Um, no. If he wants to shack up he needs to get his own place. Jesus.

ETA i have a BS who is almost 20 and he knows better than to ever ask something like that. And yes, being able to do what you want is the reason kids launch. 

No way that would be allowed in our house.  Why would this kid ever leave if he can do whatever he wants all the time, rent-free?

To bring a girl home to their parents place??

flashbacks now of the "failure to launch" movie

Well, you're already in this, so asking whether we would or not seems moot, unless you're planning on kicking him out. But, I can give you some insight into what my parents did when they were faced with this with me (yes, me) TWICE. 

First was with my XH. This situation was a little different because we were high school sweethearts, had gone to college together, and lived 5 minutes apart in the summers. His mom sold everything in her house when he graduated because she was "moving to AZ", so he had no where to stay on breaks unless he rented an apartment for three months. So my parents let him stay with us (but they loved him, and I still had siblings at home, and we were in school, and it was a big house, etc). The summer of our second year of college, his mom died before she could move, so we split that summer between staying at her house and caring for her, out of state for her funeral to be with his family, and then a month with my parents as we cleaned out her house and moved into off campus housing.

Point is: these were short stints with defined timelines. Expectations were no parties, no waking people up after 10PM, no coming in at 3AM (if we were out, we needed to STAY out), no friends over without permission, etc. This time was a bit different because we were transitioning into adulthood, and with younger siblings, it just felt like senior year of high school. But, there were rules, and we abided.

Then XH and I got divorced and I was BROKE and beaten. Met DH and we started dating (questionable whether that was smart or not). For the first 1.5 years of dating, DH and I lived with our respective parents. DH had been living on his own, but had lost his job (tail-end of the recession before things got better around here), and ET was having some other crisis that DH felt compelled to help with. DH got a job and began saving so he could move back out. 

I was coming out of a divorce and lived with my parents, but paid rent. It helped both of us out because they were able to take what I paid in rent and save it into their emergency fund and I got a structured living environment and lower rent while I dug myself out of my divorce financial hole.

Anyway, FIL would let me come by during the day, but I had to go home at night. Cool. My parents, however, were fine with DH staying over. When he started staying over, I started paying more in rent to offset the cost.

The big piece here was that my parents had a date by which I needed to move out. When I first moved in, we had a discussion about my finances and what it would take for me to pay off my debt and save up enough for a house. I managed to pay off everything early and start looking for a house earlier. DH had been saving to move back out, had gone to trade school, and had started a career-level job, so he also had savings to help find a place for both of us. It took us longer to actually get a place because we had a shoestring budget and kept getting outbid. We did eventually get a house and here we are.

My point: it was not ideal and it was embarrassing to bring around someone when living with parents. I wouldn't want to do it again. BUT, I appreciated my parents allowing it, and the only reason it was allowed was BECAUSE I hated it. I WANTED to move out, and almost did several times (likely to my detriment). There were rules, and I knew if I broke them that my SF (yeah, lived with a SP as an adult...) would toss my sh*t out my second-story window himself.

I'd be willing to extend the same offer to a kid or SK, but they'd have to live my the same rules my parents put on me. There would be no "sneaking in" someone. There would be no living here for free, or indefinitely. No secret entrances into the house. If they're staying in Case de Lt., then they will get treated like everyone else, not like a dirty little secret.

Don't dance around SS's feelings with this, and don't feel bad about expressing your concerns to him. Likely the only reason his GF feels comfortable coming over is because it feels naughty and like a secret. You start putting rules around it and making it feel like a relationship with rules and responsibilities, and my guess is that she or SS runs for the hills. It worked for DH and I because we BOTH were working to get better and we BOTH hated the situation. Having it called out just made us work faster to get TF out (and I was devestated when pre-approvals and outbids added one week after another onto the time I lived with parents).

The beauty of this is if SS doesn't like these rules, and the girls he brings in don't like the rules, then they stop coming over and/or SS moves out. It's a win for you either way.

The more you forbid something from a child, the more they want it and the sneakier they'll be to get it. Call it out for what it is, because they're adults, and it will become less "cool". Less cool = less fun.

OH, and one final tip - even though I was paying rent, I was confined to my room. And since this was a new place my parents moved into, I had to bring all my own furniture. I may have gotten a bathroom trashcan and shower curtain because my parents had an extra. But I HAD to buy my own furniture, and it HAD to fit in my bedroom. Nothing was fully furnished, and I legit spent the first two weeks sleeping on an air mattress with my TV on the ground and my clothes in bags as I waited for furniture to be delivered. I really was a roommate with parental oversight. I really cannot express how much I needed it but HATED it.

This is the kind of issue that the home owners need to decide regarding kids who may still be allowed to remain in the home once they are over 18/out of High School.

For some, this may be acceptable... for others they may not be inclined to want other guests to stay overnight... especially if there is easy access from the place that the childult is living.

If he is in a wholly separate "apartment" at the home.. it might be ok.. but it should not be assumed by him that it is ok.  This is still your home and your DH's home.  

Honestly, he may need to hear the uncomfortable truth that having overnight guests is a privilege you get when you are renting your own space independently.. and that you don't naturally have that right when still living under dad's roof.

We absolutely don't allow it, but BM does.  SD has had several different guys living with her at BM's house over the years.  When she was in high school, her boyfriends would stay all the time.  We didn't know it.  I think it was one of the ways she alienated - keeping secrets of bad behaviour- drinking, smoking pot, and boyfriends spending the night.... 15 years old.  BM did not parent.  If he wants to have sleepovers, then he needs his own place.

We do NOT allow unmarried couple sleep overs in our home.

Your ss should have more respect for her. IF he wont you should.

jmo