In my view, an offer preparation is the listing of the service and the handing over of the standard construction service description; this does not necessarily include the creation of a floor plan and certainly not the request to customize the construction service description. You simply demanded too much and generated effort, and he let it happen, which he now wants to be compensated for without a business agreement. Again, if it had been an architect, you would have had to pay for it without question even without a written agreement, solely based on the verbal request for the floor plan creation, i.e., with the wording "can you create a draft for us" (even if you cannot use it).