standard calculation window, whereas the new approach requires assessing each window at its actual size and configuration, with the results area-weighted across the dwelling. Everything within the opening that affects thermal performance is counted, which is why the change is expected to push parts of the volume market towards triple glazing.
That is principally a manufacturing and housebuilding issue, and it falls on the companies supplying windows into new build. It touches bespoke structural glazing only indirectly, though one aspect is worth noting: a calculation method that counts frame and subdivision against the result tends to favour large, minimally framed panes, and load-bearing structural couplers do not have to be included in the calculation at all.
Alongside this, a new functional requirement L3 introduces on-site renewable electricity generation for new dwellings, and the guidance has been reissued as Approved Document L (Energy and Greenhouse Gas Emissions), in Volume 1 for dwellings and Volume 2 for buildings other than dwellings.
Work to existing homes: the figures that actually apply
For new fabric elements in existing dwellings, Table 3.2 sets the limiting standards, and they carry over unchanged. Windows, including roof windows and curtain walling, are limited to 1.4 W/m²K or Window Energy Rating Band B. Rooflights are limited to 2.2 W/m²K. Doors with more than 60 per cent of the internal face glazed are limited to 1.4 W/m²K or Doorset Energy Rating Band C.
One of the notes to that table deserves more attention than it usually gets, because it speaks directly to structural work. Where other performance requirements such as wind load, safety, security or acoustic attenuation call for thicker glass, compliance can be demonstrated using an equivalent unit with standard 6mm glazing. Large structural panes routinely need heavier build-ups for entirely non-thermal reasons, and this mechanism prevents that necessity from becoming a compliance problem.
Two further details are worth having to hand. Rooflight U-values are assessed on the outer developed surface area, which is often larger than the roof opening itself, and where an upstand is built on site it should achieve a maximum of 0.35 W/m²K. And replacing glazing, or replacing a window or door within its existing frame, does not constitute the provision of a controlled fitting, so it does not need to meet the energy efficiency requirements. That has obvious relevance to repair and reglazing work on existing structural glass.
Glass extensions and the two different 25 per cent rules
This is where a good deal of confusion arises, because there are two separate rules and they are frequently conflated.
When a dwelling is extended, the total area of windows, roof windows, rooflights and doors in the extension should not exceed 25 per cent of the floor area of the extension, plus the total area of any windows and doors that no longer exist or are no longer exposed as a result of the work. The second part of that calculation is routinely overlooked, and on a glass extension replacing an elevation that already contained substantial glazing, it can make a material difference to what is achievable.
Separately, where a window is enlarged or a new one created, the area of windows, roof windows, rooflights and doors should not exceed 25 per cent of the total floor area of the dwelling, or, if it does, measures should be taken to improve the energy efficiency of the dwelling. That is the rule that tends to apply to frameless glass links and large new openings into existing houses, and it is worth noting that exceeding the figure is not a failure. It simply moves the conversation to compensating measures elsewhere.
There is also the conservatory and porch exemption to consider, which is more relevant to glazed extensions than its name suggests. Work is exempt from the energy efficiency requirements where the extension is at ground level, the floor area does not exceed 30m², the glazing complies with Part K, any wall, door or window separating the extension from the dwelling is retained or replaced, and the dwelling’s heating system does not extend into it. Where a glazed structure is not exempt and is treated as an extension, the 25 per cent area limitation does not apply to it, though the elements themselves still have to meet the Table 3.2 standards.
Commercial work: curtain walling, entrances and display glazing
For buildings other than dwellings, Table 3.1 applies to elements in new buildings, new elements in extensions, and new or replacement elements in existing buildings alike. Windows, roof windows and curtain walling are limited to 1.6 W/m²K, rooflights to 2.2 W/m²K, and pedestrian doors including glazed doors to 1.6 W/m²K. High-usage entrance doors are treated separately at 3.0 W/m²K, which is a more realistic figure for busy commercial entrances than is often assumed.
Display windows and similar glazing are treated differently again, and this matters for shopfront work. No maximum U-value is set for display windows and similar glazing, and where a building is extended, display windows may be as large an area of wall as required for the purpose. In an existing building where windows or fully glazed pedestrian doors cannot meet the standard because the character of the building has to be maintained, the alternative is a centre pane U-value not exceeding 1.2 W/m²K, or single glazing supplemented with low-emissivity secondary glazing.
Where opening areas in an extension do bite, the limits are 40 per cent of exposed wall for offices and shops and 20 per cent of roof area for rooflights, and they apply only where the extension would be more glazed than the existing building.
The transitional arrangements
For work not connected with higher-risk buildings, the new requirements will not apply where a building notice, initial notice, or application for building control approval with full plans has been given before 24 March 2027 and the work commences before 24 March 2028. For higher-risk building work, the Regulations do not apply where a valid building control approval application was submitted before 24 September 2027 and has not been rejected, and where that protection begins with an application covering one stage of the work it continues through subsequent stages.
The practical effect is that schemes running alongside one another may end up assessed under different editions, so it is worth establishing early which one a project sits under.
What this means in practice
The most useful message for design teams is a reassuring one. If you are working on an extension, a glass link, a retrofit, or a shopfront, the standards your glazing has to meet are essentially those you have already been working to, and the noise around the Future Homes and Buildings Standards is largely about new build.
What does require attention is the detail:
These are the kinds of questions that are far easier to resolve at concept stage than once an envelope is fixed, which is the same argument we have made before about treating glazing as a performance element rather than a finish.
Talk to us about your next project
If you are working on a scheme where structural or architectural glazing plays a significant part, and you would like to understand which set of requirements applies and what that means for the specification, we would be happy to discuss it at whatever stage the project has reached.
Tel: 020 8500 2818
Email: info@glasstecsystems.com
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