The study of brown dwarfs and giant exoplanets is rapidly evolving as ever-improving instrumentation becomes sensitive to cooler objects. Accurate and reliable atmosphere and evolutionary models are important for placing mass and age constraints on these newly discovered objects, and understanding the rich chemistry and physics taking place in their atmospheres. We are expanding on the widely used COND evolutionary models by developing a grid of model atmospheres (T_eff=200-2000K, log(g)=2.5-5.5) with our state-of-the-art 1D radiative-convective equilibrium code ATMO. ATMO includes the latest opacities for important molecular absorbers such as H₂O, CH₄ and NH₃, and the latest line shapes for the collisionally broadened Na and K resonance lines. These model improvements allow us to follow the evolution of 1-75M_Jup objects down to the coolest effective temperatures (T_eff=200K). We present comparisons of this new model set to those previously published, illustrating how the evolutionary tracks and the substellar boundary have changed due to improved opacities and the usage of a new equation of state. We also compare our model grid to observational datasets in colour-magnitude diagrams and investigate the impact of our new Na and K line shapes in reproducing brown dwarf spectra. Our future work will involve expanding on this initial grid, to investigate the effects of metallicity, C/O ratio and non-equilibrium chemistry in cool brown dwarfs and giant exoplanets.