Another bedroom one. Disagree with DH, AIBU?

This. 100%.

Siblings gave been sharing bedrooms for hundreds of years, why should it not be suitable in this part time situation?

If you're going to quote a post, at least read the post they are replying to.

Oh I agree.The husband needs to fund this if they have separate finances. It’s a messy situation all around but one so often repeated on these boards.

Is this your 'forever home'? If not, look at it from a purely financial perspective - all properties have a ceiling as to their 'worth'. If you did the work and then wanted to sell the house, would the sale price reflect the cost of the work? If not, then there's no point doing the work.

I see no issue with 2 sisters sharing a room. But your DH and his children do. So, if you're planning to stay in the house long-term, a compromise will need to be made somewhere. I wouldn't spend absolutely all your combined savings on an unnecessary renovation. So, I see some options as:

1. Agree together how much you are willing to spend out of savings, and how much you will need to save towards the renovation. (Don't forget to do a realistic budget for the work, including the cost of renting elsewhere, if necessary)
2. Look into the financial options available for the work i.e. using savings is obviously 1, but you could also consider extending the mortgage term or borrowing more against the existing equity in the house
3. If the loft isn't really a viable option, turn the largest room into two rooms. Windows can be moved or, again, if the budget can't stretch to this, there are very smart, elegant solutions to get light into a windowless room. (You'd still need to explore whether a bedroom with no window prevents a fire safety issue - I have no idea if there are regulations around this.) Spend a little bit to get an architect's opinion on what is possible.

How much is the loft conversion? I got my loft done for 6k from a local company. Look around for more quotes.

Its the latter part of your post that particularly makes me despair for humanity.

But to be fair, also the bit where you are suggesting that OP going into a relationship with a man with kids, has no part at all in ensuring that those kids needs are met.

because they're on a 'poor step kids having to split between two houses' train of thought that's why.

I can see the OP's point about not wanting to invest in a 5th bedroom when the 15yo probably won't want to be coming as regularly within a year of completing it.

Allowing for timescales, the planning process, and availability of builders if you do the extension you are looking at this not being completed until older one is 16/17 (ie only 12-18m left of them regularly visiting before heading to uni?). Also, would that older child not find the building work and chaos disruptive whilst studying for GCSEs and then A Levels? Tbh, it strikes me that you and DH have actually left this discussion far too late as it should have been resolved and work done before the eldest reached y10?

That said, provided there really is a good market for 5 bed properties in your area, it might still make financial to invest in the extension - but tbh I would not assume this is the case. The bigger the house, the smaller the market for buyers, the more difficult it is to sell - especially if it doesn’t have additional living space and a large kitchen/dining space to complement the 5th bedroom. And probably an additional bathroom? You’d need at least 2 for a 5 bed house. I’d speak to an estate agent so you can be very clear as to which improvements will actually add value and how much and work from there.

Then it is unreasonable to expect her to use her savings if they have separate finances to support his decision to have 4 kids and provide a separate bedroom for each.

didn't see it that way. You have a point. I still wouldn't do a loft conversion and use all my savings. I know OP is against it but in this case I would move the little ones to the big bedroom as they also have a play room.
Don't envy you OP. Perhaps just tell your DH that he has to pay a majority of it.

I would convert the downstairs playroom (which is a current luxury in the circumstances) in to a bedroom for now, assign that to one of the DSC while they’re still staying frequently then convert it back later in to a convertible sofa bed guest/ playroom so they still have somewhere to stay when visiting. Then you also have a viable guest room solution. just make sure there is a closet for the DSC to leave things in.

As soon as I moved out at 18 my parents took my room and I had to stay in the ‘guest’ room when I visited, it was fine.

That was in response to your comments before you edited the post quite heavily.

They didn’t ask for their parents to split. I’m sure they would rather just have one home like your dc. You are definitely treating them as less important than your kids and I totally understand why that is. But your DH chose to have 4 dc so should treat them equally.
I feel sorry for them.

Everyone will have their own view, mine is that having older siblings share and younger having their own room sends a very clear message to the DSC and I can't blame them for wanting less time there.

We chose a house big enough to have all dc having their own room and yes, this means one has the extra reception room and we manage with a lounge diner. Personally losing a reception room is the best option here. The 16yo WILL likely reduce his time there in time for other reasons, then you can convert it back.

The self esteem and wellbeing of the kids is more important than savings, than having less downstairs space and pretty much anything else in my opinion. I grew up in a home where kids and step kids weren't treated the same and believe me the damage can run deep for years

So he knew what size house he had and knew 4 kids = someone having to share…

I'm sorry why?

Cabin in the garden? I don't know if that solution would be cheaper. The 15 nearly 16 year old might like the independence it creates. Or I've seen an ikea hack where you use a storage unit in the middle of the room to create a temporary wall?

It's more likely to change if you don't have adequate accommodation for them.

Is this a joke?

The pp about putting another window in is an excellent idea.

Spend a bit more and have the windows sorted too then. Put a second window in and make the room into two proper rooms. Will cost a bit but an awful lot less than a loft conversion.

Agree. I feel sorry for the father. He’s one of the rare examples of a father trying to do the right thing by his children in this sort of situation.

🤦🏼‍♀️

OP I don’t blame you for your view on this, because I assume you don’t have enough parenting experience to know any better. Your have little kids. You don’t know that teenagers need their parents almost more than little kids. They don’t want to be in their pockets, but when they need them, they really need them. Just because the step children don’t come as often, it doesn’t mean it’ll continue like that. And making them share a room (when you do actually have other options) is making them feel unwelcome and surplus to requirements.

Imagine if your husband leaves you for another woman, and has a couple more kids in 10 years. Will you be happy for your kids to share a room when they don’t want to, while the shiny new kids have their own rooms?