How would you split the cost of this trip?

How do you know it is the children throwing the surprise trip? Might have been suggested by the other parent.

You are being extremely overdramatic in any case.

@GetBendyWithWendy Why?

Honestly, if you need any explanation, let alone further than what’s above, all hope is lost. You can’t explain decency or respect to someone who doesn’t appear to understand why this would be arsehole move of the century.

Just so glad you are not one of mine🫤.

But birthday parent isn't free. Assuming they have joint finances, they're paying exactly the same as if it wasn't a gift.

I'm sorry, but from OP's attitude in responses ("you do realise...") and the lack of will to find an easy solution, I think this trip is highly unlikely to be a success, and you should save your parents the trauma of it and do something else.

We did this for my dads birthday
1500 for the rental and we split it 3 equal ways between Mum, me and my brother

Mum brought Dad and they had the (fabulous) master suite
I brought my husband (1 room) and our 2 teen DS who shared 1 room
My brother and his partner (1 room), my adult niece (1 room) and my adult nephew (1 room)

I don’t know how my brother sorted the costs with his family with having grown up children, that was up to him.

I think this worked so well because my mum suggested covering proportionally more than ‘their share’.

edited to add: this wasn’t a surprise trip. I suggested a family getaway for dads birthday

Because of all the years mum and dad took you on holiday as a kid and paid for it

Exactly. If my kids threw a surprise holiday for DH or myself and expected the other to cough up, I’d wonder where the hell we went wrong. It’s a complete arsehole move and if it was for me and I found out they did that, and asked DH to pay ‘his share’, the holiday would be over pronto and I’d be out the door with some very strong words (plus some of those words would be to DH as to why he would agree to it).

The single person isn’t bringing a child

Parent - 25%
Family -50%
Sibling -25%

Birthday parent - free

I just can't imagine treating my mum to a trip for her birthday and asking dad to pay to accompany her, just as if I gave dad theatre tickets, I wouldn't expect mum to buy her own to go with him.

Whatever, if working this out is so difficult, you should find a different gift, and not consider going away together.

Because of all the years mum and dad took you on holiday as a kid and paid for it.

So 4 bedrooms required.
Split the cost by room and cover the parents room, as the repetition of ‘only a surprise for one parent’ is bizarre, surely you cover the cost for both parents irrespective UNLESS they are separated/divorced, in which case I don’t see why would be going?

Adult child 1 would pay 62.5% (for 2 rooms and half the cost of parents room/holiday).
Adult child 2 would pay 37.5% (for 2 rooms and half the cost of parents room/holiday).
Parents pay nothing as to charge 1 something when they are a couple is beyond words.

This looks like a nightmare, the question you're asking is probably the least of your worries.

Who is going to do the shopping/cooking? How are you going to split the bills for every coffee and cake and meal out? Local attractions, easily £30 per head?

Adult children each pay their own costs plus 50% of parents’ costs.
So AC with family pays own flights and bigger hotel room, AC single pays one flight and smaller hotel room. Split costs for mum and dad flights and hotel 5050.

This, wouldn't be surprised if 'child' with spouse would advocate that they get bigger room with en suite if not available with all rooms 'because there's two of them'.... but expecting a 3 way cost split including everyone else taking on the cost for their teen!

The other parent is expected to pay their share? That seems odd to me, unless he/she was organising the trip in which case they'd pay it all.

In my family this would work with either the other parent paying the whole thing or the cost being split between the two adult siblings.

And while there the adult with spouse and child should be covering food, drinks, tickets for excursions etc.

I’d split per room

The gift is from 3 people (assuming from other parent too), so why split between 4?

The cost of the gift should be split equally between the people gifting.

So 33/33/33, or 50/50 if just the adult children gifting.

Assuming the teen is having their own room and there are 4 bedrooms I would do the following…

Divide the cost of the house into each room. Family pays for two rooms. Single child pays for 1 room. The cost of the birthday parents room is divided between the family, single child and other parent. The other parent shouldn’t really be paying the same for their room as the single child, as it should be the gift of the birthday parent and they are just sharing the room.

So is the house is £1000:

Family pays £500 + £83 (1/3 of birthday room)
Single adult pays £250 + £83
Other parent pays £83 as their contribution to the gift.

If the teen is sharing with parents and there are only 3 rooms, then the cost should be equal between the siblings, unless they get a super fancy big room.

However, when we’ve been on holiday with my brother (as a couple with our kids), he has always kindly offered to pay more than his correct share as he is generous and understands how tight it is for us financially and is happy to share that burden as he likes holidays with us. So depends on sibling dynamic too!