Adding the family bathroom to other side of loft

[Gilmoregirly]

We already have one side of loft converted to a bedroom with staircase. Do you know roughly how much would it cost to add family bathroom with a velum window to the other side of the loft. We just need to add 4 stairs from the existing loft staircase to the other side of the loft. Thanks in advance.

Is there a bathroom or waste pipe below where you want to add the bathroom? Do you want a shower and a bath? Is there a window opening currently there?

No although our current bathroom is under that roof space which is not currently converted.

we need a bath, although not a shower. No windows as it is currently unconverted part of roof.