Can I divide a Jack-and-Jill Bathroom into 2? Do I need a permit?

I have a jack-and-jill bathroom in my townhouse. They are divided into 3 parts in a L-shape. There are 2 sink areas and a bathtub/ toilet in the middle (with 2 doors to lead to the 2 sink areas in the two bedrooms). I think that this is a weird design. So I want to know if it’s possible to remdel one of the sink areas into a small bathroom (with a stand shower, small sink and toilet). When I purchased this house few years ago, the seller’s agent told me I can add a shower in the sink area (the previous owner thought about that too) without a permit. But I want to know if this is true? What if I want to make it into a small bathroom instead of just a stand shower (which is more convenient but still weird)? Also how much will it cost and must I apply for a permit for that (or notify the HOA also since it’s a townhouse)? Well I am really not sure how I should ask this since I know nothing about remodeling/ renovations. I just want to give it a try and see what you think. Thanks!
Well… I am also thinking… if it is too expensive/ impossible to add a bathroom/ shower, I wonder if what I can do to lock the jack-and-jill bathroom doors better? Now the doors of my bathroom can only be locked from INSIDE. That means when I have guests stay overnite, they can just go to my bedroom via the bathroom anytime. Also sometimes people just forgot to unlock the bathroom door on my side/ even forgot to lock the doors (cuz there’re 2!). I think it is really inconvenient! Does anyone have any ideas on this? I just think this whole design is so weird!