Understanding Part L of the Building Regulations: A Complete Guide for 2025

Pemberton Timber Frame
Jun 4
3 min read

Updated: 5 days ago

If you're involved in the design, construction, or renovation of buildings in England, understanding Part L of the Building Regulations is essential. Often overlooked until it's too late, Part L governs the conservation of fuel and power—and it’s one of the most crucial parts of compliance in today’s construction landscape, especially as we move toward Net Zero goals.

In this post, we’ll break down Part L in practical terms: what it covers, recent changes, who it affects, and what you need to do to stay compliant in 2025 and beyond.

🔍 What is Part L?

Part L of the Building Regulations sets energy efficiency standards for new and existing buildings. It applies to:

New dwellings
Existing dwellings (when altered or extended)
New non-domestic buildings
Existing non-domestic buildings (when altered or extended)

It ensures buildings are designed and constructed to reduce energy demand, enhance thermal performance, and limit carbon emissions.

Part L is subdivided into four volumes:

Part L1A – New dwellings
Part L1B – Existing dwellings
Part L2A – New non-domestic buildings
Part L2B – Existing non-domestic buildings

Note: From 2021, the new format separates dwellings and non-dwellings, and further changes are due in the lead-up to the Future Homes Standard in 2025.

🏗️ Why is Part L Important?

Part L plays a key role in:

Reducing greenhouse gas emissions
Improving the thermal efficiency of buildings
Encouraging the use of low-carbon technologies
Supporting the UK’s commitment to achieving Net Zero carbon emissions by 2050

Failure to comply can result in enforcement action, delays in completion, or having to retrofit energy-saving measures at high cost.

📈 Key 2021/2022 Updates – Preparing for 2025

As part of the government's roadmap toward the Future Homes and Buildings Standards, an interim update to Part L came into force in June 2022, with a 12-month transition period.

Key changes included:

For new dwellings (Part L1A):

A 31% reduction in CO₂ emissions compared to 2013 standards.
SAP 10.2 methodology replaces older versions for energy modelling.
New requirement for photographic evidence of key construction stages to prove compliance.
Improved U-values for walls, roofs, floors, and openings.
Mandatory primary energy target, not just carbon emissions.

For existing dwellings (Part L1B):

Higher standards for replacement thermal elements (e.g., windows, doors).
Better insulation and airtightness for extensions and loft conversions.
Emphasis on minimising thermal bridging and heat loss at junctions.

For non-domestic buildings (Part L2A & L2B):

Enhanced standards for building services like HVAC and lighting.
Detailed energy modelling using SBEM or dynamic simulation methods.
Greater focus on building fabric performance and air tightness testing.

📋 What Needs to Be Done to Comply?

1. Early Planning

Start thinking about energy performance from concept stage. Choose efficient building shapes, orientations, and layouts.

2. Use Approved Calculations

New dwellings must be assessed using SAP 10.2, while non-domestic buildings use SBEM or DSM tools. These assess carbon, energy usage, and compliance targets.

3. Design to Meet U-Values and Targets

Refer to Part L’s minimum standards for:

Wall, roof, and floor U-values
Window and door U-values
Air permeability (measured in m³/h.m² @50Pa)

4. Construction Stage – Document Everything

You now need photographic evidence of insulation, airtightness measures, and major junctions. This ensures as-built construction matches the design.

5. Testing and Sign-Off

Air tightness testing is mandatory for most new builds.
Submit As-Built SAP or SBEM calculations.
Provide homeowners or building owners with an Energy Performance Certificate (EPC).

🔮 What’s Next? Looking Ahead to 2025

The Future Homes Standard (FHS) and Future Buildings Standard (FBS) are due in 2025, bringing even tighter carbon and energy standards.

Expect:

75–80% reduction in carbon emissions for new homes.
No more gas boilers—air source heat pumps will become the norm.
Enhanced performance standards for fabric-first construction.
More stringent controls on overheating and ventilation.

A full update to Part L will support these changes, aligning with the UK’s climate targets and pushing the industry toward low-carbon construction.

🛠️ Who Should Care?

Part L compliance impacts a wide range of professionals:

Architects – need to design with energy efficiency in mind.
Builders & Contractors – must follow detailed specs and build accurately.
Developers – face risks of non-compliance, which can affect saleability.
Surveyors – monitor standards and check for Part L documentation.
Homeowners – especially those extending or converting property.

✅ Final Thoughts

Whether you’re building a new home, retrofitting a shop, or extending your kitchen, Part L of the Building Regulations is a legal and environmental priority. With stricter rules already in force—and even more to come—it’s essential to understand the details and work with qualified professionals to meet them.

Stay informed. Design smart. Build better.