DSS housing that can be made comparable to the displacement dwelling. The construction costs to add features (e.g., a garage, central heating and air conditioning, improvements needed to accommodate the handicapped) to make the dwelling functionally similar to the displacement dwelling will be added to the probable sales price of the comparable before calculating the supplement. In unusual circumstances, costs to add one or two rooms to an existing house may be used. Such additions must be carefully analyzed to assure that the work will produce a dwelling architecturally and functionally compatible with the area.