68 pages, 31 figures68 pages, 31 figures68 pages, 31 figuresRecent measurements by the T2K neutrino oscillation experiment indicate that CP violation in neutrino mixing may be observed in the future by long-baseline neutrino oscillation experiments. We propose an extension to the currently approved T2K running from 7.8\times 10^{21}~\mbox{POT} to 20\times 10^{21}~\mbox{POT}, aiming at initial observation of CP violation with 3σ or higher significance for the case of maximum CP violation. The program also contains a measurement of mixing parameters, θ23 and Δm322, with a precision of 1.7∘ or better and 1%, respectively. With accelerator and beamline upgrades, as well as analysis improvements, this program would occur before the next generation of long-baseline neutrino oscillation experiments that are expected to start operation in 2026