Using scanning tunnelling microscopy we have investigated the formation of low dimensional Fe-Si structures on Au covered Si(111) surfaces. The ultrathin Au layer induces a variety of surface reconstructions, depending on the coverage and temperature: Si(111)-5 x 2, [alpha-squareroot 3 x squareroot 3,beta-squareroot 3 x squareroot 3], and 6 x 6-Au. The subsequent deposition of 0.28 ML (monolayers) of Fe at 400 degrees C results in the formation of Fe-Si nanostructures whose morphological properties critically depend on the underlying surface. All Au induced reconstructions give rise to an increase in diffusion length as compared to the bare Si(111)-7 x 7 surface, thereby allowing the growth of well-separated nanostructures at considerably lower temperatures. Ultimately, the decoupling of surface diffusion and temperature, induced by the Au layer, can be exploited to tailor the island dimensions and density. With an appropriate choice of substrate, passivating layer and deposited material, nanostructures with the desired properties can be grown in a controlled way.