Part L 2025 Is Coming: What Architects Need to Do Now
Introduction
By the time you lodge a Building Control application in early 2026, England’s new Part L will have raised the bar on domestic energy efficiency to a level most practices have never had to meet. Draft guidance published under the Future Homes Standard (FHS) signals a mandatory 75‑‑80 % cut in regulated carbon emissions for every new dwelling compared with the 2013 baseline, plus a shift to low‑carbon heat, tougher fabric backstops and a brand‑new compliance engine. The change is ambitious, but with careful design and early Standard Assessment Procedure (SAP) modelling you can hit the target without late‑stage value‑engineering shocks. This article unpacks the headline requirements, explains why starting SAP 11 calculations in RIBA Stage 2 is critical, and offers a practical checklist you can drop straight into project kickoff meetings. Throughout, we draw on the latest draft notional dwelling specification, official consultation papers and industry commentary, and we flag where Green SAP Compliance Services can keep your team ahead of the curve.
## 1 | Part L 2025 in Context: From Interim Uplifts to the Future Homes Standard
The legislative pathway is now clear. After the interim uplift that came into force in June 2022, the Department for Levelling Up, Housing & Communities confirmed a two‑step timetable: a technical consultation closed in March 2024, enabling final regulations to be laid before Parliament by May 2025, with the FHS—effectively “Part L 2025”—mandatory for all new homes from 1 January 2026 (with transition arrangements expiring after one year) Planning Portal.
Politically, the driver is the UK’s legally binding 2050 net‑zero commitment. Dwellings contribute roughly 17 % of national territorial emissions; bringing them to “zero‑carbon‑ready” status means they must operate without fossil‑fuel space and water heating and be efficient enough to absorb the decarbonising electricity grid. Consequently, new homes built to Part L 2025 must emit 75‑‑80 % fewer carbon emissions than those built to the 2013 regulations cbre.co.uktheoffsiteguide.com.
The government is backing that ambition with two additional policy levers: a proposed ban on new gas boiler connections in most homes from 2027 and non‑mandatory encouragement for on‑site renewables such as photovoltaic (PV) arrays Financial Times. Together, these changes will reshape MEP strategies and demand closer collaboration between architects, energy consultants and product suppliers.
## 2 | Draft Technical Requirements: What the Notional Dwelling Tells Us
Approved Document L has always been performance‑based: demonstrate that your “actual” dwelling equals or betters the carbon and primary‑energy results of a standardised “notional” version. The December 2023 notional dwelling tables reveal the scale of the coming uplift GOV.UK:
ElementDraft Part L 2025 Notional U‑value (W/m²·K)2021 Backstop U‑valueCommentaryExternal walls0.180.2631 % improvementParty walls0.00 (fully filled & sealed)0.20Thermal bypass eliminatedRoof (ceiling level)0.110.16Already Passivhaus‑levelFloors0.130.18Edge details matterWindows & doors1.001.60Triple‑glazing territory
Infiltration test results tighten to 4 m³/(h·m²)@50 Pa for the main notional option (5 m³/(h·m²) for option 2), signalling that anything looser than 3 ACH is likely non‑compliant without mechanical ventilation with heat recovery (MVHR) GOV.UK.
The notional dwelling is also specified with an air‑source heat pump (ASHP), demand‑controlled decentralised mechanical extract or MVHR depending on infiltration, waste‑water heat‑recovery to all showers, 100 % LED lighting at 120 lm/W efficacy, and an electric‑vehicle charging point. Although solar PV is not hard‑coded, many design teams will need at least 1.5–2.0 kWp to meet the primary‑energy target when site constraints push fabric or airtightness beyond the practical optimum.
## 3 | SAP 11, the Home Energy Model and Why Early Modelling Matters
Part L 2025 will be assessed using SAP 11—or, during a transition year, either SAP 11 or its successor, the Home Energy Model (HEM). Government guidance published in May 2025 confirms the two methodologies will run in parallel to give industry breathing space before HEM becomes mandatory GOV.UK. For architects that means your usual SAP plug‑in and EPC workflow will change:
Dynamic hourly calculation: HEM integrates a BREDEM‑based time‑step, improving overheating and heat‑pump sizing accuracy.
Revised fuel factors: Grid‑electricity carbon is slated to fall below 0.080 kgCO₂/kWh by 2030, making electricity overwhelmingly favoured.
Thermal bridging overhaul: Default Accredited Construction Details disappear; designers must model junctions or adopt psi‑values from certified products.
Because the notional dwelling already assumes best‑practice construction, late‑stage tweaks—thicker insulation, upgraded glazing—rarely claw back enough margin once form, orientation and fenestration ratios are fixed. Starting SAP 11 iterations at RIBA Stage 2 lets you “bank” fabric performance and test services packages while options are still fluid. Projects that model only at tender are averaging 3‑‑5 kWh/m²·yr primary‑energy deficits, forcing costly bolt‑on PV or mechanical cooling to counter summer gains.
## 4 | Design Strategies to Meet Part L 2025 Without Busting the Budget
1. Re‑think form‑factor early
A compact thermal envelope (Form Factor < 3) reduces transmission losses more effectively than incremental insulation in complex massing. Simple rectangles with limited articulation often shave 4‑5 % off Dwelling Primary Energy Rate (DPER) versus an identical floor area in staggered blocks.
2. Optimise orientation and window‑to‑floor area (WFA)
The draft tables cap glazing at 25 % of TFA; exceed that and the notional dwelling down‑sizes its own glazing, locking your design into a performance penalty GOV.UK. A rule of thumb is 55 % of glazing facing south and 15 % facing north with deep reveals or external shading.
3. Select heat‑pump‑ready emitters
Low‑temperature underfloor or oversized radiators sized for 40 °C flow ensure ASHP seasonal efficiencies above 300 %. Coordinate structural zones to accept UFH pipework depth before layout freezes.
4. Treat airtightness as architecture, not add‑on
Continuous air barriers, service voids inside the airtight layer, and factory‑finished cassettes are cheaper than site tapes. Projects achieving ≤2 m³/(h·m²) @ 50 Pa routinely downgrade MVHR fan power in SAP, gaining 1‑2 kWh/m²·yr headroom.
5. Value‑engineer embodied carbon in parallel
Although Part L is operational‑energy focused, clients increasingly set whole‑life‑carbon targets. A timber cassette with blown‑cellulose insulation can meet the 0.18 W/m²·K wall target with half the embodied CO₂ of brick‑and‑block, and little cost premium when procured early.
6. Exploit modular service cupboards
Integrated ASHP, DHW cylinder, MVHR and distribution manifolds in a central “energy pod” slash pipe runs, reduce thermal losses and simplify commissioning.
Case Study Snapshot
A 42‑unit scheme in the West Midlands using a masonry cavity wall originally targeted 0.21 W/m²·K but failed SAP 11 by 3 %. Switching to 140 mm full‑fill cavity insulation, thickening loft mineral wool to 450 mm, and clipping airtightness from 5 to 3 m³/(h·m²) saved £460 per unit compared with retrofitting 1.8 kWp PV across every roof. Early modelling identified those swaps at concept stage, avoiding contract variations.
## 5 | Architect’s Quick‑Start Checklist for Projects Breaking Ground in 2026
Use this as a RIBA Stage 1–2 aide‑mémoire; adapt line items to suit your QA system.
Confirm Transitional Dates
Will any plots start on site before 1 Jan 2026? If not, design straight to Part L 2025.
Appoint an Energy Consultant by Stage 1
Engage Green SAP Compliance Services or equivalent to produce draft SAP 11/HEM models alongside feasibility plans.
Calculate Form‑Factor
Target < 3 if possible; flag to client where massing compromises energy budget.
Set Fabric Targets
Internal briefing note: walls ≤ 0.18 W/m²·K; roofs ≤ 0.11 W/m²·K; floors ≤ 0.13 W/m²·K; windows ≤ 1.00 W/m²·K whole‑window; airtightness ≤ 3 m³/(h·m²) @ 50 Pa.
Model Solar Gain & Overheating
Use CIBSE TM59 or Part O routes. Orient glazing, specify shading, and size MVHR bypass modes early.
Choose Low‑Carbon Heat
Default to ASHP unless site is on a strategic heat network. Verify buffer and plantroom footprints.
Coordinate Ventilation Strategy
If airtightness < 5 m³/(h·m²), include MVHR; else dMEV with demand sensors. Check space for ducts in joist zones.
Lock Service Cupboard Sizes in GA Drawings
Allow 750 mm × 1 200 mm minimum for heat‑pump cylinder and MVHR where centralised.
Detail Thermal Bridges
Assign psi‑values: proprietary block at ground floor, insulated lintels, full‑fill cavities. Obtain third‑party certificates.
Run Pre‑Tender SAP & Cost Risk Review
Revise the model when structural, MEP and spec packages hit 90 % issue. Offer client a dashboard showing residual energy margin in kWh/m²·yr and tonnes CO₂e.
## 6 | How Green SAP Compliance Services Can Support Your Team
As specialists in Part L, FHS and net‑zero‑aligned residential design, Green SAP Compliance Services offers:
Concept‑stage SAP 11 & HEM Simulations – rapid turnaround to test massing, fabric and MEP options before planning.
Thermal Bridge & Airtightness Detailing Workshops – live sketch‑throughs with architects and contractors to eliminate psi‑value guesswork.
Heat‑Pump & Ventilation Sizing – dynamic hourly modelling for right‑sized emitters and MVHR, avoiding overspecification.
Regulatory Documentation – full SAP outputs, overheating risk assessments, Building Control liaison and Energy Statement drafting.
Net‑Zero Road‑mapping – embodied‑carbon optioneering and whole‑life‑carbon reporting aligned to RIBA 2030 targets.
Engaging us at RIBA Stage 1 typically trims at least one design iteration cycle, saving two to three weeks on the programme and de‑risking compliance surprises once construction begins.
Part L 2025 is more than just U‑values on steroids; it is a systems‑level push toward zero‑carbon‑ready homes that changes how architects compose form, specify fabric and integrate building services. Meeting the 75‑‑80 % emissions cut is perfectly achievable—but only when energy performance is a design driver from day one. Use the checklist above, lean on early SAP 11/HEM modelling, and coordinate airtightness, thermal bridges and heat‑pump logistics before drawings reach tender. When you need expert backup, Green SAP Compliance Services can plug in seamlessly, delivering the in‑depth modelling and compliance documentation that keep projects on time, on budget and on the right side of the Future Homes Standard. With smart preparation now, your 2026 sites will glide through Building Control and hand over to delighted, low‑bill occupants—proof that great architecture and rigorous energy performance go hand in hand.