Exact quote from the engineer who created the development plan for us, on the question of the maximum height of the house above street level: 20 cm too high is not a problem, no one measures that.
Exact quote from the building authority on the question of a 9 x 6 garage, etc. being exempt from approval: if we are informed by the surveyor that the garage measures 9.40 x 6.10, none of us will come out and cause trouble. You just shouldn’t overdo it.
The handling of the topic naturally varies from district to district, but for us, that would not have been a problem. We followed all the rules anyway because it was planned in compliance, but I probably wouldn’t have had major concerns about doing it if there was a significant benefit. However, we only have neighbors we know in the new development area, where definitely no one would have spoken up.
PS: I can confirm that nagging leads to faster processing.
PPS: For example, we have 22 cm roller shutter boxes everywhere. Window height from 1.1m to 2.1m. 20 cm should work then, if a 22 cm box is enough for 2.1m windows in our case. They have small aluminum slats.