This paper investigates how the existing stock of medium to large office buildings in a typical UK city performs in relation to current environmental sustainability benchmarks. The vast majority of office space in any UK city is not new; over two thirds of the stock in England & Wales was built before 1985. So, whereas current efforts to benchmark the environmental sustainability focus on the design and procurement of new buildings, this paper examines the management and occupation of existing buildings. The research focuses on speculatively built office buildings of 10,000 square feet or more that have been constructed in Bristol over the past 50 years. These buildings are located in the city centre and in out-of-town business parks. Taking the BREEAM Management & Occupation Assessment Method as a starting point, the environmental performance of a stock of over 400 buildings is examined. By classifying the stock in terms of type, age and location and analysing rents, lease terms, vacancy rates and other market information, the findings of the research reveal how potentially difficult it might be for existing offices to meet increasingly stringent mandatory and discretionary environmental performance standards without the injection of substantial refurbishment expenditure. The implication of this might be that, as the environmental performance of new buildings improves, the demand for and thus price paid by both landlords and tenants for increasingly underperforming existing buildings will drop. This raises a question: are poor rental and capital growth and early onset of obsolescence in existing office buildings unintended consequences of the current focus on environmental performance of new buildings?