Obtaining a bright and low emittance electron beam directly from the photocathode is mandatory in order to design high performance free electron lasers (FELs). To achieve this goal a clear understanding of how the emission process is influenced by structure, morphology and composition of the photocathode surface is needed. This is difficult from an experimental point of view because often the atomic scale details of the surface whose emission has been measured are unknown. A predictive theoretical approach capable of determining the effects of surface structure on emission is therefore or great interest. A model to extend the well known three step model (as proposed by Berglund and Spicer) to surface calculations is discussed in this paper. It is based on a layer-by-layer decomposition of the surface electronic structure that can be calculated through reliable and efficient DFT calculations. The advantage of this approach with respect to other existing photoemission calculations is being able to correlate directly the photoemission to the electronic, atomic and chemical structure of the surface. The proposed approach retains, therefore, the simple chemical intuition in the study of surface modifications and their effect on the photoemission. The approach is validated in calculations of the emission from clean copper and silver surfaces. The ability of the model to simulate the change in photoemission in response to adsorbates is tested by simulating monolayers of oxygen, hydrogen and lithium on the copper (1 1 1) surface.