I'd stick a loo under the stairs for downstairs. And then put new shower room in the space where the two cupboards are upstairs above the front door (between the two big bedrooms) - need to steal a bit of space off at least one bedroom and make some sort of hall to get to it not thru a bedroom (or maybe access from both bedrooms, make it a jack and Jill bathroom?). I think George Clarke did a similar house on one his programmes that I binged during lockdown - they wanted to be able to access all three bedrooms individually, rather than having to go thru one to get to another (and he added a shower room above the stairs).

Looks like an awesome house - any chance of seeing a photo of the exterior?? I prefer the bathroom upstairs being between the two bedrooms on the right. Looks like there's already a shower there so just need to expand on that and get soil pipe over there too. I prefer it as it can then serve those two bedrooms easily - perfect kid wing! Looks like those rooms are a couple of steps below the rest of the upper floor and I'd always prefer to have no steps between bedrooms and bathroom. Could always make the existing bathroom an ensuite to Bed 2 and use that as the main bedroom if that was important, but I'd personally put two kids in the right wing and keep main bathroom for parents and guests (or third child?). Always nice to keep the box room for an office too - especially now! Downstairs, if you have budget to the extension you show then I do think that would be lovely - and plenty of room for kitchen, dining and small seating area. And having a huge utility is a real luxury so I'd be tempted to keep it if possible - then it'll be laundry, bootroom and surplus storage. Could really cut down on the size of kitchen you need if you can store lots of stuff in the utility out of sight.

What's "possible" is commonly limited by the amount of money you'd like to spend. First things first ... to have a bathroom, you need both a soilpipe (drain) and water supply pipes. The drains must be connected to your main one, which from your floorplan looks like it is at the back of the house, in or near the current bathroom. So wherever you place the bathroom/ensuite, the drains must run from there. The water supply ... there's a "cupboard" marked upstairs. Is that empty or does it actually contain a hot water cylinder and/or the boiler ? In the kitchen, where is the sink ? I can basically think of two options (sizes of components not "true" only indicative for placement): One is a (really) small ensuite, given you'd have to take the drains to the rear, most likely for the rear bedroom. The space indicated there would re-use the existing cupboard; if that contains a hot water cylinder and/or boiler, this choice would mean you'd have to replace the heating system first (to a mains-fed combi boiler then to be located downstairs). You'd probably need a pocket door as this space is only about ~0.8m x 2m internally, and a door swinging inwards would take too much space. The downpipe would go through the ceiling into the kitchen below and probably be boxed in along the left side of the room all the way to the rear. Second is a "normal" bathroom, around 1.5m wide and 3m long. For this the pipework could be laid outside, on the rear of the house. It'd fit a 700mm x 1700mm bath. You'd use a normal door. Neither would, in the simplest case, have a window. You therefore would need to provision for a fan / extractor. For the centrally-located, probably straight up through the main roof, in the rear, just through the wall. Or, for the rear, have a new opening made and a window fitted (my recommend if you go for this, it makes a difference in so many ways). It all depends what your situation is wrt. to existing pipework, and what preferences are. Talk to a planner / bathroom installer ?

If you were open to losing some space out of your large bedroom then its the obvious place for another bathroom or even an ensuite. Not sure how the plumbing would work though or if it would? Where is the soil pipe? Do you have a pic of the current bathroom? I'm avoiding your stair question as I cannot think of anything clever at the mo. :) I'll mull it over and come back.