Floor area number of multiple parcels

You are certainly allowed to exceed 50% of the floor area ratio for ancillary facilities, this is common and should be stipulated in the text. This then applies to pavements, garage, and other ancillary facilities. Terrace counts towards the floor area ratio.

You have a floor area ratio of 0.25, additionally for ancillary facilities certainly the floor area ratio II, that should be enough. ?

The 50% are not excluded in the development plan. Maybe I have a thinking error? 0.25 of 557m² = 139.25m² for house and terrace 50% of that for the rest = 69.625m² Double garage, garden house, access to the house and driveway in front of the double garage... I fear that will be the problem.

Maybe I have a flaw in my reasoning?

Yep. You write that the development plan is from 1988. Then the Land Use Ordinance of 1977 applies. §19
(4) The floor areas of ancillary facilities within the meaning of § 14 are not counted towards the permissible floor area. The same applies to balconies, loggias, terraces as well as to structural facilities, insofar as they are permissible or can be permitted under state law in building law or in the distance areas.
So only the main buildings are counted. And your garden plot is also located in a WA area. You could combine the parcels if they are equally encumbered and thus add the 120 m². Since the garden plot cannot be built on anyway (completely outside the building window), the approval authority will probably allow the inclusion even without combination.

Wow, thank you very much! That’s what I was looking for, but apparently didn’t find the right thing. Does that mean everything that is GRZII can theoretically be as big as you want?

Yes, unless any restrictive conditions are mentioned in the textual provisions.

For the 1977 Building Utilization Ordinance, please be careful with the floor area. For example, if you want to convert the attic floors or plan a residential basement, the habitable rooms, corridors, and stairs in the non-full floors are included.