This paper critically reviews the current requirement for a 10% offset of carbon emission that should be obtained from on-site renewable and other carbon-saving energy sources installed in new office buildings in London built environment. Carbon saving effects of several currently available renewable-based generation technologies for use in contemporary office developments are calculated and compared in the paper. In order to allow direct and transparent comparison, the paper proposes a new index, expressed as the ratio of the total carbon dioxide offset with the total cost of technology, calculated over the lifetime period of use. The presented results include sensitivity analysis of the following parameters and factors of influence: Capital costs of technologies, availability of primary energy sources, system efficiencies, and system losses. The paper also provides a discussion of the possible changes in the current carbon offset legislation and future carbon mitigation policy implications.