Energy efficiency is a major focus in modern property standards, and two key elements in the UK regulatory landscape are Energy Performance Certificates (EPCs) and Part L of the Building Regulations. While they serve different purposes, they are fundamentally connected by a shared goal: reducing carbon emissions and improving building energy performance.

In this article, we’ll break down the relationship between EPCs and Part L — what they each mean, how they interact, and why understanding both is essential for developers, landlords, and homeowners.

⚖️ What Is Part L of the Building Regulations?

Part L deals specifically with the conservation of fuel and power in buildings. It sets out minimum energy efficiency requirements for:

  • New builds (domestic and non-domestic)

  • Extensions and conversions

  • Renovations involving thermal elements (e.g. roofs, walls, windows)

  • Changes of use (e.g. office to flat)

Part L is split into sub-documents:

  • L1A: New dwellings

  • L1B: Existing dwellings

  • L2A: New non-domestic buildings

  • L2B: Existing non-domestic buildings

🏗️ In essence, Part L dictates how energy-efficient a building must be at design and construction stage.

📜 What Is an EPC?

An Energy Performance Certificate (EPC) is the official post-construction assessment of a building’s energy efficiency, graded from A (most efficient) to G (least efficient). It includes:

  • An energy efficiency rating

  • Environmental impact (CO₂ emissions)

  • Estimated running costs

  • Recommendations for improvement

An EPC is legally required when:

  • A property is built, sold, or rented

  • Certain major renovations are completed

📊 While Part L shapes how a building is built or upgraded, the EPC reveals how it performs in reality.

🔗 How Are EPCs and Part L Connected?

Though distinct, EPCs and Part L are interdependent in several critical ways:

1. EPCs Use the Same Software and Calculations as Part L Compliance

Both EPC assessments and Part L compliance rely on the SAP (Standard Assessment Procedure) or RdSAP for older buildings. These models calculate:

  • Heat loss through fabric

  • Heating system efficiency

  • Fuel type

  • Renewable technologies

  • Lighting type and controls

The data collected during a Part L compliance check (especially SAP for new builds) feeds directly into the EPC.

✅ If your building is compliant with Part L, it’s likely to achieve a decent EPC rating — though not guaranteed.

2. Part L Requires Evidence of Energy Performance – Often via EPC

For Building Control sign-off, especially on new homes, you must submit:

  • SAP calculations (design and ‘as built’)

  • A compliant EPC lodged on the national register

  • A Building Regulations Part L Compliance Report

Without these, completion certificates won’t be issued.

3. Part L Updates Trigger Changes in EPC Outcomes

The 2021 uplift to Part L, effective from June 2022, raised energy efficiency standards. As a result:

  • SAP methodology was updated (SAP 10.2)

  • EPC scores for newly built homes shifted

  • Achieving higher EPC ratings (A or B) became harder under the new model

🔄 Keeping up with Part L updates is crucial if you want your EPC rating to reflect well on your property.

4. Part L Sets the Targets – EPC Confirms If They’re Met

  • Part L compliance is like passing the test on paper and during construction.

  • EPC is the real-world grade after the job’s done.

This distinction matters because poor installation, missing documentation, or substitutions during build (e.g. a lower-spec boiler) can lead to a worse EPC — even if Part L compliance was approved on paper.

🧱 Common Scenarios Where This Link Matters

🔨 New Builds

  • Must comply with Part L from the design stage

  • Must produce an EPC for handover

  • EPC rating affects marketability, mortgage options, and environmental impact

🏚️ Conversions & Extensions

  • Trigger Part L1B or L2B

  • May require upgrades to insulation, windows, or heating

  • EPC needed for sale or rental after works complete

🏠 Retrofits & Renovations

  • Significant changes to thermal elements = Part L applies

  • Post-renovation EPC may reveal whether upgrades had enough impact

💡 Tip: Installing new windows or insulation? Document it properly so your EPC reflects the improvement.

🧠 Why This Link Matters for Homeowners, Developers & Landlords

  • Failing Part L = Building Control delay

  • Poor EPC = Lower property value, longer time on market

  • Upgrades that meet Part L = May still result in mediocre EPC if undocumented

  • Rentals must meet EPC minimum standard (currently E, aiming for C by 2028)

📌 Final Thoughts: Understanding One Helps You Master the Other

If Part L is the energy standard during build, the EPC is your proof of performance after completion. By recognising how they’re connected, you can:

✅ Plan smarter upgrades
✅ Avoid compliance delays
✅ Maximise long-term efficiency and asset value

Need Expert EPC Help in London?

At EPCrate.co.uk, we specialise in:

🔹 Same-day EPCs for new builds and retrofits
🔹 Advice on Part L documentation and compliance
🔹 Pre-visit consultation to avoid unnecessary penalties
🔹 Landlord support for rental EPC standards