Part L Compliance:
What Developers Need to Know

What is Part L?

Part L — the Technical Guidance Document on Conservation of Fuel and Energy — is the section of the Irish Building Regulations that sets energy performance standards for buildings. The current version, TGD Part L 2019, applies to all new dwellings where a commencement notice was filed on or after 1 November 2019.

Part L 2019 represents a significant step-up from the previous 2011 version. It incorporates the EU's Nearly Zero Energy Building (NZEB) requirements and sets performance targets that are among the most demanding in Europe for residential construction.

Key Part L 2019 Requirements

Performance Metrics

Part L 2019 uses three primary metrics to assess compliance:

• Energy Performance Coefficient (EPC) — must not exceed 0.30 (meaning total primary energy must be no more than 30% of the 2005 reference building)
• Carbon Performance Coefficient (CPC) — must not exceed 0.35
• Renewable Energy Ratio (RER) — must be at least 0.20 (20% of total energy from renewables)

Elemental Requirements

In addition to overall performance, Part L sets maximum U-values for individual building elements:

• External walls: 0.18 W/m²K
• Roofs: 0.16 W/m²K
• Ground floors: 0.18 W/m²K
• Windows and doors: 1.40 W/m²K

Practical Routes to Compliance

The most cost-effective approach to Part L compliance typically involves a combination of good fabric performance (insulation, windows, airtightness) and an efficient heating system with renewable contribution. The three most common routes we see in practice are:

• Heat pump + good fabric: Air-to-water heat pump with well-insulated, airtight envelope. This is now the most common approach for housing developments.
• Heat pump + solar PV: Adding even a small solar PV array to a heat pump system makes achieving the RER target straightforward.
• Gas boiler + solar thermal + PV: Still possible but becoming more difficult and typically more expensive overall than the heat pump route.