One of the main objectives of the JET experiment is to reach near ignition conditions using deuterium-tritium mixtures so that significant heating of the plasma by alpha particles is achieved. This objective is reflected in the JET development plan which aims at one or two years of active operation and a few thousand deuterium-tritium shots. This approach, where it is believed that a sustained active operation period is necessary for a meaningful study of alpha particle heating, has had considerable design and cost implications on the whole project. The paper first reviews the impact of the active phase on the general design concepts and detailed design of the machine. Buildings, auxilliary systems, power supplies and diagnostics have also been strongly influenced by requirements resulting from the activation of the machine or the use of tritium. The paper reviews also the development programme which is being pursued vigorously in the areas of remote handling and tritium recycling, in readiness for the active phase. An evaluation of the cost of all facilities implemented for the active phase in relation to the overall project cost is also presented.