Take the total sum of all construction costs including all incidental construction costs that the construction company/prefabricated house manufacturer estimates or that is available to you as the maximum total amount after deducting the costs for land acquisition including incidental costs. Find an architect who is recommended to you or whose design style appeals to you and ask them what they could build for you with this sum, depending on your ideas about house size, building type, equipment, land, and building and planning regulations.
Prefabricated house manufacturers calculate with the strong desire of builders to save the so-called "incidental construction costs": fees for the architect, the structural engineer, official fees, etc. These costs also apply to the manufacturer; they merely charge them or actually have lower incidental construction costs because they sell a standardized product.
The architect designs a house individually tailored to you and realizes it including construction management, acceptance of construction services, billing, etc., as a trustee of your interests. The prefabricated house manufacturer/general contractor/self-contractor monitors themselves (the pot calling the kettle black) because you – like most builders – have no knowledge of construction planning and implementation. Unless you hire an independent expert/surveyor for a corresponding fee (incidental construction costs), who performs these monitoring and control services for you, which the architect naturally takes on in service phase 8 (object monitoring).
In a word: it is not more expensive to build with an architect, just different. You have to formulate your wishes more concretely and give the architect active feedback on their design, their details, and their material and equipment proposals. For this reason, the house later becomes "your house" and not the standardized cliché of a provider. Of course, there are also very high-quality prefabricated house providers with good architecture or those who have houses built according to architectural designs or have their own planning department. In that case, the costs are again comparable to those of an independent architect.