Unfortunately, this is offered only as a package, and I can understand that the developer wants to make a profit... Now, is this arrangement common?: [Grundstückvertrag+Architekt Vertrag+Gewerksvertrag].
Either A) You are the owner of a plot of land and have a house built on it. Then you are the builder and have an architect or a general contractor or main contractor. Or B) You are not the owner of a plot of land but buy land and house from a single source. That is a developer contract.
What your "developer" proposes is called by notaries "stay away from hidden builder models!". You can google that and then form your own opinion.