Heelis is the new Central Office for the National Trust in Swindon and an exemplar in sustainable office design. It is a deep-plan naturally ventilated building with exposed thermal mass both floor levels to maintain stable internal temperatures; air is introduced around the perimeter and via two internal courtyards.

Daylight is maximised via north-facing roof lights and voids in the first floor to ground level. There is careful control of direct sunlight; using the photovoltaic panels for shading in the middle of the day and the roof ventilation cowls in the morning and evening. Finishing materials such as timber and carpets were sourced from National Trust property. This project received a BREEAM 'Excellent' Rating under the BREEAM for Offices 2004 scheme.