Sustainable development is the goal of a balance between economy and the environment, whilst establishing a better spread prosperity across the world. In order to make this possible, the environmental load of our commodities needs to be reduced by a factor of 20. This factor 20 can also be translated to the office market. The PhD research presented in this thesis focussed on finding solutions effectively contributing to factor 20 environmental improvement of office accommodation. In order to determine the current environmental performance and to find the main aspects and building components causing environmental damage, twelve offices designed without a special focus on sustainability were environmentally assessed. This case study reveals that recently constructed offices are hardly more sustainable than in the reference year 1990 and that, on the basis of a lifespan of 75 years, almost 80% of the environmental load of office buildings is related to energy consumption. Heating (and cooling), lighting and use of equipment (mainly computers) together constitute approximately 90% of energy consumption. The supporting structure of a building causes almost 60% of the environmental load of building materials. Consumption of water proves unimportant to the environmental performance. The age and expected service life of a building had not yet been accounted for in environmental performance. This thesis presents a methodology for the account of these factors of time, facilitating decisions about, for example, renovation and re-use of an existing building versus demolition and construction of a new building. On the basis of this methodology it can be established that with an eventual service life of the building of around 20 years - a realistic value for modern offices - the use of building materials becomes equally important as energy consumption. Studies indicate that there are two main service life strategies that effectively improve the environmental performance of offices: design of long-lasting monumental buildings with an over-sized structure, or buildings with a demountable or short-cyclic supporting structure. The lifespan of components other than the supporting structure play no significant role in environmental performance. Theoretically, the use of sustainable energy resources can lead to more than a factor 20 environmental improvement of energy consumption. Many technologies and solutions are known and available. The supporting structure of a building can be improved by choosing optimal structural spans and combinations of building materials for the structural components. A maximum difference of a factor 4.5 can be found between favourable and unfavourable, yet common, solutions for supporting structures. Best solution found is a floor of TT slabs spanning the entire building depth, supported by timber beams and columns. For offices this is a flexible solution as well. Further enhancements will be possible through the use of new building materials. As a result of different effects, the basic shape of a building does not substantially influence its environmental performance. When comparing different building heights, however, an optimal number of stories can be found for each net floor area required for an office organisation. In the case of large buildings, the maximum difference between favourable and unfavourable solutions is around a factor of 1.6. The use of space inside offices defines the size and geometry of the building. Layout principles other than the cellular office enable substantial environmental improvement of the use of building materials. As case studies of European redevelopments around nodes of public transport reveal, on the urban scale, intensive and multiple use of space can lead to significant environmental improvement related to travel and the green area preserved outside the city. In regards to average mono-functional urban plans, stacking and mixing of functions offer the better opportunities for sustainability. Comparison of telework concepts with traditional concepts for office work demonstrate that limited improvement can be achieved in terms of employee travel and the use of space, building materials and energy. The best achievements are possible through complex forms of telework based on distributed working. In this case, office workers basically work where they are for business: with the client, in a business centre, at home, or anywhere in-between. In the near future, the best opportunities for sustainable office accommodation will be a new sustainable organisation of office work. This should be based on the concentration of general, shared office functions at nodes of public and private transport, together with home and district offices as basic workplaces for office employees, and virtual offices anywhere. In this concept, the traditional central office is transformed into a clubhouse office, where only the basic functions for management, administration and support of the office are accommodated, and where employees meet for their corporate identity and exchange of experience. This much smaller central office can be located in the heart of cities or at the already mentioned business nodes. The new organisation of office work leads to environmental improvement by a factor of 2, not yet taking into account potential additional improvements of the space, time and technology factor. Beside the environmental benefits, this new office work concept leads to revaluation and animation of urban areas, and offers more freedom with regard to the combination of business and private activities. A final assessment was done of the overall improvement potential through a combination of measures, starting with the organisation of office work, followed by solutions for an efficient use of space, optimal building designs and technologies, and finally based on sustainable strategies for the time factor. This assessment indicates that, on the basis of an optimal combination of measures, environmental improvement by a factor of 50 is possible. A less optimal combination still makes factor 10 possible. Therefore, the factor 20 seems feasible for office accommodation. Decisive to this end-result is an integral strategy, in particular involving the use of sustainable energy resources, a wiser approach to aspects of the lifespan, and a new organisation of office work. And people of course will have to do it.