Another bedroom one. Disagree with DH, AIBU?

Yes completely agree.

Because she clearly feels strongly about losing a reception room

A new kitchen …

Move your two DC into the biggest bedroom with bunkbeds. Keep as big bedroom and playroom for two youngest.

One of your DC has the small bedroom which they vacate and stay in bunk bed room when SC come to visit. It’s kept pleasant but fairly neutral and your DC keeps all their stuff in the big bedroom. Then in three years time when DSC15 is 18 and off to uni, change again.

A bit disruptive for younger child to move rooms some days per week, but less disruptive than being a stepchild and having to move homes some days per week.

It’s worse than a pita. It’s a sense of dislocation.

How about getting a quote for splitting the room professionally using a 3/4 wall with glass for the top 1/4? So that both smaller rooms have natural light. This would be a lot cheaper than the attic. Plus you could decorate the rooms at the same time to make them a bit more personalised…

Yes, and there’s a separate lounge. Plenty of options. But the OP also said she’d rather spend money on landscaping the garden than ensuring her teenage stepchildren feel comfortable in their Dad’s home. It’s clear where her priorities lie.

I think the answer is split the biggest bedroom, add another window, youngest children go in there because young children don't spend much time in their bedrooms and they have a playroom, and oldest children have 1 smaller bedroom each.

If they'll only be coming over for a few more years as people keep saying, what's the issue?

If OP is actually HOPING the stepchildren won't visit much anymore and that's why she says no to all suggestions, then she should say that to her husband, "I'm making your children feel unwelcome so I can pretend they don't exist". See how that goes.

They also have a playroom

Can’t you just make the reception/playroom a convertible bedroom space? Get a quality sofa bed, or a smart cabin bed could be a good fix. I’d ask the DSC to see if they agree this could be a good way to give them an extra bed space but still retains a temporary reception area when they’re away. Should be cheaper than a loft conversion: https://www.millieandjones.co.uk/products/trasman-mont-blanc-kids-high-sleeper-bed

To those saying - they don't need their own rooms if they have them at their mum's. I actually think that makes it worse not better for two reasons

1. It will make their mum's home much more attractive and while the OP probably doesn't care if they don't come over as much, her DH does

2. it probably makes sharing a lot harder if they haven't grown up doing it and don't have to most of the time. Kids that do it 100% of the time might well hate it as well but there's more of a chance that they get used to working around each other.

And actually there's a third one too which is that in a house with younger children, they might well feel more need for their own space than they otherwise would because younger kids are naturally louder and more present in the communal space

Would people STOP using that fact that the SC have another bedroom elsewhere as a POSITIVE! It's not a positive, it's a pain in the arse.

I don't know how people seem to miss the constant headlines saying that more and more young people are still living at home well into their twenties and thirties, and still insist that everyone moves out the day they turn 18. Half of 20-24 year olds still live at home, and 18% of 25-34 year olds. The DSC could be with OP for a decade or two yet.

Even if they go to uni, more and more DC are doing so locally and commuting. Even if they move away, uni terms are short, they'll still need somewhere to come back to.

Or they might want to move out of their mum's and move in with their dad and OP permanently once they are adults, if they get a job nearby or something.

Can you split the DSS room into 2 but also add a window so they both get a window?

I definitely think this is an option worth exploring.

I wouldn't go for the curtain type partition because it doesn't help with some of the issues with sharing - things like light, noise, privacy (e.g. you can't talk privately with a friend or on the phone or cry or anything).

And it also doesn't feel like a long term solution - if you intend to stay in the house long term, I think an extra bedroom for when the children grow up and have partners/spouses/children would be much more useful than a room partitioned with a curtain

She said it’s an extension of her kitchen area..

Is that in case the mother in law ever has the timerity to think she could stay the night? 😆

YANBU if they are regularly spending less than 3 nights a week with you. It makes no sense to convert rooms that will be empty most of the time.

If they have the biggest room and you can split that, then that is probably the cheapest option, with moving the existing window and adding another.

Careful, I got slated by another poster for suggesting a spare room.

If another bedroom were created then your DC could have the bigger rooms?

Can you put a sofa bed in the other reception room and when the sdcs come one can have that as their room, so no one else uses it whilst they're staying but it's a family room the rest of the time?

What do the sdcs suggest? I'd ask them for their ideas
Can you build a room in the garden, like a summerhouse or home office type of thing?
Would potentially cost less that a loft conversion.

Any family needs to keep a savings buffer for unexpected eventualities. It sounds like you can’t do this and convert the attic, so you can’t really afford the work. Maybe he can save hard for the next year and then pay for it entirely by himself. But as the 15/16 year old only has 2 more years of school, it doesn’t seem worthwhile.

We have 3 DC, they all have their own rooms but we did allocate it when we moved house as the youngest having biggest room and oldest having smallest room for the reason you said about toys. In your situation even with full time residency I think I would have the teens sharing and the younger ones in their own rooms, preferably with a divider for the teens if possible. Could you switch the master bedroom to being theirs space wise so they have a bit more room to divide the space with a screen?
I think having 2 younger children sharing is far from ideal as the mess from them playing and mixing up toys would be too much, as well as them waking eachother when trying to get to sleep.

Step children rarely get what they want

If it could be an option to split the step children’s current room, Would it not be possible to get a window fitted in the half of the room that wouldn’t have one?

otherwise I would be inclined to go for the loft conversion, maybe with two smallish rooms, and you would then have a nice spare room for guests.