It won't be instant, but as long as it's done with materials available in game and bought with your character's money, you don't need approval for renovations. Mundane building materials should be acquirable. Anything magical or high tech would need to be paid for with ac points currently. We don't micromanage how money works in the town, so it's up to your discretion how easy or difficult it is to afford things given what sort of income they might have. That said, the homes are found to be suited to the characters, so having some odd passages or even a fortified basement (appropriate of the 1950s level of fortification) isn't too much of an ask.

Hope this helps!