The Large Hadron Collider (LHC) at CERN will deliver an integrated luminosity of up to $4000~{\rm fb}^{-1}$ in the high-luminosity phase (HL) thanks to an increased instantaneous luminosity that will reach up to $7.5 \times 10^{34}~{\rm cm}^{-2}{\rm s}^{-1}$. The ATLAS detector will be upgraded during Long Shutdown periods. This upgrade will allow the detector to cope with the high luminosity that will be accompanied by a high pileup. A powerful new way to mitigate the effects of pileup is to use high-precision timing information to distinguish between collisions occurring close in space but well-separated in time. A High-Granularity Timing Detector, based on low gain avalanche detector technology, is therefore proposed for the ATLAS Phase-II upgrade. This new detector will improve the detector physics performance in the forward region, in a pseudorapidity range of 2.4 to 4.0. Its design was optimised to associate a time to each track with a target average time resolution of $30~{\rm ps}$ for a minimum-ionising particle at the start of the detector lifetime, increasing to $50~{\rm ps}$ at the end of HL-LHC operation.