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

This "master bedroom" is just nonsensical estate agent speak. She has the larger bedroom. So? That was agreed on, it's her home.
You're moving in, you compromise, although I can't see what's wrong with your bedroom.

Is there a basement by any chance ? Or a room beneath the stairs ? Maybe you could keep her down there to do laundry and earn her keep without getting in your way. That way you’ll not only have a walk in robe but your clothing laundered and mended too. It will also send a clear message of who the new matriarch is.

This post reminds me way too much of my childhood and how territorial new wives etc are especially when there’s prime real estate and wealth involved… oh and a wir.

If you want to mark your territory properly you need to remodel the house and redecorate in an entirely new style. You’ll also need to ensure he replaces the mattress other women have slept on. Make her knock on the door and replace the locks. Speaking from experience.

If you were all moving into the house new, absolutely.

But it's already her bedroom. To move in and immediately turf her out of her room is giving Wicked Stepmother.

If this is a long term house then you can move into that room when she moves out.

Why are you selling your house? Where will you live if the relationship breaks down in the future?

You must tell your DP that you want the master first thing today. Make it really clear how you mean to go on. If he's a decent chap all necessary decisions should be made by lunch time.

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😢

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

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.

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

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 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.

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

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?

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.

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.

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

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.

you sound like a spoilt brat

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

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.

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

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.

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

THIS!
Dint leave yourself vulnerable! Keep your home.

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.