Now the question: an architect does not plan these things for nothing. First of all: Is the topic still current or has it already been decided? If still open - with prefabricated house manufacturers, usually only the input planning service phase 1-4 is carried out by a freelance architect, whereby the manufacturers often work together with "their" architectural offices. Workshop, detail planning and production planning are done in the factory itself, adapted to the construction method, wall structures, etc. of the manufacturer. In the case of a high-quality prefabricated house, the total costs in the end are certainly no lower than for a single-family house planned by an architect with workshop planning, tendering, individual awarding and construction management. The costs are just distributed differently. I would rather focus on the question of which architect can best realize the desired house, how the house should look, etc.