Splitting cost of holiday villa

[Flockameanie]

ok, not really an AIBU, but here for traffic. And maybe I am BU for not knowing the 'fairest' way to do this.

What would you consider a fair way to split the cost of this holiday accomodation. 3 bedroom villa (2 doubles, 1 twin). Guests are a family of four (2 adults, 2 primary-aged children, who will be occupying a double and the twin) and a single adult (occupying the other double).

Would the single adult paying a third of the rental cost be fair (i.e. based on bedrooms)? Or would it be fairer to divide by 4 or 5 even?

Our multi-family holidays are usually a lot more complex than this but we have an established formula.

50% of cost is split per-head with kids counting as half a person
So in your case that's 12.5% paid by single person and 37.5% by family.

50% is split by room so 16.7% paid by single person and 33.3% by family.

Total 70.8% by family and 29.2% by single person.