We modelled particle growth by condensation around the water ice line, including the important difference between nucleation of ice on bare rock, and deposition of vapour on already icy particles. The result is a bimodal size distribution with fast-growing icy pebbles locally at the ice line and small, rocky dust particles spreading out over the disc. This is consistent with substructures forming at ice lines, and with the ice line being a preferential location for growth towards planets.