Completed in April 2012, Hanover Square marked the first UK venture for Mitsui Fudosan, Japan's leading property company, working with development managers Stanhope. The clients sought to provide a high quality exemplar sustainable building providing 91,000sq ft of office, residential and gallery space, and make maximum use of renewable energy.

Historically London rooftops have expressed the energy consumption of a building, whether it be a chimney, plant machinery or a wind turbine. Squire and Partners' early designs identified the roof of Hanover Square as an area capable of generating power for the building. The practice designed the entire top floor as a mansard clad in 77 matt black photovoltaic panels, one of the first projects in London to use photovoltaics as cladding.

Visible from the square, the mansard sits cleanly above the building displaying a sleek uniform matte black surface, avoiding the usual photovoltaic aesthetic of 'slapped on' panels. The highly efficient PV panels provide 54kWp which equates to an estimated carbon saving of 18 tonnes of carbon dioxide per year. This energy is divided up between the office building and the five residential apartments to the east of the side.

The design team conceived a well-insulated envelope and energy efficient building system, which combined with the 54kWp photovoltaic installation on the roof, achieves an 18% reduction in CO2 emissions over Part L2A 2006, and a building that is rated BREEAM offices Excellent and Code for Sustainable Homes Level 4. Implementation of these measures has resulted in a building which exceeds initial predictions for on-site energy generation.

Solar control is achieved on the masonry facades through the use of a high solid to-void ratio, recessed windows and overhangs. The double-glazed units incorporate high performance solar glass, resulting in an area-weighted average V-value of 1.8W / m'K and a solar G-value of 0.32.

A five storey living wall has been incorporated into the façade of the building which includes ferns, grasses and small shrubs, as well as bird and bat boxes to provide habitats for local species. In addition to the living wall, two areas of green roof are lined with rubble recycled from the demolished building and planted with a London wildflower seed mix. The green wall has an integral irrigation system where rainwater is collected from the roof and along with added nutrients keeps the plants suitably fed and watered.

Energy-saving measures were an important consideration for the structure. Post-tensioned slabs have reduced the depth of concrete required by over 25%. This has reduced the size of the foundations and quantity of steel reinforcement needed. In addition, more than 40% of the concrete's cement content was replaced by ground granulated blast-furnace slag - a by-product of the iron ore industry - and 25% of the aggregate content was replaced with stem - a by-product of the China clay industry, classed as a zero carbon. This resulted in a significant reduction in embodied energy. Additionally, lime mortar was chosen for the brickwork, which creates a weaker bond and allows masonry to be re-cycled at end of life, thus reducing the carbon footprint of the wall.

Condensing boilers deliver centralised heating/hot water, while cooling is provided by a compressor air-cooled chiller reducing the electrical energy demand by 40%. Specific fan power on the air-handling units has been reduced within the plant and riser confines. For individual fan coil units, high efficiencies have been achieved with electronic commutation motors and by using variable speed drives to control flow in accordance with demand. This reduces power consumption per unit flow rate by more than 50%. Within each apartment, a heat exchanger interface unit provides hydraulic separation between the commercial and residential systems.

Energy-efficient lighting is used throughout the scheme in conjunction with time switches and daylight and PIR sensors. An interactive green screen in the reception is linked directly to the BMS system, allowing building users to monitor where energy is being consumed. The screen is capable of breaking down the figures to compare floor levels energy consumption and display floor by floor league tables along with facts and comparisons with other similar buildings.