What is Fabric Energy Efficiency (DFEE/TFEE)?

Updated 20 September 2026 — reviewed against the regs as they stand.

Quick answer: Fabric Energy Efficiency (FEE) is SAP’s fabric-only heat demand metric, in kWh/m² per year. DFEE (your dwelling’s figure) must not exceed TFEE (the target — the fabric demand of a notional dwelling the same size and shape as yours, built to the reference fabric). Renewables can’t rescue it: only insulation, air tightness and thermal bridging move the number. It is one of the three Part L 2021 metrics, and it stays one under the Future Homes Standard from 24 March 2027.

What does DFEE stand for?

DFEE stands for Dwelling Fabric Energy Efficiency — your design’s fabric-only heat demand in kWh/m² per year, before any credit for renewables or heating. To pass Part L your DFEE must be equal to or lower than your TFEE (Target Fabric Energy Efficiency).

What does TFEE stand for?

TFEE stands for Target Fabric Energy Efficiency — the fabric-only heat demand of the notional dwelling, in kWh/m² per year: a dwelling the same size and shape as yours, with the reference fabric specification from Approved Document L, run through SAP under standardised conditions. Your DFEE must be equal to or lower than it.

Why a fabric target exists

Before April 2014, new homes in England & Wales were assessed in Part L against a carbon target only (DER/TER). That skewed designs in odd ways — you could build with inefficient fabric and rescue the CO₂ number with renewables or low-carbon heating. Sensible engineering (and the energy hierarchy) says reduce heat losses first, then add renewables.

So Fabric Energy Efficiency (FEE) was added to Part L in the 2013 edition (effective 6 April 2014). In SAP it appears as DFEE/TFEE — Dwelling vs Target Fabric Energy Efficiency, measured in kWh/m² per year. The dwelling’s DFEE must be equal to or lower than the TFEE of the notional dwelling.

What drives it

Fabric energy efficiency is exactly what it sounds like: the heat-loss performance of the building fabric itself —

• u-values of walls, floors, roofs and openings
• air tightness
• thermal bridging at junctions

Renewables can’t help you here — that’s the point. If your DFEE is marginal, the fixes are fabric fixes, and calculated junction Ψ-values (try ΨMonkey) are often the cheapest one available, since they improve the number without changing anything on site. Every K1 default and its R2 notional counterpart is tabulated if you want to size the opportunity first.

How the TFEE target is actually set

Your target isn’t a fixed national number — it’s built from your dwelling. SAP takes a notional dwelling of identical size, shape and orientation, gives it the reference fabric specification from Approved Document L — walls at 0.18 W/m²K, floors 0.13, roofs 0.11, windows 1.2, an air permeability of 5 — and calculates its fabric energy demand under standardised conditions: UK average climate, natural ventilation, a fixed heating pattern, so that only the fabric is being compared. That figure is your target:

TFEE = the notional dwelling’s fabric energy efficiency.

Part L 2013 added 15% to the notional figure as a flexibility allowance. Part L 2021 dropped it — the government chose the “full fabric specification” option in its Future Buildings Standard response of December 2021 — so there is no slack in the target beyond the notional dwelling itself.

Because the notional building is the same shape as yours, the target automatically reflects how compact your design is. A blocky mid-terrace has far less exposed surface per m² of floor than a sprawling detached bungalow, so its TFEE lands lower — the more exposed fabric a form has, the more heat it can lose, and the target moves with it. This is why two dwellings on the same site can have very different TFEE figures, and why “just copy the plot next door’s spec” doesn’t always work.

A worked example

Say the notional version of your dwelling comes out at 40 kWh/m²/yr of fabric demand: that is your TFEE. Your job is to design fabric whose DFEE lands at or below 40. Get your walls, roof, floor, glazing, air tightness and junctions modelled and your DFEE comes in at, say, 38 — you pass the fabric test with 2 kWh/m²/yr to spare. Come in at 43 and you’ve failed it, no matter how much PV or how efficient a heat pump you bolt on afterwards.

(The figures here are illustrative — real ones depend on dwelling type, size and compactness, so always work from your own SAP model rather than a rule of thumb.)

How FEE fits with the other SAP targets

Fabric energy efficiency is one of several tests a new-build SAP has to pass at once, not the whole story. Under SAP 10 / Part L 2021 a new dwelling must satisfy:

• Primary energy — DPER must beat TPER (the headline metric in Part L 2021).
• Carbon — DER must beat TER.
• Fabric energy efficiency — DFEE must beat TFEE (this page).
• Limiting standards — individual u-values, air tightness and fixed building services must all sit within their backstops.

Miss any one of them and the dwelling fails. FEE is the one that can’t be bought back with kit — it’s a deliberate floor under the fabric, so a home stays efficient even as the grid and its heating system change around it.

DFEE looking marginal? Where to claw it back

Because FEE is fabric-only, the levers are all fabric levers — and some are far cheaper than others:

• Calculated junction Ψ-values. Usually the cheapest win. Poorly-detailed junctions can be 20–30% of fabric heat loss, and swapping punitive SAP defaults for calculated Ψ-values improves the number without changing a thing on site.
• Air tightness. Tightening the target the build can actually achieve (and prove on test) directly cuts fabric demand.
• U-values. Better walls, roof and — often overlooked — glazing and its frames. Marginal upgrades to insulation thickness or a better window spec move the DFEE, and it is worth knowing by how much before you pay for either: U-Monkey rebuilds any wall, roof or floor to BS EN ISO 6946 and gives you the new figure on screen, free.
• Reduce needless glazing / sort orientation. Oversized north-facing glazing loses heat with little solar gain in return; the fabric metric notices.

An assessor reads the DFEE/TFEE gap to show you exactly which of these is the cheapest route back to a pass — often it’s the junctions, not another 25mm of insulation everywhere.

What the Future Homes Standard keeps

Under Approved Document L 2026 — the Future Homes Standard, published 24 March 2026 and in force 24 March 2027 — the fabric test stays. The 2026 edition keeps the three metrics of 2021 — primary energy, carbon and fabric energy efficiency — and is explicit that the target fabric energy efficiency rate “can be achieved only through fabric energy efficiency, this includes U-values, thermal bridging and airtightness”. The notional dwelling’s U-values are the same as 2021’s, but its air permeability tightens from 5 to 4, and airtightness is part of the fabric calculation, so expect the TFEE to tighten with it. Around it, the notional dwelling gets a heat pump, which means the carbon and primary energy targets are set by a heat-pump home — a far harder pair of tests for anything else to pass. The Future Homes Standard guide has the full notional specification.

Fabric matters more under the new standard, not less. As walls, roofs and floors get tighter, thermal bridging becomes a bigger slice of what’s left, and fabric performance also emerges as one of the four headline metrics on the reformed EPC.

The bottom line

Fabric energy efficiency is the test you can’t cheat with renewables — get the u-values right, tighten the air test, and nail the junctions. If a DFEE is coming up marginal, calculated Ψ-values are frequently the cheapest single fix: model yours free with ΨMonkey, then hand the numbers to your SAP assessor — no subscription, no sign-up.