(Type your query in the field above and press enter to search)

Part L1b – The Info

Where Part L1a of the Building Regulations deals with new build dwellings, Part L1b sets standards for extensions and conversions, or renovation to existing dwellings.

When Does Part L1b Apply?

Part L1b forms a large part of our workload here at Build Energy – ­ we spend a lot of time helping clients through the regs, whether it be extending their home, converting a barn or creating flats from an existing house.

part l1b

Part L1b sets minimum standards around:

Thermal Elements

The developer must ensure they meet minimum U Values for any new or replacement thermal elements, or even if they renovate an element. This applies to all floors, walls and roofs.

Controlled Services

This applies to heating and hot water systems, insulation of pipework, ventilation and cooling systems and lighting. An example might be the installation of a minimum 86% efficient boiler where you are replacing an older system, or providing a reasonable number of low energy light fittings.

Controlled Fittings

Within scope are windows, roof windows and external doors. There are differing requirements depending on whether a fitting is new or replacement.

Common Issues

One particular part of L1b which affects most homeowners is that relating to  overglazing. With the demand for large bi-fold doors and huge rooflights, this really causes some headaches.

If you are proposing to extend a home, the area of glazed openings may not form more than 25% of the new floor area. So if you propose a new floor plan of 20m² you will only be allowed to fit 5m² of glazing.

There exists a caveat which means you may add any areas of existing glazing which no longer exist, to increase your allowance.

Design Flexibility

L1b does allow for design flexibility on this point, and that is where we usually come in. By showing trade offs or compensating with improvements to the existing dwelling, you are often able to offset the extra glazing.

We show compliance in a number of ways, some of which use ‘Whole House’ SAP Calculations.

Build Energy have extensive experience in dealing with Part L1b Regulations and can provide you with all the help you need. We’ve helped hundreds of clients all over the UK and it is extremely rare that we do not find a solution – without having to change your plans.

We’ll speak to your Building Control Officer, understand what is required, and take care of everything – Easy!

Would you like to discuss your project?

Call us for a chat on 0330 055 34 05 or email

Recent Projects

I have used Build Energy for all of my SBEM assessments and EPC certification for the last few years. We have worked together on over 20 projects - I wouldn’t even consider using anyone else and regularly recommend them to our client base. I have always found them easy to deal with, helpful and prompt.

Lee Mason - DHP UK

Contact us

We are always there to help. We'd love to hear from you!

Find Us