DP’s son moving in

On the contrary, it's so late that he probably counted on OP going along feeling she had no choice and once the house is bought and the son installed, any time in the future OP complains it'll be "but you agreed before we bought the house".

In effect, he's getting OP to give tacit agreement to the house being bought as a package including the son.

He might have thought that she could refuse at that point but by dropping it in her at the 11th hour he can claim her willingness to proceed with the purchase as consent. It's really manipulative, he's coercing her; relying on the OP's worry about collapsing the chain to get what he wants.

Why on earth when he dropped this on you, did you not have a full and frank discussion there and then. If you have to tip toe around your other half like this you are setting yourself up to be a total doormat.

Have you actually both sold your own properties to fund this purchase, and are those completion dates imminent as well, or have you both given notice on separate rentals. Yo really ought to heed the advise given by literally everybody, you can postpone exchange for another week. The inconvenience to your agents / sellers should not be affecting your decision

Oh dear...at first I thought this could be worked out with a discussion but with this, you need to PULL OUT OFVTHE SALE!. are you ready to live with someone like this full time?. It will be you, your OH, his weed smoking messy son and his girlfriend in the house!. Too much to deal with!

Absolutely this. I really hope you will decide to put the brakes on, OP.

If you let this go ahead, only to find out that living with your partner, his messy, weed-smoking son and his girlfriend makes you very unhappy, what on earth will you do then? Your money will be tied up in the house, so getting out of the situation won't be quick or straightforward.

You are not his partner, just half the mortgage payment
@oscalo has nailed it.
Until this point he has been laying the groundwork, getting his ducks in a row and now he has you backed into a corner.
He thinks that you now have no choice but to finance and facilitate whatever is easiest and most convenient for him.

The DP is not even very bright. If he were, he would have waited until after exchange. Your deserve better, OP.

Stall the solicitors, don’t let them exchange contracts until you’ve sorted this out

That is a bit much at 19 tbh.

You have to put the brakes on here op.
When you do your partner will blame you but you have to put the blame right back where it belongs. Don't miss a beat, fire straight back with both barrels, tell him it's his fault, he has deliberately misled you and you no longer trust him.

I meant a third of what the mortgage costs, not adding him to the mortgage or anything.

How many bedrooms does this new house have, OP? I’m assuming 3, right? One for you and DP, one for the son and one for your daughter? Because if I were your daughter, I’m not be impressed if there was no room for me at the inn because the son and his GF had the room. You risk her not wanting to stay or visit, or at the very least feeling unwanted.

If his own mother has had enough of him, then don’t say you didn’t see the big red flag waving in your face. Don’t say you had no choice and were fooled into this. You know NOW, before the exchange happens. So how this pans out is entirely in your control here.

Yes especially as you will now be living with two adult men, sorry but this would be a huge issue if not discussed and agreed prior. Speak to your partner now

If you do decide to go ahead, make clear that the son has house rules and pays some keep. If he was messy at mum's he will be messy at dad's too so get an agreement before he moved in.

If you go ahead and it's looking likely, those two men will rule your life as long as you are in that house. It's obvious they will gang together against you if necessary.

The underhand thoughtless way your DP just decided this was happening is unreal. You are not his partner, just half the mortgage payment.

Slippery slope here. Think hard.

This is it. If your kids have left home and you have a lovely home all to yourself, I think it’s madness that you would invite these problems into your life.

I would feel absolutely terrible for the house sellers, but I would pull out of the sale unless your partner agreed his son wouldn't move in.

It's hardly a recommendation that his mum wants him to leave because he's messy. It's also hard to think of his son living there with his girlfriend, rent-free, accompanied by the smell of weed.

You need to make an instant decision, OP. I couldn't forgive my partner for this. If the sale ended because of this I'd dump him immediately.

Are you selling your own place or are you renting?

Just want to add OP, I know it’s a faff and money will be lost, you’ll piss off the onward chain

but you can say no. You can refuse to buy a house. You can say that DS moving in changes things because you believed you were in a different life stage (kids moving out and on with life not back in!) and that you believed going forward it would be your pair as a couple with kids visiting not moving in (with limited prospects)

you can tell him the lack of discussion about your SHARED home is a massive cause for concern because DP doesn’t just have himself to think about and it’s YOUR home too.

would DSS and his weed smoking potentially impact your career?

im currently applying for a job where random drugs tests are mandatory so for me anyone doing any drugs in my home would be too big a risk for me to take with regards to my potential employment.

remember you can pull out, say no.

No to messy, no to girlfriend and definitely no to weed. It stinks and your house will have it wafting around, especially if he keeps weed in his room. Mum is right to have had enough. I would never take this on.

This man has been playing nice and keeping you sweet, waiting until you're committed and he has more leverage.
Now you are seeing who he really is, he expects to be able to call the shots and you have to meekly obey him.

Don't exchange.
You have the right to live in a drug-free home.
At the very very very least, you need to tell your partner why the lack of prior discussion is an issue for you.
Also will his DC already being in residence put your DCs off and mean they have no safe space if they need it after uni? It all needs careful discussion and planning.
You're entitled to have house rules which his son needs to abide by as a condition of moving in. If he's not prepared to abode by them then it'll be your life made difficult.
In your position I'd keep my own house until all kids are away and settled.
Remember this is your life and you have to stick up for yourself. There's no reason to feel bad about doing this in response to this new situation that's been flung on you.

Well it's a spanner in the works. I would set a reasonable amount of board for starters. However thinking long term, your child finishes uni, says mum I'm coming back to live with you. Would you say- hang on, I will just ask dp? I doubt it. Feel for you, as this wasn't part of the bargain

2 major issues
Lack of any discussion re his son moving in, and no discussion about financial contribution from son
His mum moans at him a lot. So he is a bad housemate

When he is messy at YOUR house, who is going to moan at him? His dad or you?