DER TER

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.

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.

The design stage SAP calculation sets a carbon target, the TER. This is the CO₂ emissions rate the proposed dwelling has to achieve.

SAP derives that figure from 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. You then improve the design, typically by upgrading U-values, tightening the envelope, choosing a lower-carbon fuel, or adding solar PV.

What factors affect the DER?

Wall, floor and roof U-values drive the DER, along with window performance, air tightness, heating and hot water system efficiency, 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.