Flavour-changing neutral-current (FCNC) processes like $b \to s\,\mu^+\mu^-$ and $B_{(s)}^0 \to \mu^+\mu^-$ are suppressed in the Standard Model (SM) and thus sensitive to New Physics (NP) effects. This contribution summarises ATLAS results from the angular analysis of the $B^0 \to K^* \mu^+\mu^-$ decay performed with LHC Run 1 data, the $\cal B(B_{(s)}^0 \to \mu^+\mu^-)$ measurement performed with 2015 and 2016 data and projections for the $B^0 \to K^* \mu^+\mu^-$ and $\cal B(B_{(s)}^0 \to \mu^+\mu^-)$ measurements for the High-Luminosity LHC (HL-LHC). The projections are based on extrapolations of the Run 1 results with the expected statistics corresponding to assumed 3 ab$^{-1}$ of delivered luminosity for the HL-LHC and on simulations of the expected layout of a new all-silicon Inner Tracker (ITk).