The Higgs boson, discovered in 2012 by the ATLAS and CMS collaborations, is perhaps the most crucial piece in the puzzle of the Standard Model of particle physics. This theory, despite being extremely precise, cannot explain some of the biggest questions about our Universe. Several beyond Standard Model theories suggest the existence of additional Higgs bosons, which could for example explain the matter-antimatter asymmetry in the Universe or help provide a candidate for dark matter. In this seminar I will explain how we search for such new particles in the Large Hadron Collider. In particular, I will present the latest results from the ATLAS Collaboration on searches for an extended family of the Higgs boson in one of the most sensitive final states (with two photons and two b-jets).