A new Magnetic Diagnostics system has been designed for the 2005 JET experimental campaigns onward. The new system, which adds to the existing sensors, aims to improve the JET safety,reliability and performance, with respect to: i) equilibrium reconstruction; ii) plasma shape control; iii) coil failures; iv) VDEs; v) iron modelling; vi) MHD poloidal mode analysis. The system consists of in-vessel and ex-vessel sensors. The former are a set of 38 coil pairs (normal and tangential), located as near as possible to the plasma. Coils are generally grouped in rails, in order to ease Remote Handling invessel installation. The system includes: i) 2 outer poloidal limiter arrays (2 x 7 coil pairs); ii) 2 Divertor region arrays (2 x 7 coil pairs); iii) 2 Top coil arrays (2 x 5 coil pairs). Ex-vessel sensors, including discrete coils, Hall probes and flux loops (26 in total) will be installed on the iron limbs, in order to provide experimental data for the treatment of iron in equilibrium codes. The design is accompanied by a software analysis, aiming to predict the expected improvement.