We present the extraction of a 95\% C.L. limit on the production cross-section of an additional narrow Higgs-like resonance decaying into two photons, in the 150-XXX \GeV\ invariant mass range. By restricting the results to a fiducial cross-section measurement, we provide model-independent limits that can be used to constrain theoretical models predicting additional Higgs-like particles with a narrow width. The analysis uses ~3.5~\ifb\ of proton-proton collision data recorded with the ATLAS detector in 2015.