A common surprise in Part L 2026: the fabric specification is largely carried over from 2021. The SAP 10.3 notional dwelling keeps wall 0.18, roof 0.11 and floor 0.13 W/m²K unchanged; air permeability tightens modestly from 5.0 to 4.0 m³/(h·m²), and the notional ventilation system is dMEV. The FHS step-change is heating, PV and ventilation — not insulation thickness.
Why fabric tightening matters more than ever
Part L 2021 delivered a 30% CO₂ reduction vs 2013, mostly through small fabric improvements and notional PV. Part L 2026 needs 75% — and a heat pump can only do so much. The fabric envelope must be excellent for an ASHP-heated home to operate efficiently and for the dwelling to remain comfortable through the larger glazing areas typical of modern designs.
Wall U-value: 0.18 W/m²K notional (unchanged)
The notional dwelling wall value carries over from Part L 2021 at 0.18 W/m²K; the limiting (backstop) value in ADL1 2026 Table 3.1 also stays at 0.26. Standard full-fill cavity masonry, timber frame and SIPs approaches that met 2021 continue to comply.
New ground floor U-value: 0.11 W/m²K
Down from 0.18. Demands 200–250 mm of PIR or EPS below the slab, or a structurally insulated raft. The change drives heavier groundworks but improves long-term comfort.
New roof U-value: 0.11 W/m²K
Down from 0.16. Either 350 mm of mineral wool between rafters (with a cold loft strategy) or 200 mm of PIR for warm roof construction. In-roof solar arrays integrate cleanly with PIR warm-roof builds.
New air permeability target: 3 m³/(h·m²)
Down from a maximum of 8 — and from a typical built value of 5–6. Hitting 3 requires deliberate detailing of every penetration: services, eaves, party-wall junctions and windows. MVHR becomes effectively required at this air-tightness level to manage moisture and CO₂.
External door U-value: 1.0 W/m²K
Down from 1.6. Composite or insulated timber doors at this rating are now commonplace; this is one of the easier upgrades to specify.