House construction development plan weekend house area

That would need to be investigated. In all the documents I have, no justification is listed

It can very easily be the case with an older development plan that the justification for its establishment was not "digitized," then you have to blow the dust off a box at the building authority ;-)
You can show the plan here in its entirety once, maybe one can see clues in it with experience. But there must have been reasons given. And the municipality will have reasons today not to change the plan (presumably still valid ones from back then).

That will definitely be rejected. They are quick with that.

I can understand if the instinct is initially "better not to ask at all." But if you are that sure, then I think the risk of building there as your primary residence would be too high for me. Of course, ultimately, you have to decide for yourselves.