This study offers an updated overview of existing approaches used to model geometric deviations in additive manufacturing. It also proposes a new framework for representing part deviations by discretizing an ideal planar surface and accounting for both deterministic and stochastic sources of variation. Deterministic deviations are described through two main components: surface waviness and overall orientation. In contrast, stochastic deviations are introduced through automatically generated variations following a normal probability distribution. The second section of this work presents a numerical investigation of a prismatic component featuring a functional planar surface produced using the Fused Deposition Modeling (FDM) technique. As an initial step, a reference specimen is fabricated to verify essential parameters associated with the mathematical models used for the FDM process. The geometric deviation model relies on converting the nominal planar surface into a mesh of nodes, after which a deformed surface—representing the actual manufactured geometry is generated using the deviations computed by the proposed approach. Finally, a Monte Carlo analysis is performed to examine how these geometric variations influence the evaluation of surface parallelism tolerance. To support interpretation of the results, a correlation is established between the simulated deviations, the specified tolerance limits, and the resulting non conformity rate calculated for each tolerance range.