We report a combined polarized and unpolarized neutron diffraction study on a multiferroic Sr2CoSi2O7 (SCSO) single crystal below and above the antiferromagnetic ordering temperature TN = 6.5 K. Unpolarized neutron diffraction measurements at 15 K confirm the melilite-type tetragonal P4 over bar 21m space group as the parent structure of SCSO. The low temperature study at 2.3 K, in contrast, reveals symmetry lowering with the orthorhombic Cmm'2' and P212'12' magnetic space groups being equally possible. In these Cmm'2' and P212'12' magnetic space groups we obtain a very similar ordered magnetic moment about 2.86 and 2.94 mu B/Co2+, respectively, which lies in the ab plane. Our spin polarized flipping ratio measurements under an applied magnetic field of 6 T in the paramagnetic state support the results of our bulk magnetization data, indicating strong easy-plane spin anisotropy, responsible for the in-plane order below TN.