Building plan not followed by architect - consequence

because we actually don't need the room.

Then better just build what you need and don't bring in strangers

If the [Einliegerwohnung] is part of the financing, it must exist. The bank just needs to ask or want to see the building permit. Apart from the unfriendly neighbor.

The neighbors are the ones who drag the authorities on you because of the rental. And then it gets really expensive if the tenant possibly has to be compensated and the rental income fails to materialize.

The alternative would be to rent out the attic.

Honestly: start planning again completely from scratch without a granny flat and one size smaller.

To your question:

The question was whether the architect violated his duty here.

No one can answer that easily for you. In which service phase are you? The initial meeting with authorities and the review of approval feasibility is in service phase 2.

that the preliminary inquiry to the building authority has led to

This half-sentence makes me suspect that essential details are missing in the description of the case. Why are final plans already being worked on when the decision regarding a preliminary building inquiry is still pending? Why was a preliminary building inquiry made in the first place? Who initiated this? What agreements (written/oral) exist regarding this?

Does it make sense to seek legal advice here?

If the relationship with the architect is completely broken and the feeling of injustice causes you sleepless nights, yes. Otherwise, can surely give you the right tips for communication with the architect. I still see chances here to achieve a satisfactory plan with considerably less money than a legal dispute would cost. However, both parties would have to be willing to compromise.

Architect owes an approvable design. [...] This is not an unlimited flat rate for indecisive clients

We hired the architect on an hourly basis.

That, on the other hand, is a flat rate for hours consumed off limits until the approvable design ;-(

Are 1.5 stories acceptable for you?

Otherwise, the only conclusion is that you bought an unsuitable plot of land.