I had this issue as a teen. 17, Dad moved his new girlfriend in (of 3 months…) and her 15YO son. Me and my Dad had lived alone for about 6 years at this point, after I’d been removed from my mother/step fathers chaotic home.
He thought he could just walk into my bedroom whenever he wanted, despite my Dad being very firm about bedrooms being a place where you knock and wait for a response before even touching the door handle.
It should be noted that the bathroom was downstairs so he was watching me leave the bathroom in my robe and following me upstairs.
He was bollocked. He did it a second time. He was bollocked. The third time he tried, I’d shoved my chair under the handle. He started kicking the door to get in, I was screeching at him and my Dad had just got home from work during this.
He came upstairs, lost his shit, phoned his girlfriend and told her she needed to leave work, pick her son up and they both needed to stay at her house (luckily, it hadn’t been sold yet).
A bolt and chain was installed on my door, but I only ever showered when my Dad was in the house after that, or went to my Uncles.
Didn’t stop him though. The fourth time (although I’m sure he’d done this multiple times, this just happened to be the time I saw him) I caught him looking at me was through my bedroom window - it backed onto the neighbours garden (weird layout outside, very elderly neighbour who didn’t use the garden, I had one of those vertical slatted office blinds) and he was stood there, staring up at me.
Unfortunately she’d sold her house by then (6 months…) I no longer felt safe in my own home, my Dad refused to make them leave and I ended up moving in with my Uncle whilst I finished my A Levels, my relationship with my Dad 20 years on is still very, very strained.