MASS -- modified assignment algorithm in facilities layout planning. We have proposed a semi-heuristic optimization algorithm for designing opimal plant layouts in process-focused manufacturing/service facilities. Our proposed algorithm marries the well-known CRAFT (Computerized Relative Allocate of Facilities Technique) with the Hungarian assignment algorithm. Being a semi-heuristic search our algorithm is likely to be more efficient in terms of computer CPU engagement time as it tends to converge on the global optimum faster than the traditional CRAFT algorithm – a pure heuristic. We also present a numerical illustration of our algorithm.