I wouldn't go less than 1200 width if you can avoid it. Depends on the scale of the house, eg a small apartment 1000 may be suitable, a larger, more grand home, you'd want it to be wider to be in proportion to the rest of the house. Without seeing the whole plan it's hard to comment too much - I'd hand it over to your Architect or Designer to come up with a few options :) A couple of ideas maybe are to make the other side of the shower (facing the vanity) glass - so glass 2 sides. That'll make it feel bigger than a solid wall. Also a wall hung vanity will give the illusion of more floor space and make the space feel bigger. All the best!

I love your idea of extending, but understand your concerns re over-capitalisation. I think your budget will need to reflect how long you plan to stay in the house, and whether you consider the enhancements to your lifestyle as valuable an investment as the actual financial cost vs possible capital gain. Have you thought of getting in a real estate agent to discuss what effect any potential extensions or changes will have on the value of the property? I agree with MB above that the laundry is under-utilised space, and using the entry space would also be a very cost-effective solution. A way to really modernise the home with less expense than a large extension and new ensuite bathroom would be to close off the door between the existing bathroom and toilet and add a toilet to what would now be the master ensuite. The laundry and current toilet could become the family bathroom, and the kitchen could be extended very slightly by pushing out the exterior wall to create a scullery with back door, which could be closed off from the kitchen with a cavity slider. If you pushed out the laundry/bathroom wall to match, you could add a bath, too. Best of luck.

my suggestion includes a free standing or set in bath up to 1800 x 800 (the "overhang" would be a feature not a restriction) and any gap between the set in bath and the walls can be filled in with an extended tiled ledge, twin 900 x 500 or an 1800 wide double vanity with matching mirrored wall cabinet, a 1200 x 800 shower and the toilet tucked behind the door with a generous window over the bath, subject to view and privacy, a window down as low as the bath with plantation shutters for privacy could work very well in the alcove

Thanks Siri.... having it drawn to scale certainly highlights the inefficiencies in my idea unless compromises/sacrifices are made. Your banquette dining might work in lieu of a normal dining table Siri? Either way, I think a walk in pantry in this house demonstrates that space will be a problem. But good for AJ to see that as they explore a range of options before potentially seeking out a pro :)