I I wouldn't have allowed the overnights to start with. If they want the perks of being adults, they get their own place. Is he even paying rent? If he is then double it because there are two of them. Frankly, I'd put a stop to the overnights, put a lock on the kitchen door, put her gear into a bag and hand it to her next time you see her, and tell your SS he has until the end of next week to find a place to live. This is your home and the pair of them have overstayed their welcome. 

... placating and catering to his offspring at the expense of his wife. We see this sh!t here all the time.

Everyone else in the household appears to be perfectly happy with the situation. You're the one bothered, so you're the one who's going to have to change. That's a sad old story too. You have lots of company here. There is nothing unique about your problem and nothing unique about its solution.

Lay down the law. This advice sounds simple almost to the point of being flippant, but it's the basis for your future and it's what you're going to have to do if the situation is to change.

Basically you must demand that they GTFO. Tell them what they have to do and when and then use your superpowers to see that they do it. You have some good suggestions above. Do not tolerate refusal or stalling. It is not going to be easy--everyone, including you, is in the habit of tolerating this, and nobody likes to change. But change is imperative here. Though he won't admit it, it will even benefit your DH. He will complain bitterly about your abuse of his beloved son, but make sure all parties including your DH understand that you mean business. Stand your ground. You're signing up for a life of misery if you don't impose your will and enforce your boundaries now.

You can do this.

You need to sit your DH down and tell him about the picking up after her and his son.  And the fact that you can't relax in your own home.  He is prioritising his son's happiness over his wife's.

Its time for SS to move out.  He can go share a space with a bunch of roommates his own age.  I can tell you from experience the roommates will put a stop to the 6 nights a week free loader girlfriend fast.  I had to do it when I shared after university.

Edit

Just seen you are in the UK.  If the rent is really expensive in your area, consider if it would be ok for you and DH to subsidise SS.  We did this for my YSS.  I couldn't bear to live with him another day but rent is so high in our city we subbed him.

Another thought is that you need to turn your home into somewhere uncomfortable for the pair of them.  Just some suggestions below.  You've already laid down the law on laundry.

• no cooking priviledges
• no using communal spaces (can't lounge around in the living room hogging the tv)
• no wifi priviledges for visitors or household members who give the code to visitors
• no netflix, amazon, sky access.  this is reserved for household members only and only for those who can be trusted not to had the passwords out to visitors

I assume you help pay the bills, so you get a say in who lives in your house, and that includes his son.  Have you directly expressed your displeasure to your DH about the GF essentially living there? If you have and he doesn't care how you feel, then you have a larger problem, which is that your DH cares more about his son's wants than your needs. Having your GF live with you at 19 is a want; for me, at my age, having a calm and quiet home in which I control who lives there, is a need.

For me, this would most certainly be an issue to have a fight about (and a hill to die on), including letting him know that if she stays, I go.  I'd be okay with SS staying for a fixed amount of time to achieve a fixed goal (ie, 6 months so he can finish school or save X amount of money), but not an open ended situation where SS gets to live there forever supported by Daddy.

I've seen this happen quite a lot. A girl with parents who don't want to be responsible for an adult child (for whatever reason) are happy to see her move out with a BF. They probably laid down the boundary so why do you have to pick up the slack. I really wonder about her family situation. Would it be possible for you and your DH to talk to her parents?  

Tiem to make it less comfortable for everyone. Stop cleaning up after them. Stop giving them the privacy to do whatever they want. Start charging rent. Make it a nominal amount at first then create a schedule to ramp it up. They are adults not children.

As others said the problem was allowing overnights. Everyone else with kids that are less than adults, take notice. Never allow overnights with a SO, it's a huge motivator to move out if you don't allow it from the start 

I would take the average room for rent in your area- divide it by nights. Let her know that starting Oct 1 you will be charging $___ per night for her to stay there. 

I wouldn't even talk to my husband about this, I would just do it. Right or wrong- I don't care. I am not having some SO of a kid or SK stay the night at my house and not contribute. Make them uncomfortable to the point where they either move out or start staying at her parents house. 

One, I sure hope birth control is in effect here.

Two, my sister-in-law had this situation, too.  She was the nice mom who let her son's GF stay there quite a bit because GF's parents were "too mean".   Long story short, her son and GF ended up getting married.  Her son was a passive type of person and altho he was having second thougjts, wasn't strong enough to end their relationship.  Fast forward and he's living with a shrew who is verbally abusive to her kids.

My SIL would have done her son a favor not to let GF move in.  It would have made it easier for him to break up with her if she hadn't been almost living there.

You and DH would be doing your SS a favor to not let GF stay there so much.  If they still love each other, it will endure.  If not, it makes a break easier.

Recommendations?  GF leaves now and has no more over nights.  Playing hide the sausage in a parents home, while I am fully aware that it happens and did it myself as a late teen and in my early 20s, is not something that IMHO should happen and for damned sure should not be sanctioned and supported by parents.

When I did it, the young women would enter and leave my room via my window after everyone else was asleep and they were gone before everyone woke up.  There was also no discernable passionate vocalizations going on.  That is what a pillow over the face is for.

The only times I had overnight female guests, they stayed in a guest room.  There was no hanky panky going on when they were officially guests.

You are the adult, it is YOUR home. You make the rules. So, make them, enforce them, and do not let these two rude young people make you uncomfortable in your own home. My guess is that once you end the overnight booty calls, SS will launch in a hurry, along with his F-buddy.

My DW and I have lived with my parents intermittently for extended periods over the years.  But, we are married.  Teens humping out their hormonal urges are an entirely different scenario.  

When I was a dependent, I clearly understood that I would not have a live in GF in my parents home.  Not even for a single night. Unless they were invited guests who had their own room.   As for the window visitors, well, that is another story.

Yes, I understand the hypocritical slant.

Take control. Quit being uncomfortable in your own home.

The  rules in our house ARE

No sleep overs for un-married adults. PEROID...kids knew this before they were in HS

Revisit the family home rules. That ^^^ may fix the current problem all together. No sleepovers with his gf should result with a moveout

A common issue - you are definitely NOT alone in this.

Skids that treat their parents home like THEY are the ones that own it - even while they dont pitch in and they dont pay a dime towards anything - they are a common issue. The parent needs to lay down the rules and create/enforce boundaries and here they often fail.
Then because YOU are uncomfortable, YOU become the problem.

So, you have to put your foot down and give out the rules about the GF. Stop cleaning up after them. Stop cooking for them. Charge the SS. If he is adult enough to have GF come over he is old enough to support that.

Is for adults with thier own domiciles.  Not for young adults still living with parents.

If he wants that privilege of having GF stay overnight, then they need to move out and find their own place pronto.

I wouldn't go along with them paying rent - it sounds like their presence feels intrusive so the only solution realistically is 1) no sleepovers anymore, which will likely tick off SS since its already been unofficially allowed or 2) they have 60 days to locate a place where they can rent together (if they don't like that, then go back to door #1).

It's very important to have clear rules for adult kids once they finish high school, My SD16 finishes high school at the end of next year and I've already started talking to her about the changes that we will put in place then that all revolve around increased responsibility. 

As for skid's partners, while I hate the idea of anyone staying over (as I like my privacy) I'm open to the idea but there will be strict boundaries. No more than 2 nights per week and they can't be consecutive nights (that way they can't spend the whole weekend at our place). The situation must also be reciprocal ie if we host skid's partner for two nights per week then they most host skid at their place for two nights per week. If they want to spend more than 4 nights together per week then they should be moving into their own place. 

I think you need to sit down with DH and draw up some rules/boundaries. Make sure he understands it's not negotiable - it's your house too and you get equal say. If you can't agree then do a mediation session as it always helps to have professional support and they would likely point out your DH is being unreasonable giving you no say in your own house and being unwilling to compromise. 

I say, make her pay rent. She's using your electricity and water. That stuff isn't free. 

Stories like this make me really glad my SK's moved out of state. When they come back there is lots of drama but at least it's not in my house day in and day out.