The spin transition in a square lattice nanoparticle, whose the primitive cell is constituted of 2 metallic centres, is investigated using a two-variable elastic Ising-like model solved by Monte Carlo simulations. We show that the specific lattice geometry can lead to an important surface relaxation of both the structure and the spin-state, which are analysed by mapping the local strain. The consequence of this surface relaxation is the occurrence of a stepwise spin transition where the surface and the core switch at different temperatures. (C) 2017 Elsevier B.V. All rights reserved.