FBS vs Part L 2021 – What Changed?

The Future Buildings Standard (FBS) replaces Part L 2021 as the energy performance standard for new non-domestic buildings in England. Published on 24 March 2026 as Approved Document L Volume 2 (2026 edition), the FBS raises the compliance bar across solar PV, heating systems, lighting efficacy, and compliance metrics, while retaining most of the 2021 fabric backstop values (AD L2 2026; Consultation Response 2023).

Side-by-side comparison

Fabric standards – continuity with context

The FBS retains the same limiting (backstop) U-values as Part L 2021 for walls, roofs, floors, and windows. The government's consultation response explained that non-domestic fabric standards were "mostly the same fabric requirement as currently required under the 2021 standards", with the principal exception being enhanced airtightness for warehouses and sports halls (Consultation Response, Section 5).

However, the effective fabric requirement is tighter because the notional building specification has changed. The notional building now includes low-carbon heating and solar PV, which lowers the TER and TPER. To compensate, designers will often need to specify better fabric performance than the backstop values, even though those backstops have not formally changed.

Solar PV – the biggest new requirement

Part L 2021 had no mandatory solar PV requirement for non-domestic buildings. The FBS effectively mandates on-site renewable generation for non-domestic buildings through the notional building specification, which includes solar PV equivalent to 40% of foundation area for both side-lit and top-lit buildings (consultation response, para 3.11, p18). The current NCM 2021 Equation 9 still uses a weighted formula (20% side-lit / 40% top-lit) - this will be updated when the 2026 NCM is published. If 100% of heating is from heat pumps, no PV is required (NCM 2021, para 83).

The government originally consulted on 75% coverage for top-lit buildings (warehouses, sports halls), but reduced this in the final standard following industry feedback on cost and practicality. Higher-risk buildings exceeding 18 metres in height are exempt entirely (Consultation Response, Section 3).

Heating – the shift to low carbon

Under Part L 2021, the notional building assumed gas boiler heating, making it straightforward to comply with a gas-heated design. The FBS fundamentally changes this: the notional building now specifies heat pumps for side-lit spaces and radiant electric heating for top-lit spaces (Consultation Response, Section 4).

Additionally, the FBS introduces an explicit fuel restriction: new heating and hot water systems must use fuels with a CO₂ emission factor no greater than 0.086 kgCO₂/kWh and a primary energy factor no greater than 1.969 kWhPE/kWh. This effectively prohibits fossil fuel heating in new non-domestic buildings. Heat networks providing "sufficient clean heat" remain an acceptable alternative (AD L2 2026, para 4.4).

Lighting - higher efficacy standards

General internal lighting efficacy rises from 95 lm/W under Part L 2021 (NCM 2021, para 71) to 105 lm/W under the FBS. Display lighting efficacy rises from 80 lm/W to 95 lm/W. The government originally consulted on 150 lm/W for general lighting (consultation response, para 12.55, p114), but adopted 105 lm/W following industry feedback that the higher target was not cost-effective (AD L2 2026, para 5.65; Consultation Response, Section 7).

Compliance metrics – same framework, harder targets

The FBS retains the dual-metric compliance framework from Part L 2021: BER must not exceed the TER, and BPER must not exceed the TPER. Respondents to the consultation supported retaining these familiar metrics over alternative approaches such as delivered energy (Consultation Response, Section 2).

The critical difference is that the targets are now significantly harder to achieve because the notional building specification includes low-carbon heating and solar PV. A building that would have comfortably passed under Part L 2021 may fail under the FBS without design changes.

Building services and controls

The FBS introduces new requirements for building automation and control systems (BACS): buildings with space heating or air-conditioning systems exceeding 180 kW effective rated output must install a BACS complying with BS EN ISO 52120-1 Class A (AD L2 2026, para 5.76). New wet heating systems must be sized for a maximum flow temperature of 55°C (AD L2 2026, para 4.12).

Transition from Part L 2021

Projects with a building control application submitted before the enforcement date (March 2027 for standard buildings, September 2027 for HRBs) may continue under Part L 2021, provided work commences within 12 months. Transitional protection applies on an individual building basis, not site-wide. See the transition arrangements page for full details (Building Circular 01/2026).

Cost impact

The government's impact assessment estimates additional build costs ranging from 0.6% for hospitals to 2.6% for shallow-plan naturally-ventilated offices and hotels above Part L 2021 levels. In absolute terms, cost uplifts range from £20/m² for retail warehouses to £79/m² for hotels (FBS Impact Assessment, Table 16, p64). The net present social value is estimated at £874 million over 70 years, with total carbon savings of 11.8 MtCO₂e (FBS Impact Assessment, Table 16, p64).