Don't make it more difficult or complex than neccessary.  When you get home you get the living room and the kitchen and SD and her BF go to her room.  Since you earn the living for the household, you get the common space for your relaxation and peace of mind for the few hours before bed time. They of course can join you, but... you control the music and TV and they sit calmly and quietly.

Being a wilting violet does not resolve any of the discomfort you are experiencing each evening.

We have a similar small house, and I am an introvert. Thankfully, DH and SS are too. When SS21 was younger and still visiting, if he was allowed to play video games,it was fine, he'd be in his room. But if not, he'd be in the living room.

I also generally went into my bedroom, and had a little space in the basement I could hide away in too. I didn't really mind that, I have a laptop and didn't care where I was watching/reading.

That being said, my SS was a minor. Your SD is grown and there is no way in hell I'd agree to her AND her boyfriend living there, even for half the week.  If they are old enough to play house, they can go fiind their own apartment.

I see no reason for you to give up your peace and quiet just so your SD and her bf can take over the house. They occupy the living room, the kitchen and a bedroom? Time for them to move out and find out what independence is all about (hint: it's not about parents footing the bill and putting up with their whims all the time). Nope, if they want to be grown ups, then let them be grown ups ... in their own home.

Charging them rent might motivate them to move ... OTOH, it might also saddle the bf's parents with them full-time but that is not your problem.

Introvert for sure here, and I struggled a lot when my DH and I combined households 9 years ago.  We have a good sized house, but every common area is in a "traffic pattern", so I did not feel like I had a space for solitude.  Other than our bedroom, which I did take advantage of but that made my over analyzing mind feel like I looked withdrawn ( or maybe it really did look that way)  DH is not an introvert and didn't really get my feelings, he grew up in a home with friends over all the time and people in and out, which honestly gives me anxiety just thinking about.  It caused conflict, but eventually he was able to recognize this is part of what makes me who I am, not that I am trying to be difficult, and he started trying to put in effort to accommodate my space if needed, limiting nights there were people over, etc. I also made an effort to be more relaxed about things, and honestly, feeling like he understood and was trying helped me a lot.

With that said, in your case I would have to speak up.  To have 2 adults, even young adults, taking over your space would disturb anyone (except possibly their own parents,, but even then I bet they would be bothered but just not wanting to say anything).  Have you been able to talk to your partner about how you are feeling?  I would start there, and give her the opportunity to address it with your SD - if everyone is ok with them being there half the week, they are going to need to find a way to make their bedroom or whatever their hang out spot.  If your partner is not on board with you, I think it is time to have a conversation with your SD.  That is something I never did, I always left things to my DH, trying to stay out of it, but that muddied the waters and in the beginning he wasn't good at addressing ornfollow through which just built resentment.  You have a reasonable request, hopefully your SD is a reasonable and respectful person and maybe just didn't think about how space might be impacting others?  I think getting it out in the open instead of holding it in and letting it build will help you regardless of what happens after that.  Good luck to you, you are definitely not alone!

So often its hard to communicate with others. It's easy to say nothing and then feel stressed or upset when the other person keeps doing the thing that bothers me. It leads to resentment and all kinds of problems.

I always find when I just say it, whatever it is, I feel better. But it doesn't have to always be some big discussion or heavy handed type of conversation.  Sounds like you all get along. If the dynamics are good, relationships normal, then it shouldn't really be an issue. 

Just a simple statement when you get home that you want to watch your show for awhile tonight or whatever. A polite little, 'hey kids after I get a shower, can you go hang in your room a bit.' Or 'hey I'm gonna see if (insert partners name) wants to watch that Netflix show and snuggle on the couch awhile....hey honey, wanna binge some episodes with me?'

If there leaving messes just toss out there, hey can someone clean up here so I can make my sandwich.

But the kids won't know what you want if you don't say it. Your partner may feel like it's all good too. I'm an introvert as well, sometimes really hard to say something, but dang what a relief it is when I do.

I totally get that feeling of overcrowding.  We had a small place with only one bathroom.. it seemed to be overwhelming some weekends with the two girls (we had loft rooms for them.. but we only had one bathroom).

Is the 18 year old paying rent? What is the reason for them still living at home... ? is it because they are still in HS? or going to college? if they are working and graduated from HS.. what are their plans for finding a home of their own.

In my home, the paying heads of household get primary dibs and the free ability to take priority over a space and kick someone out of it.  That doesn't mean I would necessarily do that all the time.. but they should be very willing to defer to you if you tell them you want the LR for the evening.. or if you want to cook a meal.  It may mean some scheduling so that everyone is able to use the kitchen for example, but to make the home comfortable for everyone will be a little bit of work.

If they are decent kids as you say, they will be willing to shift aside for you and your partner.  It also may make it more appealing to start looking for their own space.  If the BF does have his own space?  I would ask that there are limits to how many days per week they can "hang" especially overnight at your home.

2 nights a week is plenty to have a guest... if he has his own space.. they need to spend more time there.

Hey Barry, 

You are the bill payer, so YOU have priority over who gets what space.

Honestly, I just tried to imagine my teen self monopolising the kitchen and lounge while my dad was forced to hang out in his bedroom after a long day at work.

Hahahaha NFW would that scenario EVER have happened!!! 

I would have got a clip round the ear and a lecture on respect...

This is more than likely the size of your place getting to you. Hell, I felt the same way with me and my own kids. Especially after the covid restrictions started.. we were driving each other crazy and it was more than time to do something different. We barely made it through the summer and I'm not sure if we would have been able to make it through the winter.