EPC Ratings in Hertfordshire: What They Mean and How to Improve Yours

An Energy Performance Certificate (EPC) is a document that rates the energy efficiency of a property on a scale from A (most efficient) to G (least efficient). It is produced by a qualified Domestic Energy Assessor (DEA) who visits the property and records information about the construction, insulation, heating system, windows, and lighting. The assessor enters this data into the SAP (Standard Assessment Procedure) software, which calculates the energy efficiency rating and produces the certificate.

EPCs are required by law whenever a property is sold, let, or newly built. They are valid for 10 years. The certificate includes a current rating, a potential rating (what the property could achieve with recommended improvements), and a list of recommended measures with estimated costs and savings.

The EPC matters for three main reasons. First, mortgage lenders are increasingly using EPC ratings as a factor in lending decisions — some lenders offer preferential rates for properties with A or B ratings (green mortgages), and some are beginning to restrict lending on properties with F or G ratings. Second, landlords are legally required to achieve a minimum rating of E for new tenancies, with a proposed increase to C from 2025. Third, buyers and tenants are increasingly using EPC ratings as a factor in their decisions — a higher rating means lower energy bills.

For Hertfordshire homeowners planning a house extension or loft conversion, the EPC implications of the project should be considered from the outset — a well-designed extension can improve the rating, while a poorly designed one can reduce it.

The SAP calculation considers five main factors: fabric performance (insulation in walls, roof, and floor), glazing (window U-values and area), heating system (boiler efficiency, heat pump coefficient of performance, controls), hot water (cylinder insulation, solar thermal), and renewables (solar PV, wind). The relative weighting of these factors depends on the property type and size.

For most Hertfordshire properties, the heating system has the largest single impact on the EPC rating. Replacing an old G-rated boiler (60–65% efficiency) with a new A-rated condensing boiler (90–94% efficiency) can improve the SAP score by 8–15 points — potentially moving a property from D to C. Installing a heat pump instead of a gas boiler can improve the SAP score by 15–25 points, because heat pumps have a coefficient of performance (COP) of 2.5–4.0, meaning they deliver 2.5–4 units of heat for every unit of electricity consumed.

Insulation is the second most important factor. Cavity wall insulation can improve the SAP score by 4–8 points. Loft insulation top-up (from 50mm to 270mm) can improve the score by 3–6 points. Solid wall insulation can improve the score by 8–15 points. For a detailed breakdown of insulation options and costs, see our Retrofit Insulation Guide.

Renewables — particularly solar PV — can have a significant impact on the EPC rating. A 4kWp solar PV system can improve the SAP score by 5–10 points. However, the EPC rating is calculated on the basis of the primary energy consumption of the property, not the carbon emissions — so solar PV has a larger impact on the Environmental Impact Rating (EIR) than on the energy efficiency rating.

A house extension or loft conversion changes the floor area of the property, which directly affects the SAP calculation. The impact on the EPC rating depends on how well-insulated the new space is relative to the existing house.

A well-insulated extension built to the 2026 Part L standards (wall U-value 0.18 W/m²K, roof U-value 0.15 W/m²K, triple glazing) will typically have a better fabric performance than the existing house — particularly for 1930s–1970s properties with unfilled cavities and single glazing. Adding a well-insulated extension to a poorly-performing house can therefore improve the overall EPC rating.

Conversely, a large glazed extension (such as a conservatory or orangery) with poor thermal performance can reduce the EPC rating — particularly if it is connected to the main house heating system. TCM always advises clients on the EPC implications of their extension design before work begins. For detailed guidance on energy-efficient extension design, see our guide on energy-efficient extensions and loft conversions.

For loft conversions, the EPC impact depends on whether the conversion is a warm loft (insulation at rafter level, bringing the loft into the thermal envelope) or a cold loft (insulation at joist level, leaving the loft outside the thermal envelope). A warm loft conversion typically improves the EPC rating because the new insulation at rafter level exceeds the performance of the existing joist-level insulation. See our Loft Conversion service page for details.