  The large-scale properties of homogeneous states after quantum quenches in
integrable systems have been successfully described by a semiclassical picture
of moving quasiparticles. Here we consider the generalisation for the
entanglement evolution after an inhomogeneous quench in noninteracting systems
in the framework of generalised hydrodynamics. We focus on the protocol where
two semi-infinite halves are initially prepared in different states and then
joined together, showing that a proper generalisation of the quasiparticle
picture leads to exact quantitative predictions. If the system is initially
prepared in a quasistationary state, we find that the entanglement entropy is
additive and it can be computed by means of generalised hydrodynamics.
Conversely, additivity is lost when the initial state is not quasistationary;
yet the entanglement entropy in the large-scale limit can be exactly predicted
in the quasiparticle picture, provided that the initial state is low entangled.
