Width of the property driveway deviating from the building permit

Or would that be regulated in another law or regulation?

Yes.
Road Act - StrG §13
...
There is no common use if the use of a public road unreasonably impairs the common use by others.

Does permeable paving not count towards the floor area ratio?

Yes, it does. However, the development plan may then allow further exceedances, or the authority can grant exemptions.

Normally, the floor area ratio regulates the sealed area.

No. Although the amendment of the Land Utilization Ordinance in 1990 was justified by the sealed area, this is not reflected in the ordinance text, so that the areas mentioned in §19 are to be added regardless of the surface condition.

Nothing is stated about the width of the driveway in the development plan, so does that mean I have no restriction here at all? Or would this be regulated in another law or ordinance?

I am not aware of any state building code that makes this a management issue. As far as I see, the development plan is decisive here.
[Edit: was faster and knows more precisely]

Does permeable paving not count towards the floor space index?

It is inconsistent down to the municipal level which floor space index discounts are given for grass pavers, etc. However, paved surfaces are never counted as "0," regardless of how wide the joints/gaps are.

Ah, damn! It seems to me that the road law is going to throw a wrench in the driveway calculation. With a property width of 17.5 m, a 7 m driveway would probably be too wide. Is it worth asking the building authority about it?

Regarding the permeable pavement: this is prescribed anyway according to the development plan; there is nothing stated here about the floor area ratio.

Is it worth asking the building authority?

Yes, unless you are simply considering making the change. Do many vehicles park on the street?

Regarding the infiltration paving: according to the development plan this is mandatory anyway, nothing is stated here about the floor area ratio.

I thought the floor area ratio wouldn't be an issue either way. Gradually, a site plan could be helpful to clarify the situation.

Yes, unless you are considering making the change just like that. Do many vehicles park on the street?

It is a new development area, with detached single-family houses and a handful of terraced houses. So far, no one is parking yet; the construction approval for the area will only be granted at the end of August, the land consolidation has just been completed.

I thought the floor area ratio (FAR) wasn’t a problem either way. At some point, a site plan could be helpful to clarify the situation.

Yes, well I still have about 70 m² free in terms of FAR (about 290 possible, 220 used). The type of paving stones (something pervious or grass grid stones or similar) is specified by the development plan, but not regarding the FAR, rather because of the environment.

Attached is a meager sketch (currently only from my phone).

The city will rightfully have some objections if your driveway costs a street parking space. Often, families with small children move into the new development areas. Then the children grow up, and when the first ones start driving cars, it gets crowded in the area. Well, hopefully by then we will have better mobility concepts than "everyone their own car." Cars stand around idle most of the time and are rarely used more than 5% - that would be 72 minutes - per day.

Permeable surfaces are not sealed.

In our case, they are; they were even included in the calculation of the size of the infiltration trench. The head of the lower water authority is aware of the decreasing permeability and the problems on our street. Everything was understandable, and the infiltration trench was made three times as large as originally planned.