Attic with gable roof 22 degrees roof pitch

The degree is determined firstly by the development plan, secondly by the roof tile manufacturer "many are only watertight from 22 degrees"

And last but not least, the builder.

Does it make a difference in terms of storage space whether you have a 22 or 25 degree hipped roof?

Yes.
10/2 * TAN(25°) = 2.56m maximum height under the ridge, which is a good half meter more than at 22° - meaning you can still stand in the middle and store and retrieve a larger volume more comfortably.

We have 8.2m x 12m with 22° SD over two full stories as a warm roof. You can’t stand in the middle, but the space is simply huge and in my opinion worth its weight in gold if you don’t have a basement.

Ok thanks. The construction company proposed 22 degrees to us. A neighbor will also build with 22 degrees. 25 degrees are allowed. We had increased it from 22 to 25 degrees. We are building without a basement. Then we did everything right. I just didn't know how big the difference is and whether the additional cost is worth it.

I just didn't know how big the difference is and whether the extra cost is worth it.

Neither did we, because we don't know your extra cost. At the exit of your pull-out staircase, I don't see it making a significant difference, and for the storage space it's not the standing height there that matters, but the critical area is at the edge: at 22° DN a 50 cm height gain is reached 1.24 m from the edge, at 25° DN after 1.07 m – so only 17 cm earlier. That’s peanuts, the difference can only be cluttered up with a lot of sporty effort. I would see an added benefit at most where someone like treats themselves to a "proper" staircase to the attic – but for that, consider your stair location not suitable.

We have a mansard roof with also 22° in the hip roof area. We are having a folding attic ladder installed there as well and even have only 1.3 m finished dimension at the highest point. We are doing this for maintenance reasons alone and gladly take the additional storage space with it. The highly insulated folding attic ladder including tongue and groove paneling and installation costs you under 1000 euros. In my opinion, the price/performance ratio is pretty good.