Another stepchildren & bedrooms one…

I'd let the boys have the big rooms, daughter gets the small room and space elsewhere in the house to build legos and draw etc (and also get the high-bed/wardrobe/desk combo in there now too)

When older boy is 18 he gets to swap with DD.

but i still say: no bath and build a 2nd toilet.

If your daughter spends more time at this house then she is entitled to a bigger room. Your DP should not have got ahead of himself, he will need to explain this to his sons

I would let the teens have the big rooms on the understanding that they have to swap rooms when they leave home. At 9 does your DD really spend a lot of time in her room? Can’t you have all her toys and drawing stuff downstairs? Or maybe get her an exciting built in high sleeper bed with room under it for her things? My Youngest 2 share a room while teen DSD (here 50%) has her own as she spends more time in her room.

We never use a bath took it out for a shower
I think your creating problems that don't exist
Leave everything be , problem solved

Or as others have said, do away with the bath and big shower - if you are expecting other family members to cope with less space then why can't you? Do the bathroom up when they've left home.

YANBU. DH has been a dick.

DD gets a double room. The boys can choose between sharing and having the single as a den/gaming/homework room, or they have a double/single and swap every six months.

Growing up, I always had the smallest room in the house but honestly preferred it. It just seemed cosier.

If you go to Pinterest and look at ideas for small rooms there are some incredibly genius ways to design a small room. It is amazing where storage spaces can be fitted.

Maybe it is a case of your daughter having the smaller room, because I understand the need for older teens, for many reasons, to have more space. Perhaps more can be taken from your budget to make your daughter's bedroom special and fit her needs.

These are just a few I found, that might give you some inspiration.

https://www.pinterest.co.uk/pin/103160647705666662/

https://www.pinterest.co.uk/pin/103160647705667163/

https://www.pinterest.co.uk/pin/103160647705689288/

I think you need to reframe your thinking. The boys wouldn't be getting anything your daughter doesn't get. By the time she's their age, she'll be in one of the big rooms just like them. At her age, they shared. They already have the short straw having to move homes every week. I get that you want the best for your daughter, understandably, but you can't do that at the expense of the other children. This is a situation of your making, it's not a stepchild problem.

Sorry, but how many of you need to be in the bathroom at a time?

It sounds like your daughter needs a double more than you all need a spacious luxurious bathroom when you already have a perfectly functional shower room.

As others have said, post a floor plan of the whole house and let's see if there's a better way to reconfigure it. Or ask an architect.

As a practical suggestion, I have a DC with a big room and a DC with a small room. We "compensate" by letting the smaller room DC have priority access to a downstairs space (i.e. it's mostly hers unless we have lots of guests or something) which she uses for studying in and entertaining friends. Is this an option for you?

Her bedroom is mostly for sleeping and storing her clothes. She has a hanging rail as her room is too small even for a single wardrobe, so I'm struggling to understand why your DC needs a double wardrobe.

you're putting a bath in? you don't need it. Sacrifice the bath (and water bills) for the sake of your daughter

I would suggest either:

a) DD has one double, boys share other double, single is used as a gaming/TV room;

or

b) DD has one double, boys have the other double and the single on a turn-and-turn-about or toss-of-a-coin basis.

Take turns choosing rooms for your kids. You can be generous letting DP choose first 😁

I have one DC who WFH and one DC who is a teen and still at school.
We thought they could share a desk. Believe me, this plan doesn't work.

Teens want to work in school holidays when adults are still working. At sixth form level they don't go in for whole days and they want to work at home during the day. They may go to a sixth form that only requires them on site 2 days a week. They want to leave their stuff spread out over the desk and not have to tidy it up every night. They want their IT equipment where they want it and not where someone else prefers it. They want to stick history timelines and zillions of post it notes on the wall (surely my DD is not the only one). They want to study when they want to - which might be at 3.30pm when they get in from school (when a WFH person is still working). That's before you even get into bad weather and cold and darkness making a garden office not very appealing in the evening. Are you happy to pay to heat the garden office constantly in the winter, or will they be freezing in there?

Teens also want to invite friends round. Do you have a downstairs space they can take over, if their bedroom is too small?

The older in the single, the youngers in the 2 doubles and you and your husband in another double.
That is the way I would do it.

this option would mean no one is being treated differently, they are all getting a bedroom and a playroom / games pit and they all have their own space.

Lots of people on here, including the OP, are acting like the stepchildren are barely there. This is their home 50% of the time. They don't "visit" or have a main home elsewhere. There's no reason to think they will come less before they move out. This stuff is why 50/50 is difficult as it can make kids feel like they don't have a home at all.

OP, you need to have an open discussion with all three children. Your DH shouldn't have promised them doubles. They should get to choose to share with a separate gaming room, or decide which gets the smaller room. Your DD doesn't get automatic right to a big room: she should also be part of the discussion.

The issue here is reducing the size of the 4th bedroom - if the older kids were only there every other weekend I would be more inclinded to agree one should have smaller room - but with being 50/50 I think this changes things.
Dh shouldn't have promised them the bigger rooms, but then you have also assumed that your DC is getting a bigger room too. Drawings straws is a possible solution, after explaining the situation or offering an incentive for someone to take the smaller room.
Personally, I wouldn't reduce the size of the room at this point and look for an alternative solution there.

He's being a total twat but please don't settle for the mess he has created. Your DD only has your to advocate for her. He should be ashamed of how grabby he's been, especially as he has two boys so close in age.

FWIW I'd put DD in the smallest double and get the boys to toss a coin for their rooms.

Get an en suite in the biggest double and keep it for yourselves.

Divide the next biggest double into 2 singles.

Give the kids the option of either 2 boys have a single each and share a single as a games room and girl gets the double as her bed and play room, or they get a single each and all share the double playroom.

Am I missing something .... you say they've been grumbling about sharing and want their own rooms - but they WILL be getting their own rooms, just that one would be a single. Surely they'll be so pleased to finally have their own rooms that they won't mind drawing straws for the larger one and/or rotating annually?

Ridiculous for people who are only there half the time to get the 2 biggest rooms. Outrageous actually. The DSs obviously don't keep ALL their stuff at your house and have 2 rooms EACH, one in each house, while your DC's stuff IS probably mostly at your house but she gets much less space. No.

DP was wrong to promise larger bedrooms from plans anyway given your DD is there most. Even if they all remained doubles. It seems his DC needs trump yours and he is trying to be Disney Dad.