Is paving a parking lot outside the building boundary allowed?

Caution is advised with grass pavers... according to our development plan, these are also considered sealed surfaces...

If without deduction, I therefore consider it contestable.

Commonly, development plans for privileged buildings (such as garages, equipment houses) allow a maximum of 9m boundary construction on one border and up to 15m in total. This would also include a carport. However, municipalities can regularly deviate from this. If the development plan does not specify anything, I would initially assume that you can build your carport, provided that no other privileged buildings have already exhausted the maximum boundary construction. The distance to the street often also plays a role. It is best to simply ask the building authority with a small sketch.

Common in development plans for privileged buildings (such as garages, equipment sheds) is a maximum of 9m boundary construction on one border and up to a maximum of 15m in total. This also includes carports. However, municipalities can regularly deviate from this. (...)

Yes. However, garages and carports are only allowed within the building envelope in our area.

Deducting for the grass pavers or not, they count as sealed surfaces. If someone wants to argue about this with their building authority... go ahead.
One can use their time more sensibly.

Whoever wants to argue with their building authority because of this

Who wants to argue? Do it, let it turn green, and enjoy a parking space that looks like a garden.

If the development plan does not say anything about it, I would initially assume that you can build your carport,

It is probably only about a parking space or paving, not a carport

I find the appearance of the concrete grass grid stones rather questionable. If partially paved, then rather as grass hole bricks. Or alternatively as flower-gravel-lawn, in which case the argument about sealing would ultimately fail.