It's probably an all-or-nothing situation here. For a start, you will be applying for the protection to be placed somewhere which I would assume not a lot has been built yet quantity/quality-wise and even then you are only applying for half of your total city to be protected. It would make sense for that city to be fully protected, not half protected. If this were a real life situation, if you were rejected having filled in your full application, you wouldn't re-apply with half of that - it wouldn't make sense.
My advice would be keep building, make sure it is good quality and apply for your whole city to be protected at a later date.
Hope this helped