Bristol office lifts NABERS UK rating from 3.32 to 4.43 stars without replacing gas boilers

A 2018-built office in Bristol has raised its NABERS UK Energy for Offices Base Building rating from 3.32 stars to above 4.4 stars without major capital spend or replacing its gas-fired heating, according to a case study published by CIBSE on 26 October.

The Chartered Institution of Building Services Engineers (CIBSE), which administers NABERS UK, said the Aurora building reached a 4-star rating of 4.42 in July 2025 and sustained it at 4.43 after a second 12-month measurement and verification programme.

Royal London Asset Management commissioned the NABERS UK-focused energy tracking and optimisation programme after Aurora received its first rating of 3 stars in autumn 2024. CIBSE said the aim was to close the performance gap, cut emissions, improve building control and stretch the rating.

Owen Porter, licensed NABERS UK assessor at BIP.Verco, which audited the building, wrote: "Significant reductions in energy use do not have to come from major capital investment."

The building

Aurora is a 9,000 sq m office in Bristol that relies on gas-fired space heating and domestic hot water, CIBSE said. Willmott Dixon, which built it for developer Cubex at Finzels Reach on the Floating Harbour, describes it as 95,000 sq ft of Grade A office space.

Willmott Dixon said Aurora was the first BREEAM 2014 Outstanding rated office outside London and its highest-scoring BREEAM project at 91%, designed for a 37% cut in carbon emissions and 59% less water demand than a typical office. Under NABERS UK, which CIBSE said rates metered performance over 12 months against comparable assets, 3 stars represents a typical building.

The programme

BIP.Verco first audited Aurora as part of a wider net zero audit programme for Royal London Asset Management, CIBSE said. The team set a 4-star target with 4.5 stars as a stretch, and broke the target into system-level energy budgets tracked monthly, with issues flagged to the facilities team at regular meetings.

BIP.Verco worked with JLL's Sustainable Operations and Asset Experience teams to implement the measures. Plant run times were cut by aligning tenant schedules with occupancy, heating and cooling deadbands were widened, and a temperature hold-off was added to the boilers and chillers.

CIBSE said a minimum-demand threshold for heating and cooling proved a large saving. It stopped the central plant running at periods of minimal demand, or when a faulty fan coil unit wrongly called for heating or cooling and activated the whole central system.

An AI-enabled optimisation platform was trialled for about six months between April and October 2025 to support optimum start-stop times, demand-based control and fault detection. CIBSE said the system worked effectively, and that after its removal in October 2025 there was a short period of elevated consumption while the team restored BMS controls.

Tenant comfort

After tenant engagement during the cold period, the minimum heating setpoint was raised to 21°C and plant run times were adjusted to give a longer morning conditioning period. CIBSE said this increased gas consumption in January and February 2026 but the building kept its 4-star performance.

Space heating is the building's largest energy consumer, CIBSE said, and the winter engagement showed it had been fully optimised within the limits of tenant comfort.

Rating point	NABERS UK stars
Baseline, October 2024	3.32
Re-rated, July 2025	4.42
After further 12-month M&V	4.43

CIBSE described the 1.1-star gain as a proof of concept achieved through optimisation alone.

Background

NABERS UK is based on Australia's NABERS programme and launched in 2020 through a partnership between UK industry organisations and NABERS Australia. CIBSE was announced as administrator on 2 April 2024, taking over from BRE. Ruth Carter, Chief Executive of CIBSE, said at the time: "We are delighted that CIBSE is now overseeing the administration of NABERS UK."

CIBSE Certification said NABERS UK offers two products for offices, Design for Performance and Energy for Offices, and that Energy for Offices ratings are "Deemed to Satisfy" the operational energy requirements of the UK Net Zero Carbon Buildings Standard, giving an alternative route to conformity based on verified in-use performance.

BIP.Verco said combining NABERS UK with building optimisation programmes is proving a cost-effective route to higher ratings. Aurora's second rating of 4.43 followed a further 12-month measurement and verification period after the July 2025 result.