An case Study - an 120 year old single-tenant building - an former railway head quarter had to be transformed under use – into a multi-tenant building to give another lease of live. Up to recently the consumption of energy could not be metered and billed to the tenants. It was decided to split up the building into letable areas of 400 sqm each. Infrastructure shafts, for water, sewage, heating and electricity had to be broken through on going rented spaces. New district heating ducts – with energy form a waste incineration plant - were implemented with better insulation. The plant rooms were totally new laid out with new variably adjusted heating pumps, to reduce the carbon footprint. A whole new efficient low temperature water heating system was built in. All energy consumption is being metered to save the utmost energy. Water saving fixtures and urinals are introduced to save water. Water retaining artificial trenches under a car park were introduced to retain more water from the partly new turfed roofs. New cables for electricity and new meters were introduced. To save further energy presence detectors for the energy saving lighting fixtures were introduced. Highly insulated windows were implemented, to save energy, yet to get thermal gain, through the window openings. The transformation of the building structure is accompanied with of the first German BREEAM in use and certification.