Extension of an annex

hello and best regards from southern Hesse

we have bought a house, built around the 1950s, and have already gutted it. In the mid-1960s, a narrow bathroom (W 1.70 x L 2.70) was added, which essentially connects the house with the garage and adjacent summer kitchen. We would like to widen this bathroom by about 1 m - the plot is large enough - about 400 sqm - the floor plan of the house is about 80 sqm. Do I need an architect for this or is a structural engineer enough? It is only supposed to move the house wall and add a staircase to the yard. Can I submit a self-created plan to the building authority? Can someone tell me something about the costs (architect, building authority)? I already thank you for ideas and suggestions.

Building applications may only be submitted by authorized persons. You cannot avoid involving an architect or structural engineer. In general, for all projects that affect exterior walls and other elements, I would definitely consult a structural engineer.

That depends on whether the structural engineer is authorized to submit building applications. ;)