What is Part L of the Building Regulations?

Part L is the section of the Building Regulations (in England and Wales) that deals with the conservation of fuel and power — in plain terms, energy efficiency and carbon emissions. It sets standards for insulation, glazing, air-tightness, heating efficiency and, increasingly, low-carbon measures. It applies to new buildings and to most extensions, conversions and significant alterations of existing ones.

What is a U-value and why does it matter?

A U-value measures how quickly heat passes through a building element such as a wall, roof, floor or window, expressed in W/m²K. The lower the U-value, the better the element resists heat loss and the less energy you need to keep the space warm. Part L sets maximum U-values for each element, so meeting these targets is a core part of complying — and a major driver of how comfortable and cheap-to-run your finished space will be.

Does Part L apply to my extension?

Yes. When you build an extension, the new external walls, roof, floor and glazing must meet the current Part L standards for thermal performance. There are also limits on how much glazing you can have relative to floor area. Your designer and builder should specify and construct the extension to meet these requirements, and Building Control will check the insulation before it is covered up.

Do I have to upgrade my whole house to comply?

No. Part L applies to the new or altered work, not to the rest of an existing house. Building an extension does not force you to re-insulate the original property. However, when you carry out certain works — such as replacing windows, a boiler, or re-rendering a wall — the new elements themselves have to meet current standards, and it is sometimes sensible to upgrade insulation while the opportunity is there.

How does RCB make sure my project meets Part L?

We design to the correct U-values from the outset, specify compliant insulation, glazing and heating, and build to the detailing that air-tightness and thermal performance require. Because we manage design and build together, compliance is considered from the first drawing rather than discovered on site, and we coordinate the Building Control inspections through to your completion certificate. Contact us to discuss your project.

Understanding Building Regulations Part L: Energy Efficiency Explained

What Part L Is — and Why It Exists

The Building Regulations are organised into parts, each lettered A to S, covering a different aspect of how buildings must be built. Part L deals with the “conservation of fuel and power” — in everyday language, energy efficiency and carbon emissions.

Its purpose is straightforward: to make sure buildings keep heat in, use efficient heating, and waste as little energy as possible. For you as a homeowner that translates directly into lower bills, a more comfortable home and a project that holds its value. Part L has been tightened over successive updates as the UK works towards lower-carbon buildings, so the standards your project must meet are the current ones, not the ones in place when your house was built.

What Part L Covers

Part L sets standards across several areas of building performance:

Thermal insulation (U-values)

Part L sets maximum U-values for walls, roofs, floors and windows. A U-value measures how much heat passes through an element — the lower the number, the better the insulation. New extension walls, for example, are expected to achieve around 0.18 W/m²K.

Glazing and doors

Windows and external doors must meet minimum energy standards. There are also limits on the total area of glazing relative to floor area, to prevent excessive heat loss through large expanses of glass.

Air-tightness

Uncontrolled draughts waste energy, so Part L expects the building fabric to be reasonably airtight, with controlled ventilation provided instead. New dwellings are tested; extensions are expected to be built to a sealed standard.

Heating systems and controls

Boilers, heat pumps and heating controls must meet minimum efficiencies. When you replace a heating system, the new one generally has to comply with the current standards, not the old ones.

Thermal bridging

Junctions — where a wall meets a floor, roof or window — can leak heat if poorly detailed. Part L expects these "thermal bridges" to be designed out so the insulation performs as intended.

Low-carbon and renewable measures

The latest standards push towards lower-carbon heating and, for new homes, on-site measures such as solar PV. For extensions, the emphasis is on a high-performance fabric.

When Part L Applies

Part L is triggered by a wide range of common projects, including:

Building a new extension — the new elements must meet current Part L standards
Converting a loft, garage or basement into habitable space
Replacing windows or external doors (these must meet minimum energy ratings)
Replacing or installing a new boiler or heating system
Re-roofing or re-rendering where it is reasonable to upgrade insulation at the same time
A material change of use — e.g. turning a commercial unit into a dwelling

Part L applies to the new or altered work, not to your whole house. Building an extension does not force you to re-insulate the original property — but the new elements must meet today's standards.

How a Project Complies with Part L

Compliance is a sequence that runs from design through to sign-off:

Design to the right U-values

Your designer specifies wall, roof, floor and glazing build-ups that meet or beat the Part L targets. This is decided on paper long before anything is built.

Choose compliant products

Insulation, windows, doors and heating appliances all carry performance figures. The specification should name products that meet the required standards.

Build it as designed

Insulation only works if installed correctly with no gaps, and air-tightness depends on careful detailing on site. Good workmanship is what turns a compliant design into a compliant building.

Building Control inspection

Building Control checks the work at key stages, including insulation before it is covered up. This is part of why insulation is inspected before plasterboard goes on.

Certificates and sign-off

Replacement windows and heating systems are usually self-certified by a registered installer (FENSA/Gas Safe), and the project receives a completion certificate confirming compliance.

Why Part L Is Worth Caring About

It is easy to see Part L as a box-ticking exercise, but building to a high energy standard pays you back:

Lower energy bills — a well-insulated, airtight space costs far less to heat
Greater comfort — fewer draughts, more even temperatures, less cold-spot condensation
Compliance — non-compliant work can be flagged at sale and may need putting right
Resale value — energy performance increasingly affects what buyers will pay
Future-proofing — building to a high standard now avoids costly retrofits later

We Build to the Standard from Day One

Because RCB manages design and build together, Part L compliance is designed in from the first drawing — the right U-values, the right products and the detailing to make them perform — and coordinated through Building Control to your completion certificate.

Learn how Building Control works

Understanding Building Regulations Part L