DER TER

DER and TER are acronyms relating to carbon emissions.

You may have seen reference to a DER or TER within a planning condition or building regulations documentation, and more than likely be completely baffled as to what it actually means! The terms relate to the targeted and actual carbon emissions of a new home.

Energy assessors use these emissions figures to guide the design team, and establish where a building may be underperforming against UK compliance requirements.

DER vs TER

The Dwelling Emission Rate (DER) and the Target Emission Rate (TER) are the headline CO₂ figures which SAP Calculations measure. These figures will determine whether a new dwelling passes or fails on its carbon emission targets set within Part L of the building regs.

As a result of the design stage SAP Calculations, a carbon target (TER) will be arrived at – this is the target CO₂ emissions rate which must be achieved by the proposed dwelling.

This figure is arrived at based on a notional dwelling of the same type, size and heating fuel as the one proposed.

Although there are several compliance criteria which determine a pass or fail within SAP (fabric, design, controlled services and fittings etc) if this headline target is not achieved the dwelling simply will not pass. The figure is measured in kg of CO_² per m².

Many design factors can affect the DER, from fabric efficiency to fuel choice and air tightness.

Frequently Asked Questions

What does DER stand for in SAP calculations?

DER stands for Dwelling Emission Rate – the calculated CO2 emissions (kg CO2/m²/year) of the proposed dwelling. The DER must equal or beat the TER for the dwelling to pass Part L.

What does TER stand for?

TER stands for Target Emission Rate – the maximum CO2 emissions permitted for the dwelling, based on a notional reference building of the same size, type and heating fuel.

What happens if the DER is higher than the TER?

The dwelling fails Part L and cannot receive building regulations sign-off. The design must be improved – typically by upgrading U-values, improving air tightness, choosing a lower-carbon fuel, or adding solar PV.

What factors affect the DER?

The DER is influenced by wall, floor and roof U-values, window performance, air tightness, heating system efficiency, hot water system, thermal bridging, and any renewables such as solar PV or heat pumps.

Do DER and TER apply to extensions?

DER/TER compliance applies primarily to new dwellings under Part L1A. Extensions fall under Part L1B, which uses a different framework – though SAP whole-house calculations are sometimes used for complex conversions.