  In this document, we present the Technical Design Report of the Upgrade of
the T2K Near Detector ND280. The goal of this upgrade is to improve the Near
Detector performance to measure the neutrino interaction rate and to constrain
the neutrino interaction cross-sections so that the uncertainty in the number
of predicted events at Super-Kamiokande is reduced to about 4%. This will allow
to improve the physics reach of the T2K-II project. This goal is achieved by
modifying the upstream part of the detector, adding a new highly granular
scintillator detector (Super-FGD), two new TPCs (High-Angle TPC) and six TOF
planes. Details about the detector concepts, design and construction methods
are presented, as well as a first look at the test-beam data taken in Summer
2018. An update of the physics studies is also presented.
