Don't know how I feel about it...

So my BF and I are finally moving together. I'm so very excited. However, the place is a duplex and his mom will be living in the other house/ unit...she's contributing a significant amount of money, so I can't say anything. Its not a negative or bad thing...I absolutely love her...but idk why I feel weird about it. Like we are both grown with careers and this is a time where we are establishing ourselves as a couple and as adults. It just seems like there are some lines or boundaries that can be crossed really easily. Maybe its just because this isn't how I saw things going? Maybe I'm just being ridiculous?