Office Building FBS Compliance

New office buildings in England must comply with the Future Buildings Standard from March 2027, delivering significantly lower carbon emissions than the 2021 Part L baseline. Offices are classified as side-lit buildings under the FBS impact assessment, but how they perform – and what compliance costs – depends heavily on whether the design is deep-plan or shallow-plan (FBS Impact Assessment, Section 3).

Deep-plan vs shallow-plan offices

The FBS impact assessment models two distinct office archetypes, reflecting the very different energy profiles of each:

• Deep-plan, air-conditioned offices – typically city-centre buildings with large floor plates, sealed facades, and mechanical cooling. The impact assessment estimates an additional £40/m² GIFA, a 1.2% cost uplift relative to Part L 2021 (FBS Impact Assessment, Table 16, p64).
• Shallow-plan, naturally ventilated offices – buildings with narrower floor plates that rely on openable windows and cross-ventilation. These face a higher proportional cost uplift of £65/m² GIFA ( 2.6%) because the baseline build cost is lower, so the additional measures represent a larger percentage uplift (FBS Impact Assessment, Table 16, p64).

Solar PV on office buildings

The FBS policy requires PV equivalent to 40% of the building foundation area for all non-domestic buildings, regardless of lighting type (consultation response, para 3.11, p18). However, the current NCM 2021 Equation 9 still calculates a weighted figure of 20% for side-lit buildings like offices - this will be updated when the 2026 NCM is published. If the office uses 100% heat pump heating, the notional building has no PV requirement at all (NCM 2021, para 83). In practice, flat-roofed offices are well suited to PV deployment – the roof is typically unobstructed and can accommodate east-west or south-facing arrays with optimal tilt angles.

Key considerations for office PV installations:

• Roof plant competition – air handling units, chillers, lift motor rooms, and communications equipment all compete for roof space. Design teams should coordinate rooftop layouts early to ensure adequate PV area remains after accommodating essential plant.
• Shading analysis – neighbouring tall buildings in urban settings can significantly reduce PV yield. The SBEM model accounts for overshadowing, so realistic shading data is critical for accurate compliance modelling.
• Multi-storey considerations – in tall office buildings, the ratio of roof area to total floor area decreases. Meeting the 40% foundation area requirement may require higher-efficiency panels or consideration of facade-integrated PV on south-facing elevations.
• Higher-risk building exemption – office buildings at least 18 metres tall with 2 or more residential units (i.e., mixed-use) may qualify as HRBs and be exempt from the PV requirement, though they must still meet the TER and TPER through other means.

Heating strategy

The FBS effectively mandates low-carbon heating for all new buildings, with CO₂ emission factor and primary energy factor caps that rule out fossil fuels (AD L2 2026, para 4.4). For offices, this means:

• Air source heat pumps (ASHPs) – the most common solution for side-lit buildings. The notional building assumes heat pump heating for side-lit spaces (FBS Impact Assessment, Section 6). Rooftop ASHPs integrate well with flat-roofed office designs, though noise from outdoor units needs careful acoustic planning, particularly in mixed-use urban contexts.
• Variable refrigerant flow (VRF) systems – popular in multi-zone office buildings as they provide simultaneous heating and cooling with heat recovery between zones. VRF systems can achieve high seasonal efficiencies, and AD L2 2026 sets a minimum SEER of 5.5 for VRF systems (AD L2 2026, Table 5.7).
• District heat networks – permitted as an alternative where 4th-generation networks are available (FBS Impact Assessment, Section 6). Connection to an existing or planned district heat network can simplify rooftop plant requirements.

New wet heating systems must be sized for a maximum flow temperature of 55°C, which favours underfloor heating or oversized radiators in offices where radiant ceiling panels or fan coil units are not used (AD L2 2026, para 4.12).

Air permeability challenges

Air permeability is a critical compliance factor for offices. All non-domestic buildings must be pressure tested (AD L2 2026, Section 6), and the assessed air permeability feeds directly into the BER and BPER calculations.

Office buildings face particular air tightness challenges:

• Curtain walling junctions – the interface between curtain walling and floor slabs is a common leakage path. Detailed specification of perimeter sealing is essential.
• Service risers and penetrations – multi-storey offices have numerous vertical risers for mechanical, electrical, and plumbing services. Each penetration through the air barrier must be individually sealed.
• Entrance lobbies – high-usage entrance doors have a limiting U-value of 3.5 W/(m².K) (AD L2 2026, Table 3.1), but the air leakage through revolving or automatic sliding doors also needs consideration in the energy model.
• Loading bays and car park interfaces – where offices include basement car parking, the thermal and air barrier at the transition between heated and unheated zones needs careful detailing.

Lighting and controls

Lighting is typically the largest single energy end-use in office buildings, making it a key compliance lever. AD L2 2026 requires general lighting to achieve an average luminaire efficacy greater than 105 luminaire lumens per circuit-watt (AD L2 2026, para 5.65).

Mandatory lighting controls for offices include:

• Automatic presence detection – to switch off lighting in unoccupied spaces (AD L2 2026, para 5.68)
• Daylight dimming – photo-switching and dimming controls for general lighting in spaces with natural light, particularly beneficial in shallow-plan offices with good daylighting (AD L2 2026, para 5.68)
• Display lighting – must be on dedicated circuits, switched separately from general illuminance (AD L2 2026, para 5.68)
• Metering – internal general and display lighting must be separately metered (AD L2 2026, para 5.68)

For deep-plan offices relying heavily on artificial lighting, investing in luminaires well above the 105 lm/W minimum can significantly improve the BPER calculation and ease overall compliance.

Building automation

Office buildings with heating or air-conditioning systems rated above 180 kW effective output must install a building automation and control system (BACS) compliant with BS EN ISO 16484 (AD L2 2026, para 5.76). This applies to the majority of new office buildings over approximately 2,000 m². The BACS must continuously monitor, log and analyse energy use, and be rated to at least Class A under BS EN ISO 52120-1 (AD L2 2026, para 5.84).

Cost impact

The FBS impact assessment provides the most authoritative figures on cost uplift for offices. The preferred option costs relative to a building constructed to Part L 2021 are (FBS Impact Assessment, Table 16, p64):

• Deep-plan, air-conditioned office: £40/m² GIFA, equivalent to a 1.2% cost uplift
• Shallow-plan, naturally ventilated office: £65/m² GIFA, equivalent to a 2.6% cost uplift

These figures are in 2025 prices and cover capital costs relative to Part L 2021. The primary cost drivers are the heat pump installation, enhanced fabric performance (particularly for naturally ventilated buildings), and the solar PV system. The impact assessment notes that buildings built to the FBS will be zero carbon ready, avoiding future retrofit costs that are estimated to be 66 - 80% more expensive than meeting higher standards at the point of construction (FBS Impact Assessment, Section 2).

Practical tips for office projects

• Engage the energy assessor at concept stage – early SBEM modelling can identify whether the proposed form and fabric will meet the TER and TPER before the design is locked in
• Coordinate roof layout early – map out PV zones, plant locations, and maintenance access routes on the roof plan during RIBA Stage 2 to avoid conflicts later
• Consider the air barrier strategy from day one – define the continuous air barrier line on drawings and ensure every junction and penetration has a specified sealing detail
• Optimise glazing ratios – excessive glazing increases cooling loads and reduces the benefit of daylight dimming in deep-plan zones. Balancing daylighting with solar control is critical, particularly for east and west facades
• Use the Compliance Route Finder to confirm which regulations apply to your project and whether transitional provisions are available
• Budget for commissioning – AD L2 2026 requires a comprehensive commissioning plan (AD L2 2026, Section 7) and the design-stage BRUKL report must be submitted before work begins