Based on general legal principles that would apply in the United States, renting out a home is not in any rational world considered a "business" use. Unless Scotland has gone batshit crazy, it's a total waste of your time to engage in such casuistry. The apartment (or "flat" as you call it, despite the fact that it appears to have three dimensions) is going to be inhabited by someone living there. If you said the owner was going to turn it into a Bengali restaurant you'd have a good zoning case. As it is, I doubt it. Same thing with getting a permit for "change of use".

If, however, there is a legally binding requirement that the owner get permission from all apartment owners to make the change for the door, why don't you just refuse to give the permission on the grounds that you are concerned about your child's safety and be done with it? If she doesn't stop after that, seek an injunction on those grounds.