“Fabric first” is the phrase every energy statement comes back to, and the Future Homes Standard has just made it more demanding. Published in March 2026 and coming into force on 24 March 2027 after a transition period, the standard requires new homes in England to produce at least 75% less carbon than a 2013 Part L baseline. Meeting that target starts not with technology but with the building fabric — the walls, roof, floor, windows and airtightness that determine how much energy a home needs in the first place.
This post explains what fabric first means under the new regime, the U-values and airtightness figures now in play, and how to present them in a planning energy statement.
Fabric first: the top of the energy hierarchy
Every energy statement follows the energy hierarchy — Be Lean, Be Clean, Be Green. Fabric first sits firmly in the Be Lean stage: reduce demand before you add heat pumps or solar. A well-insulated, airtight, thermally-bridge-free envelope cuts heating demand permanently, needs no maintenance and cannot be switched off, which is why planners and Building Control both reward it. Our overview of the energy statement sets the hierarchy in context, and our guide to the Future Homes Standard 2026 covers the wider package.
The numbers that now apply
The Future Homes Standard notional dwelling — the reference specification your design is compared against — tightens fabric performance across the board. In broad terms the direction is:
- Airtightness around 4 m³/h·m² at 50 Pa, down from roughly 5 under Part L 2021 — a meaningfully tighter envelope that demands careful sealing and testing.
- Low fabric U-values for walls, roofs and floors, with high-performance glazing, continuing the trajectory set by the 2021 uplift toward a genuinely low-demand envelope.
- Reduced thermal bridging at junctions — lintels, jambs, eaves and floor perimeters — using accredited construction details or calculated psi-values.
Because the fabric is so much better, the standard assumes a heat pump as the default heating system and rooftop solar PV covering roughly 40% of ground-floor area. Fabric first is what makes those systems small, affordable and effective — a heat pump in a leaky house is an expensive mistake. For the heating comparison, see our post on heat pumps versus gas boilers.
Airtightness and ventilation go together
Tightening a home to 4 m³/h·m² only works if it is ventilated properly. The notional specification uses decentralised mechanical extract ventilation (dMEV), though many housebuilders opt for mechanical ventilation with heat recovery (MVHR) to claw back warmth from extracted air. Whichever route you take, the energy statement should show that airtightness and ventilation have been designed as a pair — a tight house without adequate ventilation risks poor indoor air quality and condensation.
Do not forget overheating
A highly insulated, airtight, south-facing home can overheat in summer. That is why Part O (overheating) sits alongside Part L: the fabric that keeps winter heat in can trap summer heat too. A good energy strategy anticipates this — sensible glazing ratios, shading and purge ventilation — so the Part L and Part O solutions do not fight each other. Getting the fabric right is the foundation for both.
Presenting fabric first in your energy statement
For planning, your energy statement should tabulate the proposed U-values and airtightness against the notional building, quantify the carbon saving at the Be Lean stage before renewables are added, and confirm the SAP 10.2 (moving to the Home Energy Model) methodology behind the figures. For London schemes, this feeds the London Plan Policy SI2 requirement for a 35% on-site carbon reduction and any carbon offset contribution. Demonstrating a strong fabric-first baseline is the most persuasive way to show a scheme is genuinely low-carbon rather than reliant on bolt-on technology.
Why fabric first pays beyond planning
Reaching for a fabric-first specification is not just about passing Part L or satisfying a planning condition. A genuinely low-demand envelope means a smaller, cheaper heat pump; lower running costs for occupants; and a home that is comfortable and resilient as energy prices move. It also future-proofs the building: because you cannot easily re-insulate or re-seal a home once it is built and occupied, the fabric decisions made at design stage are effectively permanent. Bolt-on renewables can be upgraded later; the wall build-up cannot. That is why a strong Be Lean baseline is the most durable form of carbon saving a scheme can offer, and why demonstrating it clearly in the energy statement carries so much weight with planners. For flats and larger schemes the same logic applies at scale, where communal heat losses and party-wall detailing become significant — small improvements to the fabric multiply across every dwelling.
How Fortress Associates can help
Fortress Associates produces automated energy statements for UK planning applications, free of charge and ready in minutes. We build your Part L 2021 / SAP 10.2 baseline, apply the full energy hierarchy — Be Lean, Be Clean, Be Green — and calculate the CO₂ saving at each stage, including the fabric-first contribution. For London sites we address the London Plan SI2 35% target and carbon offset. Whether you are building one home or a mixed-use scheme, contact us or see our planning services to get started.
Sources & further reading
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