What’s the Difference Between Part L And Part O Building Regulations?

There are many building regulations when it comes to our homes and workplaces, which all aim to keep us safe and comfortable in our homes.

We focus on two of these - Part L and Part O - to see what they mean and how they interact with one another.

Part L for energy efficiency

Part L focuses on performance targets such as CO₂ emissions and low carbon heating systems.

There are two volumes in Part L providing guidance on energy efficiency requirements with Volume 1 applying to homes only while Volume 2 applies to all other buildings.

It sets standards for insulation in walls, floors and roofs to reduce heat and energy loss and install clean energy technology such as solar panels.

Other areas covered by Part L include heating and ventilation systems and lighting.

All these measures aim to reduce CO2 emissions by 31 per cent and 27 per cent for other buildings and are set to be reduced further when the Future Homes Standard comes into effect.

Future Homes Standard

Yes, that’s right! March 21 is the official Solar Shading Day with the motto ‘Save Energy, Be Comfortable, Protect the Planet’.

It has been running for two years and is organised by the European Solar Shading Organisation, a not-for-profit organisation that supports the EU’s ambitious energy-efficiency goals.

The day is a call to action for governments, architects, engineers, and individuals to integrate solar shading into their daily lives and long-term planning to save energy, keep buildings cool in the summer, and warmer in the winter.

Part O for overheating

Part O provides guidance around preventing overheating in houses, flats, care homes, schools and student accommodation.

It covers solar gain, ventilation and removing excess heat so we stay cool in our homes.

This is of great importance as our climate warms and we experience hotter temperatures and more frequent heatwaves as seen with 2025’s warmest spring on record.

The regulations ensure windows and glazed areas are limited to a certain size depending on the room size, the direction they face and cross-ventilation and where the building is located.

For example, a building in a city is deemed as a high risk of overheating because of reflected heat from office and apartment blocks.

The guidance states that installing solar shading like external blinds and awnings is crucial to stop the risk of overheating in homes, especially those with windows, doors and roofs facing south.

How do Part L and Part O interact?

The two sets of regulations interact with each other targeting energy efficiency, helping households to slash electricity bills.

While Part L is concerned with cutting CO2 and reducing the energy we use, Part O aims to stop overheating occurring so that residents don’t have to resort to energy-guzzling measures such as air conditioning units.

Air conditioning units are responsible for ten per cent of electricity consumption worldwide, equating to almost four per cent of the world’s greenhouse gas emissions.

The cost of running air conditioning units is also prohibitively expensive for many people and can double a household’s energy bill.

You cannot have Part L (high levels of insulation) without considering Part O, as your home will heat up to an uncomfortable temperature very fast, even on cooler days. While Part L tries to stop heat escaping your home, Part O prevents heat coming into your home which without adequate ventilation cannot then escape.

This is a common problem with extensions, particularly if they have lots of glazing; highly insulated but without solar shading built in, the indoor temperature can increase to a sweltering 28C or more very quickly on a warm day.

External blinds are a sustainable solution to overheating

External blinds are a much more cost-effective and sustainable solution to overheating, reducing inside temperatures, sometimes by up to seven degrees Celsius once installed.

This is the difference between stifling and comfortable for residents as our case studies demonstrate.

Even when controlled via an app, external blinds use a minimal amount of energy to raise and lower.

The heat-reflecting fabric is made with a special weave so plenty of light is allowed through, negating the need for artificial light during the day, and occupants can still enjoy the view outside.

External blinds can help keep your home warm

In the winter, external blinds can be raised, allowing you to let as much solar gain into your home as is comfortable, providing free heating.

There is plenty of anecdotal evidence that external blinds can help reduce energy bills because it provides a layer of insulation which stops the heat inside escaping as quickly as well as any drafts. However, this has not been officially tested.

If you’d like to find out more about how external blinds work, our experts would be very happy to speak to you.