Tidal stream energy projects involve high amounts of capital, for which ex-ante economic assessments are crucial. Unfortunately, the existing models to estimate the capital costs of a tidal stream energy project are often oversimplified. Focused on single estimates on a per installed power basis, such models fail to capture the all-important sensitivity of the capital costs to project site-specific characteristics (water depth, distance to the shoreline, etc.) The objective of this work is twofold: (1) to develop a new formulation for estimating the capital costs of a tidal stream farm, and (2) to implement this formula in a Matlab-based tool coupled with a Navier-Stokes solver and thus deliver the spatial distribution of this cost. As a case study, the new tool is applied to the Bristol Channel – the single largest resource area for tidal energy in the UK. Here, the best sites to install tidal stream farms are identified, with a cost comparable to that of offshore wind energy. Simulation results show that the proposed method is more efficient than the existing approaches. The results of this work have relevant policy implications in that they serve as decision criteria for allocating tidal stream projects and for designating supporting mechanisms.