— reviewed against the regs as they stand.
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Assemble the element layer by layer from a library of 418 materials, with the Part L limiting and notional checks, condensation-risk screening and the thermal-mass figures applied as you build.
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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), a fabric test (DFEE ≤ TFEE) and the limiting values.
What does DER stand for?
DER stands for Dwelling Emission Rate: the carbon dioxide emissions of your dwelling as calculated by SAP, in kgCO₂/m² per year — the annual emissions from space heating, hot water, fixed lighting, pumps and fans, less anything generated on site, divided by the floor area. It is calculated for the building as designed, and again as built.
What does TER stand for?
TER stands for Target Emission Rate: the emission rate of the notional dwelling — a version of your dwelling with the same size, shape, orientation and opening areas (capped at 25% of the floor area), built to the reference specification in Approved Document L. Under Part L 2021 that specification is walls at 0.18 W/m²K, floors 0.13, roofs 0.11, windows 1.2, an air permeability of 5, an 89.5% gas boiler, waste water heat recovery on the showers and a PV array sized at 40% of the ground-floor area divided by 6.5; the full set is on the reference page. SAP works the notional dwelling through the same calculation as yours, and its result is your TER.
Because the notional dwelling is your dwelling with a standard specification, the TER is different for every design. A compact mid-terrace and a sprawling bungalow of the same floor area get different targets, which is why there is no national pass mark to design to, and why the target only exists once the SAP has been run.
The carbon pass/fail
The design-stage SAP calculation produces both numbers at once. Building Control needs the design-stage figures before work starts and the as-built figures within five days of completion — regulation 27 of the Building Regulations, with 27A and 27C doing the same for the fabric and primary energy rates — and the as-built calculation is also what the EPC is lodged from.
Under SAP 10 the notional dwelling has a demanding specification — including the assumed PV array — which is why the TER is much harder to beat than under older versions of SAP: a gas-heated design with no PV of its own will not pass on carbon, whatever its fabric.
DPER and TPER: the primary energy test
Part L 2021 added a parallel pair, the Dwelling Primary Energy Rate and Target Primary Energy Rate, in kWh of primary energy per m² per year. Primary energy counts what it took to generate and deliver each kilowatt-hour, so a unit of grid electricity is worth 1.501 kWh of primary energy in SAP 10.2 against 1.130 for gas, and the primary energy test is the headline test of the 2021 edition. The two tests pull in different directions for electric heating — a heat pump wins both, a panel heater passes on carbon and fails on primary energy — and SAP 10.3, the Future Homes Standard methodology, moves the electricity factors further apart still: carbon down to 0.086 kgCO₂e/kWh, primary energy up to 1.969. What changed in SAP 10.3.
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 psi value calculator to see how much headroom your specification gives the DER — often the junctions are the cheapest place to find it, because SAP’s default Ψ-values are often several times a calculated psi value — the Table K1 versus R2 comparison puts a number on the gap for each junction.
The four tests together
CO₂ isn’t the only criterion. A new dwelling under Part L 2021 passes four tests at once — DPER ≤ TPER, DER ≤ TER, DFEE ≤ TFEE, and the limiting values for U-values, air permeability and services — and missing any one of them is a fail. The fabric test is the one renewables cannot rescue; the carbon and primary energy tests are the ones a change on the heating side moves most. The Part L guide walks through all four, and the Future Homes Standard keeps the same three metrics against a heat-pump notional dwelling from 24 March 2027.
Common questions
What does DER stand for?
Dwelling Emission Rate — the calculated carbon dioxide emissions of the dwelling as designed, and again as built, in kilograms of CO₂ per square metre of floor area per year. It is produced by the SAP calculation.
What does TER stand for?
Target Emission Rate — the CO₂ emission rate of the notional dwelling: a dwelling the same size and shape as yours, built to the reference specification in Approved Document L and run through the same SAP calculation. It is the number your DER must not exceed.
What is the difference between DER and TER?
The DER is your building's calculated emission rate; the TER is the target. Both are in kgCO₂/m² per year and both come out of the same SAP calculation. Pass = DER ≤ TER. The gap between the two is the design's margin, and it is where an assessor looks first to find the cheapest improvement.
What are DPER and TPER?
The primary energy pair added in Part L 2021: the Dwelling Primary Energy Rate and the Target Primary Energy Rate, in kWh of primary energy per square metre per year. Primary energy counts the energy used to generate and deliver each kilowatt-hour to the dwelling, so grid electricity carries a higher factor than gas — 1.501 against 1.130 in SAP 10.2. DPER ≤ TPER is the headline test of the 2021 edition; the carbon test sits alongside it.
Is the DER the same as the EPC rating?
No. The DER is a carbon figure used for Part L compliance. The SAP rating on the EPC is a cost-based score from 1 to 100, and the EPC's environmental impact rating is a separate carbon-based score. A dwelling can pass its DER comfortably and still sit in EPC band B, because the two measure different things.
Does an extension or a conversion have a TER?
No. The target emission rate, the design-stage and as-built notifications and the BREL report apply where a dwelling is erected. An over-glazed extension is checked by a notional-versus-proposed comparison of its own, and a conversion is assessed against the standards for work to existing dwellings, with a SAP of the whole dwelling used to show the trade-off where an element cannot reach them.