We report on a project aimed at simulating the morphological and spectral evolution of young supernova remnants undergoing particle acceleration. If a sizable fraction of the energy is channeled into energetic particles, noticeable modifications of the shock structure are expected - and are indeed suggested by recent observations with Chandra and XMM. We are thus developing a time-dependent 3D code which enables us to study the shrinking of the region between the forward and the reverse shocks, concomitantly with the development of the Rayleigh-Taylor instability at the contact discontinuity. To model acceleration of particles by the shock, we use a semi-analytical non-linear kinetic model. As a first step to take into account the back-reaction of energetic particles, the adiabatic index of the fluid is varied accordingly at the shock front. The next step will be a multi-fluid approach, in order to derive both the thermal and non-thermal emission from the remnant.