Mixed-Use Building FBS Compliance

Mixed-use buildings – developments combining residential and non-domestic uses within a single structure – present the most complex compliance scenario under the Future Buildings Standard. Both AD L Volume 1 (dwellings, governed by the Future Homes Standard) and AD L Volume 2 (non-domestic, governed by the FBS) may apply simultaneously, with different requirements, calculation methodologies, and enforcement timelines for each portion of the building (AD L2 2026, Scope).

When both Volume 1 and Volume 2 apply

AD L2 2026 is explicit about the boundary between the two volumes. The key distinctions are:

• Dwellings – self-contained residential units (flats, apartments) are assessed under AD L Volume 1 using SAP (the Standard Assessment Procedure). Each dwelling type requires its own SAP calculation demonstrating compliance with the Future Homes Standard.
• Non-domestic portions – commercial spaces (offices, retail units, restaurants, gyms) are assessed under AD L Volume 2 using SBEM or an approved dynamic simulation model (AD L2 2026, Section on calculation methodology).
• Rooms for residential purposes – hotel rooms, student halls of residence, and similar accommodation that is not self-contained fall under Volume 2, not Volume 1 (AD L2 2026, Scope). This is a common source of confusion – a serviced apartment block may be assessed under either volume depending on whether units are self-contained.

Common heated areas

Mixed-use buildings typically include shared spaces that serve both residential and non-domestic portions. AD L2 2026 provides specific guidance on how these areas are treated:

• Heated common areas in multi-dwelling buildings – lobbies, corridors, stairwells, and communal lounges that are heated follow Volume 2 for their energy assessment (AD L2 2026, Scope). This means these spaces must be included in the SBEM model alongside the commercial areas.
• Unheated common areas – where common areas are not directly heated, they follow Volume 1 for fabric requirements but Volume 2 (Sections 4 and 5) for any fixed building services installed within them (AD L2 2026, Scope).
• Plant rooms and service areas – central plant rooms serving both residential and non-domestic portions must be assessed carefully. Shared heating plant may need to be apportioned between the Volume 1 and Volume 2 calculations.

Higher-risk building classification for mixed-use

Mixed-use buildings are particularly likely to qualify as higher-risk buildings (HRBs) under the Building Safety Act 2022. A building qualifies as an HRB if it is at least 18 metres tall or has at least 7 storeys, AND contains at least 2 residential units (Building Safety Act 2022).

Many urban mixed-use developments – with commercial space on lower floors and residential apartments above – meet both criteria. HRB classification triggers several important provisions:

• Later enforcement date – HRBs must comply from September 2027, six months after the March 2027 date for non-HRBs (Building Circular 01/2026). Both the residential (Volume 1) and non-domestic (Volume 2) portions of an HRB share the later timeline.
• PV exemption – HRBs are exempt from the solar PV requirement due to the practical difficulties of installation on tall buildings (Building Circular 01/2026, Section 4.4). However, the TER and TPER targets are still derived from a notional building that includes PV, so the non-domestic portions must achieve compliance through other means – superior fabric, highly efficient heat pumps, and excellent lighting performance.
• Building Safety Regulator – HRBs are regulated by the BSR rather than local authority building control. All Building Regulations applications, including Part L compliance, go through the BSR gateway process.

Assessment coordination

Mixed-use buildings require separate but coordinated energy assessments:

• Volume 1 assessment (dwellings) – each dwelling type requires a SAP calculation. This is typically carried out by a SAP assessor and produces a Predicted Energy Assessment (PEA) at design stage and an on-construction EPC at completion.
• Volume 2 assessment (non-domestic) – all non-domestic areas, including heated common areas, require an SBEM or DSM calculation. This produces design-stage and as-built BRUKL reports (AD L2 2026, Appendix C).
• Shared systems – where a single heating plant serves both residential and non-domestic portions, the assessors must coordinate to ensure consistent assumptions. The heat pump capacity, distribution losses, and control strategy must align between the SAP and SBEM models.
• PV allocation – if PV is installed (i.e., the building is not an HRB), the generation must be allocated appropriately between the Volume 1 and Volume 2 assessments. Double-counting PV output across both calculations is not permitted.

Heating strategy

Mixed-use buildings frequently use centralised heating plant serving multiple uses. Under the FBS:

• Centralised ASHP or GSHP – a common approach, with a central plant room distributing low-temperature hot water to both residential and non-domestic zones. Flow temperature must not exceed 55°C (AD L2 2026, para 4.12). Heat interface units (HIUs) in individual dwellings and zone controls in commercial spaces provide independent temperature regulation.
• District heat networks – mixed-use developments on larger masterplans may connect to 4th-generation heat networks. The non-domestic portions can credit the low-carbon heat source in the SBEM calculation (FBS Impact Assessment, Section 6).
• Zone control requirements – the non-domestic portions must be subdivided into separate control zones for areas with different solar exposure, pattern of use, or type of use (AD L2 2026, para 4.13). In mixed-use buildings, this typically means separate zones for retail, office, restaurant, gym, and common areas.

Fabric considerations

Mixed-use buildings must meet the limiting U-values in AD L2 2026, Table 3.1 for non-domestic portions (roof 0.18, wall 0.26, floor 0.18 W/(m².K)) and the equivalent Volume 1 standards for residential portions. Key fabric considerations include:

• Thermal boundary between uses – where non-domestic spaces adjoin residential spaces (e.g., a commercial unit below a flat), the separating element must be assessed. If both spaces are heated, the thermal performance of the separating element is less critical, but acoustic separation requirements may dictate construction that also improves thermal performance.
• Air permeability – the entire building must be pressure tested (AD L2 2026, Section 6). In mixed-use buildings, achieving good airtightness at the interfaces between different construction types (e.g., concrete residential frame meeting steel-framed commercial podium) requires careful detailing.
• Curtain walling and glazing – non-domestic windows and curtain walling have a limiting U-value of 1.6 W/(m².K) (AD L2 2026, Table 3.1). Where the same facade system spans both residential and commercial floors, it must meet the requirements of both volumes.

Practical tips for mixed-use projects

• Appoint both assessors early – engage a SAP assessor (Volume 1) and an SBEM assessor (Volume 2) at concept stage to coordinate the energy strategy across both portions of the building
• Check HRB status immediately – determine whether the building meets the 18 m / 7 storey and 2+ residential unit thresholds. If it is an HRB, plan for the September 2027 timeline and the PV exemption challenge from day one.
• Define the Volume 1/Volume 2 boundary – produce a drawing showing exactly which spaces fall under each volume, including common areas and plant rooms. This boundary drawing should be agreed with building control before detailed design.
• Coordinate shared plant assumptions – ensure the SAP and SBEM assessors use consistent data for shared heating systems, including heat pump COP, distribution losses, and control strategy
• Allocate PV generation carefully – for non-HRB mixed-use buildings, agree the PV output split between Volume 1 and Volume 2 calculations before commencing detailed modelling. Document the allocation methodology.
• Plan for multiple BRUKL submissions – the non-domestic portions require both design-stage and as-built BRUKL reports, while the residential portions require SAP calculations and EPCs. Allow time in the programme for sequential submissions.