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

My suggestion is that you just stay in your flat. Thos isn't going to end well. Your tone and outlook is all wrong.

I also would not sell your flat. You have no security and step off the housing ladder in London and you may regret it - rent it out for a few years, keep your options open. You won't be on the mortgage you aren't getting married and your thoughts towards evicting his dd...I think you may have many storms ahead.

You are right but surely you can see this is a total non starter!!

Do you also live in London? If you can avoid it I wouldn't sell your home. Rent it out if you can. Do you not hope to leave something to help your kids in the future? What happens if you split up, could you afford to get back on the property ladder again? Your partners house is his daughter's home and it will be for many years unless she feels forced out. The last thing you should be doing is trying to change things that impact his kids, small or not. It would be entirely different if she had that room whilst at uni but had siblings who didn't have their own room.

I cannot IMAGINE having my clothes in separate closets, either. Ridiculous

oh yeah the inhumanity. It’s on a par with being locked in a prison cell.

She has her own more or less self-contained accommodation. He and you will have the rest of the house. It’s a perfectly sensible arrangement.
What is concerning is that you don’t recognise the importance to him of his daughter’s need for privacy, comfort and emotional security.

I think it’s actually sensible that the 18 year old uni student has a bigger room. It means she can have a bit more independence. Sounds like her room is big enough for her to hang out in, have friends etc.

it would actually give you ( as a couple) a bit more privacy/ alone time in the evenings.

Do you want her to instantly despise you? You are being VVU

Seriously? The girlfriend moves in and the first thing she does is turf out the resident daughter from her room? It would be a recipe for disaster.

No, he's perfectly correct. You don't have a right to kick her out of her bedroom. It will cause tension all around. His solution is reasonable, although not ideal.

Okay you’re being really stupid. Why are you selling your home? Are you going on his deeds or something? Keep your home and rent it out if you must.
Also, no, you can’t start throwing your weight around as regards bedrooms when you are moving into their home. He’s right with the office and she probably spends a fair bit of time in her room. Unless you want her to hate you, pipe down.

So is she a young woman or a girl? You call her both to try and suit whatever your narrative is.

Why is it not her home?

Whether she has lived there a couple of years or her whole life the house was clearly bought with her in mind.

She may also contribute financially.

Also, he isn't expecting his girlfriend of FOUR years to play second to an 18 year old. He's expecting his girlfriend not kick his daughter or EIGHTEEN years out of her bedroom because she has the better wardrobe.

His daughter having the Master bedroom is not your decision—moving in and evicting someone from their room is a poor way to start things. Sure, if everyone was moving in on the same day, you might have more sway, but the Master bedroom has been taken already. Your demands are unreasonable.

Thankfully your views are in the minority here. Since when does a girlfriend trump children (no matter how old they are)? I wouldn’t be wanting your views anywhere near my home.

I cannot IMAGINE having my clothes in separate closets, either. Ridiculous

I'm sorry, what?

It's to to you, but if I was you, I wouldn't sell your home. If you split up, you'll probably be unable to buy a new place, with the was property prices increase.

Even if you don't split up, it's good to have your own place for financial security.

Regarding her moving out of her room? No, YABU.
That's been her room for years and this won't go down well.

Whoa, don't sell YOUR property! Rent it out or let a relative move in but don't sell it! You may never be able to buy anywhere else again if this relationship falls through

My DC’s both have bigger rooms than I do, they spend a lot more time in them, I just sleep in mine. There is enough room for storage for my clothes

Woman. You are greedy.

Yes it is!
Only in the linked post I copied, she had already sold her home.

No normal person would reasonably sell a 250k property in London, to then move in with a partner. They would most likely rent it out to generate a passive income.

Even if it wasn't a 250k home in London. Why give up your home aka freedom to be at the mercy of somebody else, who can put you out their home at any time.

I'm sorry, but just no! Who goes in and takes over some teens bedroom? Are you guys married? If not then I'm afraid your just the GF and his daughter takes priority. That's it. If my dad had dated a woman for 4 years, then suddenly turfed her in, and she decided to become queen of the house by hijacking my bedroom/ sanctuary, then I would have been gutted and started WW3. Stay in your lane, you're just the gf, enjoy the new bedroom and leave her room alone.

Oh my goodness. You sound like the typical evil stepmother. Do not turf his daughter out of her room

I wouldn't be selling your house. Rent it out. This relationship isn't going to last. Youll be out on your ear trying to compete with his daughter (rightly so)

When my son lived at home he had the master bedroom from about 16 years old ( we had just moved into the house so he was first one in the room ) when he left to get his own place my daughter had that room.

it was big as in huge, lovely bathroom room and enough room for a pair of 2 seater sofas, a fridge and sink unit (no oven but a microwave) and still plenty of floor space.

it meant he could have friends round and do his model making and whatever he wanted and we could watch tv in the main room,

I agree with your partner you only really sleep in your bedroom, to a teenager a bedroom is much more