How would you split the cost of this trip?

Or the three adults: the two adult children, plus the one adult child's spouse, split the cost for both their parents.

But the adult child + spouse pay for their teen child, especially if that teen child has his/her own room.

How many rooms? is the main question ...

We do this kind of thing a lot. Three siblings, with 0/2/3 DC respectively. It's always split between the adults. No one has ever mentioned one sibling paying less/more because of the number of DC. The headcount for the purpose of paying is adults only.

In this case, we'd pay for both parents and split everything three ways. It's worked quite happily for about 20 years now!

Same with food/drinks. We do a big order/delivery of whatever we need and split it three ways. Otherwise you're into "Robert had two rashers of bacon! I'm a vegetarian, and carrots are cheaper, and your DD just ate another bagel' territory and that would be the end of the happy times

Yeah but presumably they’re all having their own room. So it doesn’t matter if the spouse is going or not, especially if they’re paying extra for the teen.

But there are 4 adults:
the other parent
two adult children
spouse of one of the adult children

Teen is paid for by the teen's parents

Personally I think we’d split three ways, unless there was a huge difference in price between a 3 bed vs 4 bed. In that case I think the adults with a teen should pay the difference .

What is being proposed by the various family members, OP? You’ve had lots of opinions on what’s fair - but I don’t think we know yet what is the source of the dispute?

Or are you just gathering views before cost is discussed?

In our family the two adult children would split it 1/3 & 2/3

We wouldn't expect the other parent to pay - if we invited one parent we would assume they wanted their spouse and were treating both.

And we would roughly recognise that couple and teen used more rooms than single sibling and do a rough calculation and not argue about the cost basis. We'd see it as an opportunity to all have a great time together, and would only do something that we could all budget to do without quibbling over the basis of the split. If one family was struggling the other would just pick up more of the bill. No one takes the piss, no one begrudges, no one is mean.

The birthday-parent spouse would probably show up with a case of wine or something.

That's what we do in my family. If we are buying tickets for something and the other parent is accompanying them (particularly if they don't like it), they get the tickets as a freebie.

We bought a hotel stay and tickets once and the other parents just got the freebie.

Divide the total cost by 5, other parent pays their portion, sibling pays their own, couple pay their share each and split between them the cost of their teen so that is the 5th portion.

We done similar for my mum’s birthday! Dad didn’t pay! We just paid between us siblings. 3 siblings, 2 marriaged. Married couple have 3 kids between them.

we split it 5 ways between 3 siblings and the 2 spouses. Kids didn’t pay and single sibling was happy to split the cost of their nephews

Yes.

This.

whose idea is it? I’d bet at least one of the family units is joining this out of obligation rather than genuine enthusiasm!

In a typical family dynamic the non -birthday parent would pay for your accommodation if they are requiring your presence for the weekend

The obvious answer

It can obviously be split between the adults but I would also look at the finances of each. My brother was kind enough to pay for the majority of a holiday for my parents anniversary a few years ago and I paid what I could. At no time did he tell my parents or hint at me it wasn't enough.

Assuming you are the adult with no spouse or child. Yes, it’s unfair to expect you to pay 1/3 but represent 1/6 of the people in the house.
To make it more palatable/easy to understand for the person who I assume is your sibling, I would split per occupied bedroom.
1 - you (you pay)
2 - their teenager (they pay)
3 - their couple (they pay)
4 - your parents - either the other parent pays, or the other parent pays half and then the birthday parent’s half of that room is split a further 3 ways so you are all paying for them.

If it were me I would say three ways if it was both a surprise and a gift.

How many rooms are there? I would split per room and split the cost of the parents room between the families or have the parent who’s birthday it isn’t pay for the room. If teenager has own room then the family pays for two rooms.

Yes, because they wouldn’t need as many bedrooms

@GetBendyWithWendy I think splitting it 3 ways is treating it like a mutual shared holiday - a scenario where it does seem fair that the single person pays less.

However this is a birthday gift. If the gift was something other than a trip away how would you split the bill? 3 ways between the adult children and the other parent? Or would you expect the adult child's spouse to contribute as well?

Would the cost be any less if the spouse and the child didn't come?

I would first off get everyone to pay their own way in terms of flights/travel (not including birthday parent) then in terms of the air bnb the family have 2 rooms they pay their share. Then the other two one pays their share of one room and the other half (again not including birthday parent)

then the birthday parent costs are split 3 ways between spouse and 2 children

your way @GetBendyWithWendy has the person with a partner penalised and paying more and the partner in effect supplementing a present for a parent that isn’t theirs

@GetBendyWithWendy

I assume you’re the parent who knows about the gathering or the childfree person?

Do you think you’ll all be able to work this out fairly? It would be a shame to go away together and there be the potential for any fall outs or arguments.

Each household pays for their own element of the trip. Why make it more complicated than it needs to be?