What The Replacements to Building Regulations L1a & L1b Mean For Glass

From June 15th 2022, new building regulations Document L – which addresses the conservation of fuel and power – have  come into effect, overriding previous iterations of the regulations.


22 Jun 2022


Simon McAuliffe

Approved Document L covers the efficiency standards for new and replacement energy thermals in buildings, with part 1a covering new build dwellings, part 1b in existing dwellings and L2 covers non-residential buildings. Naturally, this has implications for glass and the glazing industry.

Introduction to Document L

Approved document L is an interim uplift that focuses primarily on insulation and energy conservation, and has come about following two public consultations on previously proposed changes. Part 1 deals with residential buildings (‘dwellings’), with L1a being for new dwellings and L1b being for existing buildings. 

In a nutshell, this is to limit heat and energy gain or losses within the thermal envelope of any one building, whether that be a new building or an update or extension to an existing building. This is just the latest, and definitely not the last, update as we as an industry move ever closer to the Future Building Standard of 2025

The new document L regulations are twofold: a new energy compliance metric that all builds must pass, and new minimum fabric efficiency standards for new or replacement thermal elements, doors and windows in existing or new homes.

While not all of the L regulations relate directly to glazing, it’s still important, as principal designers and contractors will have to work collaboratively as it is the overall performance that will have to pass the new regulations, not just individual elements. For example, for a dwelling with a large amount of glass, designers will have to make up for the thermal loss by overperforming in other parts of the building.

Glazing and the new document L1

On initial briefings of the documents, and the replacements to parts l1a and l1b, the new building regulations could sound alarming for the future of highly glazed extensions. Primarily the rules will be that any glazing or opening areas must not exceed a total area of 25% of the total floor space. 

This includes windows, rooflights, roof glass and all doors. At first glance, this looks restricting, but there are ways to account for this rule, thanks to the stipulation that your building as a whole will be judged by the performance of the thermal envelope and the total DER – Dwelling Emission Rate, not just the Dwelling Fabric Energy Efficiency Rate. 

The performance of the glass chosen will also play a big part in achieving compliance, and with the coatings and options available from Cantifix – such as our Solstice Glass or triple glazing options – there is little to worry about with the new regulations.

Arguably, the three most significant changes for the glazing industry, and specialists such as Cantifix, are as follows:

  1. U-values for rooflights are now calculated in their actual installed orientation rather than considered , as previously, in a vertical orientation.  The actual Uw-values are higher this way, but give a more accurate prediction of in-situ performance.
  2. A minimum air permeability rate of 8.0 litres of air per hour per m2 has been implemented for all opening elements.  Although this is not difficult to achieve, it clearly indicates the direction in which subsequent regulations will go from now on.
  3. For the first time, replacement windows and doors require lower u-values than those being installed into new buildings.  This is very much a sign of things to come, to deal with the challenge of significantly reducing energy loss throughout our existing housing stock.
Glazed box extension on the rear of a limestone country house

Specific criterions and targets

The biggest change for this update as opposed to previous regulations is that all new dwellings must create a 30% reduction in their carbon output, something that has been much-discussed since it was first announced. 

There are 5 main criteria stated in the document, each with its own focus.

Below are two tables from the document that show the target U-values needed to achieve compliance. (We will go into further detail later on how these are open to change, depending on the specifics of each build.)

Table 4.1 Limiting U-values for new fabric elements in new dwellings 

(the number in brackets relates to where you can find information on that element in the official approved document)

Element typeMaximum U-value(1) W/(m2 ·K)
All roof types(2) 0.16 
Wall(2) 0.26
Floor 0.18
Party wall 0.20
Swimming pool basin(3) 0.25
Window(4)(5) 1.6
Rooflight(6)(7) 2.2 
Doors (including glazed doors) 1.6


Table 4.2 Limiting U-values for new fabric elements in existing dwellings

Element type Maximum U-value(1) W/(m2 ·K)
Roof(2) 0.15 
Wall(2)(3) 0.18 
Floor(4)(5) 0.18
Swimming pool basin(6)0.25
Window(7)(8)(9) 1.4 or Window Energy Rating(10) Band B minimum
Rooflight(11)(12) 2.2 
Doors with >60% of internal face glazed(13) 1.4 or Doorset Energy Rating(10) Band C minimum
Other doors(13)(14) 1.4 or Doorset Energy Rating(10) Band B minimum


*Also, the new thermal element cannot perform worse than what it is replacing, even if it outperforms the table above. 

Another part of the new regulations (criterion 5) is the upkeep and running of the new build or extension. Every project must teach the owners and users of the space how to use it optimally and maintain it in good working order. This is to ensure that the space runs efficiently when occupied. To achieve this there needs to be an O&M manual created, that details how to operate and maintain the building as well as any integrated technology such as solar control methods or auto rooflights, something that has been a Cantifix standard for many years in either case.

Approved document L1 - new dwellings

This is where the future of eco-friendly buildings lies. The purpose of document L is to ensure that all new buildings are as close to zero energy loss as possible, paving the way for carbon-neutral living environments. 

When building new dwellings, it will now be essential to make sure your plans meet the Target Emission Rate (TER), Target Primary Energy Rate (TPER) and the Target Fabric Energy Efficiency (TFEE). To directly quote from the paper: ‘The TER is expressed as the mass of Co2 emitted in kilograms per square metre of floor area per year. The TFEE is expressed as the amount of energy demand in units of kilowatt-hours per square metre of floor area per year.’

These are figured out by the Standard Assessment Procedure (SAP). This works by creating a ‘notional dwelling’ – a hypothetical copy of your proposed project but with all the changes needed to make it regulation compliant. From this, the targets mentioned above are created. It is an ideal scenario that architects and contractors can replicate in order to maximise compliance in their designs.


Design implications and considerations


While the use of a notional dwelling makes it easier to create a building that hits all targets, it drastically limits design options, and few architects or clients will want to create something that is generic and by the book. 

Fortunately there are ways that you can with the regulations that pretty much add up to proving that glazing can perform as well as – or better than – other building materials, or that the pros of glazing outweigh the cons of whatever other thermal element is used. 

The use of simple payback calculations can come in handy here too. It is always good practice to make sure your materials outperform the targets, allowing you a degree of margin for error – this is something Cantifix have been offering our clients for many years, as the majority of the glass we create exceeds the requirements of the regulations in terms of U-values and thermal performance. 

Once plans have been drawn up for your build, with a focus on the thermal envelope, they will be tested and will need to meet or outperform the TER and TFEE.

If the rate targets are not met, construction cannot go ahead. If approved, the tests will be performed again on the actual building once completed, to make sure that plans have been followed and that the building is performing as planned. 

As well as the SAP test, there will be the need for a physical test, the pressure test for air permeability. If this cannot take place, a rounded-up estimate will be produced, which will mean that higher targets will need to be met. The target permeability rate is to be no more than 8.0m3/(h.m2) @ 50 Pa and 1.57m3/(h.m2) @ 4 Pa.

This particular test will not apply to structural glazing, which by its bespoke nature, is designed to make sure the glazing is airtight. It will be assessed on the performance of its parts instead.

Approved document L1 - existing dwellings

When it comes to renovations or extensions to existing dwellings there are three key points to consider:

  • To limit heat loss/gain through the entire building envelope.
  • Ensure any new building services outperform their previous iterations and are energy efficient.
  • That any new elements introduced comply with the minimum performance targets.


However, not all extensions have to comply. If the work is at ground level, has a floor area equalling less than 30m², has no independent heating or heating from the house and the separating wall is maintained, then it is classed as a glass porch or conservatory and is exempt.

If not, such as in the case of many highly glazed extensions, then it has to go through a similar process for building regulations as a new dwelling would, but taking into consideration the rest of the house too. This could mean that if someone wants a lot of glazing, they will have to look at how their roof is performing thermally, for example. 


Renovations need to abide by regulations L1b if they:


  • Involve up to half of the area of either a wall, floor or roof that forms part of the dwelling’s thermal envelope, that separates the dwelling from the outside or another part of the building that has a significantly different temperature.
  • If there is a change of use that will significantly change the amount of energy needed or lost.
  • They require replacing more than a quarter of the surface area of the dwelling.
  • The extension is not fully outside of the building’s thermal envelope.


The regulations also extend to any fixed building services that are being replaced, such as lighting or heating. You could argue that too little glazing could require a higher output of electric light or the need for greater ventilation. Any fixed building service that is replaced has to conform to the targets set out, as well as out-performing the item it is replacing. As an added extra, wet-space heating systems will be required to operate to a maximum flow temperature of 55°C, to try and keep the use, and cost, of heating as low as possible. 

Exemption for the replacements to L1a and L1b

As with any regulation that includes a wide range of variables, there are exemptions to the rules. Fire doors, for example, will be allowed a higher u-rating of 1.8 w/m².

If a building renovation is taking place on listed buildings, scheduled monuments, in a conservation area or in historical and traditional protected buildings, they do not need to comply with all the regulations if it is detrimental to the value of the building. These cases will be looked at and assessed by the local authority, as is currently the case, and structural glazing remains a powerful design tool for historic properties. They will however have to prove that all work shows reasonable opportunity to adhere to the regulations where possible. 

Also, any work that has given initial notice before June 2022 or has been approved will not need to comply, as long as the work begins before June 2023.

Impacts of Approved Document L1 on the industry

These new regulations will affect every part of the construction industry, but will hopefully lead to innovations in how different parts of each build performs. There will be cost implications – for everyone – so changes will have to be made.

A takeaway from the changes is the emphasis on detailed drawings of insulation plans, with specific sealing methods described in detail. If every element is performing to standard, and every gap or loss has a contingency, then there is less to worry about.  Work must be carried out as planned and stated, as poor quality work and resources will result in a lower performance than calculated. 

For glazing specifically, this will mean more of a focus on the placement of windows and doors, with plans to maintain the insulated plane as much as possible, and considerations for thermal bridging. This will most likely lead to a rise in glazing overlapping the inner face of the external leaf and the use of continuous cavity closers in openings. However, as with all the other parts of these regulations, there may be wiggle room, with the need for natural light for health being a factor, as well as air permeability. 

Glazing options will have to perform better than ever before: with e-coatings and argon filling, and double glazing, if not triple glazing, as a minimum. Plus if there is framing it will have to be fully thermally broken. 

For historical aesthetics we will be seeing a lot more low emissivity secondary glazing on older buildings too, where newer glass would be detrimental to the look of the façade.  

Finally, it would do any prospective architect or contractor to also take a look at other regulations coming into effect: Documents F and O, the former regarding ventilation, and the latter addressing overheating: something that could be especially pertinent when considering arguments for glazing over brickwork. 

Final thoughts

With more emphasis than ever on climate impacts, and more products available to help reach lower U-value ratings, the regulations are becoming tighter and will continue to do so. Cantifix welcomes these changes, as we all have to re-evaluate how the way we live and work impacts the world we inhabit.

It is crucial to remain up to date with the building regulations as they change, especially with the 2025 Future Buildings Standard meaning that technology and practice will need to develop to match the rapid pace at which we are now seeing tighter restrictions coming in: a full technical consultation is planned to start in 2023, necessary legislation to be introduced in 2024, all ready for implementation in 2025.

This is something Cantifix’s clients should be reassured about. We always work collaboratively with our clients, providing glazing solutions to meet their design requirements, meaning that put simply: this new update does not worry us. As long as principal designers and contractors are au fait with the new regulations, and are prepared to design to meet them, Cantifix are as ready as ever to provide glazing to realise your projects.