The building authority has now given confirmation of how incorrect the assessors' statement regarding the windows was.
The publicly-law correct answer that I can give you on this will not please you: I do not have to examine this in the simplified procedure, especially not for a single-family house of low height.
In the simplified approval procedure for low-rise buildings, the architect is responsible for fire protection. He signs for this with the building application.
There were no objections to the architects' proposal in your construction project, so I also gave no indication regarding the escape routes.
To shed some light on the matter, I would now like to give you a practical answer.
According to the building regulations NW, there must be a second escape route from every floor with habitable rooms.
This is secured if a window opening is 90cm wide and 120cm high in the clear. The sill of this opening may be 120cm high, and the location of the window in roof slopes may be 120cm away from the exterior wall (dormer or roof window!).
It is important that one can make oneself noticeable from there to a public traffic area, as these openings are usually not intended for self-rescue.
I would have found it more clever to designate the children’s room window as an escape route, since one could then also flee onto the garage roof (or one of the other windows directly on the street front) and thus be directly on the street, but the intended solution is also acceptable.
It is possible to independently escape onto the roof terrace and, since one is already outdoors there, to make oneself noticeable and wait there until the fire brigade arrives with a ladder. If this is considered too unsafe, one could also create a possibility to get from the roof terrace into the garden. However, a second structural escape route is not required by the building regulations for buildings of this class.
Because according to building code §40 paragraph 4 only one window is necessary, but you even offer an improvement in the form of a door opening without a sill to be overcome, which also allows partial self-rescue, I do not understand the concerns of your expert.
The cited DIN standard merely serves as an aid during construction to determine the ratio of door opening size to door to be installed.
That the clear opening of a patio door is smaller than that of a normal apartment door with the same rough construction dimension is probably normal due to the threshold and the upper frame and does not change its qualification as a second escape route.
“Escape doors” are roughly speaking doors from, for example, assembly halls, shops, or similar. But not as a second escape route in a single- or two-family house. I assume that your first escape route, the front door, complies with the common standard.
I would prefer to simply reduce the bill.