We use the formalism of Generalised Geometry to characterise in general the supersymmetric backgrounds in type II supergravity that have a null Killing vector. We then specify this analysis to configurations that preserve the same supersymmetries as the D1-D5-P system compactified on a four-manifold. We give a set of equations on the forms defining the supergravity background that are equivalent to the supersymmetry constraints and the equations of motion. This study is motivated by the search of new microstate geometries for the D1-D5-P black hole. As an example, we rewrite the linearised three-charge solution of arXiv:hep-th/0311092 in our formalism and show how to extend it to a non-linear, regular and asymptotically flat configuration.The dictionary is the following: H=H, FhereIIA=-FthereIIA, FhereIIB=FthereIIB, K=-32K, χ=-32K, ΨhereIIB=32ΦthereIIB, ΨhereIIA=-32ΦthereIIA.