The paper presents a combined optimisation of the indoor environment and the energy consumption using the Eco-factor. It enables environmental assessment of different energy sources and techniques in the design and planning of energy efficient buildings with low environmental impact and desired indoor comfort. A typical office building is selected for further investigation and several solutions are investigated and optimised. It is found that the combined optimisation of indoor environment and energy consumption may influence otherwise obvious solutions significantly. Furthermore, it is found that the Eco-factor is a very efficient tool to perform the combined assessment.