Thinking it’s weird that his 18 year old will have the master room

Oh no, however will you cope without a walk in wardrobe 🙄🙄🙄

Wow, no way. If someone tried to unsettle my DC from their home they would not step foot into the house again esp if you cannot even understand why you are wrong to even suggest this and not be able to empathise with his daughter.
The fact that you had to ask an online forum (and expect support) suggests to me you have no concept of how to handle the dynamic of moving into an established family set up.
This is a conversation to have 3-4 years down the line.
You are not coming across well in this thread.

You’re moving from a 250k house into a 2.5 million pound mansion, be grateful for the room you have. If you want a walk in wardrobe, then get yourself a better job to pay for one

Sorry what? You are upgrading to a much nicer home than you can afford yourself, and are moaning that you don't have a walk in wardrobe? In addition, you want to turf out a person who lives there full time from a room it makes perfect sense that she currently has due to usage/study facilities. You sound awful and I'd be rethinking this if I were your DP.

You’re selling your home to move in? How long are you trialling living together before you take that leap? When you do this are you keeping finances separate and ring fencing that money?

Our son has what would be classed as the master bedroom, he has lots of toys, sleepovers and will soon need somewhere to do homework. We sleep in our room and put away our clothes, we don’t need space for a trainset or for friends to sleep on air beds. When our daughter was at an age of needing her own bedroom she was given the next biggest room for the same reason.

If my husband had suggested changing his room when he moved in originally not only would he not have moved in, it would have led to the relationship ending.

THIS!
Dint leave yourself vulnerable! Keep your home.

Yes. Keep your house and rent it out. Otherwise you will have no security if you split up.

A totally valid reason for her to have the bigger room.

Also nice for her to have somewhere to spend time as she hasn't moved out for uni, I expect most of her friends have their own accommodation.

If you feel any resentment towards the fact that she's still living at home (or has the bigger room) then you mustn't move in, it's a recipe for disaster.

His logic is spot on.

but if your not happy don’t move in. He has made his position clear and is rightly putting his daughter first

I live in the SE in a small house with three young adult DC (no partner).
My only boy has the en suite master bedroom.
They live in their bedrooms. He WFH every day. Has his girlfriend over a lot. It made sense.
Am fully expecting them to be living at home until about 30 so you’d better prepare yourself for that too. I can’t believe you suggested moving her out and installing yourself in the best room of a house that’s not yours and probably never will be.

Do not sell your home. Rent it out. You would leave yourself in a very vulnerable position.

you sound like a spoilt brat

You would so be 100% unreasonable if there had been a vote on this one!

Are you jealous of your partner's daughter? If you want her to hate you then you're going the right way about it.

What's with all the wicked stepmother trying to get between fathers and daughters stories on here lately?

I agree with every other poster. A well established household is not to be meddled with by a 'newcomer'.

More concerning is you are selling your home to live in a very wealthy part of the world which, as you say, you couldn't afford to buy in even jointly. I suggest you seriously look financially into what you are doing here.

Good luck OP.

My advice is to keep your property- it may turn out to be a bolt hole for you in the future. Otherwise you may find that you’re in a position of trying to maintain a relationship purely because of lack of anywhere else to go.

This isn't going the way you want is it? Only responding to those who agree with you.
Is the daughter's name Cinderlla by any chance?

That has always been her room and it wouldn't be nice of you to come barging in and expect her to move her room because you don't like the fact its bigger than yours

“you sound like a spoilt brat”

Yes, this or a gold digger, I am sorry to say but hope that I am wildly off the mark… even before moving from a £250k home to a gorgeous 4 bedroom house worth £2.5m you are making demands.
He is right, you have lost your mind.

The right approach here is to move in with sensitivity to his daughter - don't try to change or even question how they have lived for years. She will soon graduate from uni and likely move out to be independent, by when your relationship with her should be on stronger ground and you can talk about the use of rooms if you still think it matters.

However the biggest flag in your OP is this: Tonight I suggested that since I’m moving in we maybe switch the bedrooms around and he looked at me like I’d lost my mind. You and your DP are not on the same page in terms of expectations and this is a big step to take. I would be having a much fuller conversation about how you both envisage the move. I also think it's unwise to sell your own home until you know that the new setup works - can you rent it out in the short term?

You're a girlfriend of 4 years, this is his daughter, she's always going to come first so don't even suggest it.

I don't know.

My DS's and I are moving and I've told my eldest he can have the master bathroom with en-suite.

That way he can have some privacy and more of his own space.

I really don't mind as it's on the third floor and I'm done with climbing extra stairs.

Please keep your house! It’s very easy to fritter away money. Rent it out and use the income.

The fact that you have only responded to the one and only post that agrees with you, says it all OP. (Unless you've fell asleep). Please don't impose yourself on this family. It's doomed. Keep your own modest house, where you can be queenie. There's no way I'd let you move in, after that comment. His poor daughter😢