Reliably reducing the emissions in the building sector plays a crucial role if the 1.5°C climate target from the Paris Agreement is to be met. The observed trends show a significant increase in building energy use, especially in emerging economies. Counteracting these trends is absolutely essential, especially in the light of urbanisation, population growth and changing lifestyles. In terms of mitigating the climate impact of buildings, ensuring high levels of efficiency (i.e. very low energy needs, especially for heating and cooling) has the greatest potential for saving energy and emissions, and is at the same time the prerequisite for effective use of energy from renewable sources. Clearly defined targets and suitable metrics are essential to enable appropriate design decisions. Implemented projects clearly indicate that quality assured design and construction lead to reliable in-use energy performance. Effective policy packages to address opportunities and challenges are important drivers to support the uptake of state-of-the-art efficiency measures in the urban building sector.