The UK’s Future Homes Standard (FHS) will reshape the way houses are designed and constructed. The objective is simple on paper but demanding in practice: new homes built from 2025 must emit 75–80 % less CO₂ than homes built to the 2013 regulations. To achieve that reduction, fossil‑fuel boilers will give way to electric‑led systems, buildings will need much tighter fabric performance and renewable technologies will become the norm. This represents a significant change from previous standards, especially with the transition from the Standard Assessment Procedure (SAP) to the Home Energy Model (HEM) for assessing home energy performance in line with upcoming regulations. Planning for these changes requires more than a last‑minute swap from gas to heat pumps; it demands early design thinking, better data and careful balancing of cost versus carbon.
Below is a guide to what’s changing under Part L 2025, including how the Standard Assessment Procedure (SAP) is evolving into the Future Homes Standard (FHS) and the Home Energy Model (HEM) and what developers, architects, and energy consultants should be doing now. Where relevant, links to Darren Evans’ services and resources have been provided to help you navigate the transition. These changes are significant, and the housebuilding sector will need time to adapt.
The Standard Assessment Procedure (SAP) has been the UK’s official method for assessing a home’s energy use and carbon emissions for decades. It underpins compliance with energy regulations and supports certification processes.
While SAP 10.2 is currently in use in both England and Wales, the Future Homes Standard consultation outlines a transition to SAP 10.3 as an interim update ahead of a more significant shift to the new Home Energy Model (HEM).
This evolution reflects the need for more accurate, responsive modelling in line with net zero targets. HEM will introduce half-hourly energy profiles, improved appliance efficiency data, and more detailed inputs for systems such as pipework and ducting offering a much closer representation of how homes perform in real life.
We predict the response to the Part L consultation will be published in summer 2025, legislation will be laid before Parliament in December 2025, and both SAP 10.3 and the Home Energy Model will become available to support compliance during 2026. These tools will be used for energy assessments, demonstrating compliance with the Future Homes Standard, and achieving energy performance goals for new homes. There will then be a 12‑month transitional period from December 2026 to December 2027 before full enforcement begins in January 2028. Projects that register plots before December 2026 and start on site by December 2027 can follow Part L 2021, but any plots registered from January 2027 must comply with the FHS.
The Home Energy Model will play a key role in assessing energy performance and supporting compliance with building regulations. It will be used during design and planning stages to inform energy assessments and help demonstrate alignment with the Future Homes Standard.
The Home Energy Model (HEM) underpins the Future Homes Standard (FHS) and will reshape how design teams demonstrate sustainability in construction, energy performance and compliance. Below is an overview of what’s changing and what it means for architects, developers and M&E leads.
Under the FHS consultation, compliance for new dwellings references two main notional options (non–heat-network):
Why it matters: Early selection of ASHPs, space planning for hot-water storage, and roof strategy for PV (especially low–mid-rise) become integral to concept design, not late-stage bolt-ons.
HEM uses new emissions factors that reflect a lower-carbon grid (e.g., electricity 0.086 kgCO₂e/kWh; mains gas ~0.214 kgCO₂e/kWh). This reduces the relative carbon “credit” of PV alone versus SAP10-era factors and strengthens the case for high-efficiency heat pumps and fabric.
Why it matters: To hit carbon targets cost-effectively, prioritise fabric efficiency + heat pump performance; treat PV as part of a balanced strategy rather than the primary decarbonisation lever.
WWHR features in Option 1 (typical efficiency ~50% in the notional), signalling the growing importance of domestic hot water loads in well-insulated homes.
Why it matters: For homes with high occupancy and shower frequency, WWHR can materially improve compliance margins and running costs when paired with a heat pump.
HEM replaces monthly profiles with half-hourly simulation, improving representation of heat pump COP variability, thermal storage, and time-of-use tariffs. This better captures flexibility and interactions between systems.
Why it matters: Designs that use load shifting, weather compensation, and cylinders/buffers will be rewarded more transparently supporting both compliance and operational cost outcomes.
Expect more specific inputs (e.g., bedrooms, wet rooms, system details) and assessments referencing a HEM Product Characteristics Database (PCDB) for consistent, real-world product performance.
Why it matters: Early manufacturer engagement and validated performance data become critical to avoid rework at gateway stages.
HEM moves compliance from a tick-box, monthly average to a dynamic, system-level view. Teams that integrate heat pumps, hot-water recovery, fabric performance and load flexibility from RIBA Stages 1–3 will find it easier to evidence sustainability in construction, achieve compliance margins, and deliver lower bills and emissions in use.
Fossil‑fuel heating will no longer meet compliance. The FHS consultation proposes that no fossil‑fuel or biofuel heating systems will be allowed in new homes. Heat pumps (air or ground source) become the default for space and hot‑water heating. Darren Evans’ net‑zero consultancy warns that as the grid decarbonises, electric heating and heat pumps will be twice as effective at cutting carbon as PV panels.
Under SAP 10.3 and the FHS, WWHR is one of the most cost‑effective measures to improve hot‑water efficiency. Research cited by the WWHR supplier Showersave shows that integrating WWHR with heat pumps can reduce hot‑water energy use by up to 55 % and cut electricity bills by £150–£200 per year. WWHR reduces the size of hot‑water cylinders, saving space and material costs. It also eases pressure on the electricity grid. In the FHS notional building, Option 1 includes WWHR alongside a heat pump and PV.
Solar PV will remain a key part of sustainability in construction, but its relative carbon benefit is falling as the grid decarbonises. The consultation suggests coverage of around 40% of foundation area for side-lit spaces and 75% for top-lit spaces in non-domestic buildings. For dwellings, PV is increasingly paired with heat pumps, and while not yet officially mandatory, many in the industry anticipate a “sunshine bill” approach where PV becomes unavoidable. Developers should integrate PV as part of a balanced strategy, not rely on it as the sole compliance measure.
The Future Homes Standard (FHS) is expected to build on Part L 2021, with tighter limits on air permeability and stronger emphasis on low U-values across floors, walls, roofs, windows and doors. While the consultation indicates that backstop values may be reduced further, the exact figures are not yet confirmed. It is also likely that reliance on generic psi values will be phased out, requiring designers to calculate bespoke thermal bridge values. For now, the direction of travel is clear: achieving sustainability in construction will demand better fabric performance and early coordination of details.
As the Future Homes Standard tightens air permeability targets, the focus shifts from design intent to site delivery. Achieving lower air test scores will require site teams to build with greater precision and consistency. This makes ventilation strategy a critical decision: while MEV and DMEV can be modelled with product-specific fan powers, and MVHR offers heat recovery benefits, none of these systems will perform as intended unless the underlying airtightness is achieved on site. Natural ventilation alone is unlikely to be viable in such airtight homes, reinforcing the importance of buildability and quality assurance throughout construction..
These dates may shift slightly depending on legislative processes, but the broad outline signals that anyone with projects starting in late 2026 or beyond should be designing to the FHS now.
Darren Evans has been guiding developers and designers through Part L and SAP changes since the 2000s. As a trusted partner in the industry, they play a key role in supporting clients to enhance energy efficiency and adapt to evolving standards. The transition to the Future Homes Standard and HEM will require more modelling and technical input than ever. Here’s how their services align with the challenges outlined above:
Darren Evans helps clients stay ahead of regulatory changes and industry standards, ensuring they remain competitive and compliant as the industry evolves.
The Future Homes Standard is not just a tweak to the building regulations; it is a wholesale shift toward zero‑carbon‑ready dwellings. Developers who start planning now—integrating heat pumps, waste‑water heat recovery, airtight fabric and smart ventilation—will be ready for the new regulations and will deliver homes that are comfortable, cost‑effective and future‑proof. Those who delay may find themselves scrambling to redesign projects when the transitional period expires.
Darren Evans stands ready to help. By combining technical rigour with real‑world pragmatism, their consultants can guide you through SAP calculations today and prepare you for the complex modelling demands of SAP 11 and the Home Energy Model tomorrow. Get in touch early, and together you can build homes that meet the 2025 standard and beyond.
