Environmental sustainability is a topic of increasing concern among scholars, city planners, engineers and policymakers. Electricity consumption constitutes about two-thirds of a city's carbon emissions. In high-density Hong Kong, the building sector accounts for 89% of total electricity consumption, with commercial buildings alone accounting for 67% of the total. This paper reports the results of an in-depth case study examining how Langham Place Hotel Mongkok Hong Kong (LPHKG) has successfully reduced its CO2 emissions by implementing a range of sustainability design strategies. LPHKG is a five-star hotel situated in Mongkok - one of the most densely populated districts in the world, with an estimated 130,000 people per square kilometre. In particular, the paper discusses how the hotel, in its efforts to achieve net zero-energy, has made wise use of technologies to maximise energy consumption efficiency in its existing lighting and air-conditioning systems and in the mechanical devices of the building itself. The overall implication of LPHKG's efforts is that, although the initial investment required for such technologies is usually large, the resulting long-term cost savings can be significant. (C) 2012 Elsevier Ltd. All rights reserved.