Creating energy-efficient, low carbon homes continues to be a major driver for self-builders, but there are also legal requirements when building new homes that must be considered. The legal framework is incorporated in Building Regs, and guidance on how to meet these regulations is outlined in Approved Document L – Conservation of Fuel and Power. 

With 2021 updates to Part L now coming into play, here’s all you need to know about the changes. 

What is Part L? 

Building Regulations Part L covers the conservation of fuel and power in the building of new homes and establishes how energy-efficient homes should be. 

Changes to Part L were announced in December 2021 by the government in a move to help the UK deliver net zero. 

When do the changes take effect? 

It largely depends on where in the country you’re building. In England the changes came into force in June 2022; in Wales they took effect from November 2022; and in Scotland – which has made similar updates to its Section 6 (energy) rules – they were effective from February 2023. 

As it typically takes a year or more to get from concept to starting on site, we’re only now beginning to see the impact on projects breaking ground. 

What are the differences? 

The updates set significantly stricter targets for CO2 emissions from new housing – a reduction of 31% compared to the previous standard. 

Every new home in the UK must now be assessed against the regs at the technical design stage via a Standard Assessment Procedure (SAP) calculation. This is the same wherever you are in the UK – England, Wales and Scotland have adopted the latest SAP10.2 specification to measure it. 

An energy assessor must measure your proposed design’s performance in SAP10.2 software to ensure it complies. The baseline for comparison is set out in Approved Document L via the ‘notional building’. To meet the minimum regulatory requirement, you must intend for the home to perform better overall than the notional building model. 


Part L provides notional and limiting values, addressing U-values for walls, ground floors, roofs, external windows and doors, as well as targets for areas like airtightness. 

The good news is the roof, wall and floor values aren’t changing by much – mainly because we’re already at a very good level of performance for these elements. So, achieving the required U-values is more straight-forward than you would expect – all three of our TechVantage systems deliver these levels with ease. 


SAP10 puts a huge amount of emphasis on thermal bridging, so the real change you’ll notice with the 2021 Part L updates is Psi-values. Thermal bridges are weak points in a building envelope where heat can escape more easily. They are measured as Psi-values (for individual components) and y-values (total heat loss due to thermal bridging across an entire property).  

Under the new Part L, you must ensure that your building system provider can supply Psi-values for your energy assessor.  

At Frame Technologies, we have an in-house energy assessor. We’ve done extensive modelling work on our systems  and have invested in the software to provide this information for our clients. The standard details are already on record, ready to go, and we can run the detail to deliver the ultimate performance based on different scenarios. 

As every self-build house is bespoke, we’ll always need to develop some more that are specific to the project, but this detail is guaranteed, because our timber frames are factory-built, which provides added value. 

What do the changes to Part L mean for me as a self-builder? 

If you fail to follow the latest regulations, the building inspector can ask you to take down work and ultimately refuse to provide you with a completion certificate, so it’s crucial that you are aware of the updates. 

You should take some time to understand the key Building Regs energy performance factors and what it means for your home. You should also carefully consider your project team, as you’ll need to bring in an energy assessor (unless your system provider has one in-house as we do), build system provider, window supplier, heating engineer etc to liaise with your architect or designer to get the design through Building Regs.  

When it comes to deciding on a building system, make sure you’re not just focusing on insulation and U-values. It is now vital to find out how airtightness is dealt with and whether the provider has modelled detailed Psi-values that can be used in the SAP process. If they say no, don’t accept their quote, regardless of how cheap it is. Make sure you do your homework and buy with your head rather than your pocket. 

What should I do next? 

Compliance to these regs is crucial and without Psi values for your system, you will automatically fail the application.  

Get to know the updates; speak with your building system provider and keep up conversations with them, as well as your building control officer and building inspector. Working together in partnership has never been more important.  

As always, we’re here to help navigate through the ups and downs of your project, so if anything is unclear or if you have any questions, please give us a call and we can talk it through.  

From a friendly chat and reassurance to expert technical advice, Frame Technologies will work closely with you to achieve your dream home. 

If you have any questions about these new changes, please get in touch here or email Simon Orrells on