— reviewed against the regs as they stand.
Quick answer: DER is the Dwelling Emission Rate — your design’s calculated CO₂ emissions in kgCO₂/m² per year. TER is the Target Emission Rate, calculated for a notional dwelling of the same size and shape. Pass = DER ≤ TER. Both come out of the SAP calculation, and under Part L 2021 they sit alongside a primary-energy test (DPER ≤ TPER) and a fabric test (DFEE ≤ TFEE).
The carbon pass/fail
The Dwelling Emission Rate (DER) and Target Emission Rate (TER) are the primary CO₂ figures measured by SAP calculations, expressed in kgCO₂/m² per year. The design-stage SAP sets the target (TER); the proposed dwelling’s actual rate (DER) must be equal to or lower than it to pass Part L.
The TER is derived from a notional dwelling of the same type and size as yours. Under SAP 10 the notional dwelling has a demanding specification — including an assumed PV array — which is why the TER is much harder to beat than under older versions of SAP. A parallel primary energy target (DPER vs TPER) was also added in Part L 2021.
What moves the DER
Plenty: orientation, fuel choice, u-values, air tightness, thermal bridging, heating system efficiency and renewables. Energy assessors use the DER/TER gap to show the design team exactly where a dwelling is underperforming — and the cheapest places to claw it back.
The fabric side is the part you can test yourself before an assessor runs the full SAP: model element U-values in the free U-value calculator and junction Ψ-values in the free thermal-bridging calculator to see how much headroom your specification gives the DER — often the junctions are the cheapest place to find it.
Note that CO₂ isn’t the only criterion: fabric energy efficiency, limiting u-values, services and air tightness standards must all be met too. But miss the DER target and the dwelling fails regardless.