Moving a toilet and converting a garage

[badgerread]

I wondered if anyone has done anything similar and if so, what sort of costs were involved?

I'm thinking of converting the tandem garage so that the front half is a normal garage (there is currently a sink in there as per the floor plan) with my washing machine, tumble dryer and chest freezer in. Then the back half (putting up a stud wall with a door to the garage) turning it into a kind of den for my DC's with a TV, sofas, games console etc, putting a couple of roof lights into the flat garage roof.

I'd also like to move the toilet to make the kitchen a bigger space. Could I potentially move it across the hallway using some of the utility space without encroaching too much on the den space? maybe the other side of the new stud wall (marked in red)? there is currently a shower tray and plumbing in there as the current owner has a dog grooming business and the other side is used as beer brewing!

Obviously it depends on soil pipe etc, but I'm just looking into costs at the moment..