Prefab house construction plans as a template - does that cause problems?

Copyright can only be asserted by an architect if their design has a certain level of individual creativity. For this, the design would have to stand out from the mass of average houses. A plain single-family house with four walls and a pitched roof certainly does not count as such initially, but in individual cases, an expert opinion may be required in a dispute to clarify the question of copyright eligibility.
If a design is eligible for copyright, you are not allowed to make any modifications later without the architect's consent. Attention must then be paid to the contract design, otherwise the copyright remains with the architect.
This can even go so far that a photo of such a building may not be published. We once had a case in the photo forum where the owner of a mill invoked the architect's copyright so that a photographer removed the image from his online photo gallery. However, the photographer did not want to risk a legal dispute, so I do not know whether this would have been successful. There is also something like [Panoramafreiheit], but this is beyond the scope here.
With an 0815 house, I would not worry much about copyright, but I would be more cautious with a more distinctive design.