  Recent 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
3$\,\sigma$ or higher significance for the case of maximum CP violation. The
program also contains a measurement of mixing parameters, $\theta_{23}$ and
$\Delta m^2_{32}$, with a precision of 1.7$^\circ$ 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.
