I don't see much possibility on the ground floor
Why not? You have windows to the outside… others would build a conservatory at the back. Accordingly, a second office can be added.
I don't want to sacrifice the open space.
You wouldn't either, it stays, just with stairs
Sacrificing the garage is also bad because the entrance to it is at the front by the gate.
Only an uninformed person suggested that at first… it was not meant as advice.
Somehow everything is not optimal
Well, just because of a "larger" office… Is it about storage space for file folders, a larger or second desk? I would address and optimize at this point before shooting sparrows with cannons. For example, the existing office could be equipped with a bay window. That would gain 4-6 sqm. The bay window could be extended forward and to the side or both. Or an extension that goes into the front garden? That would also add some excitement to the facade. If the open space is that sacred, then the need is not set high, but rather a wish. If necessary, put a garden shed or even better, a mobile home in the garden. A caravan is available new from €12,000 with gas or electric heating, an older mobile home maybe for €30,000… That has more charm than spending €50,000 on an ugly extension that barely has more value than a small little room plus access (or rather minus, because the dressing room is sacrificed) plus a lot of construction work where the costs are practically thrown out more than invested.