How would you add a bathroom to this floor plan?

[Lazerquest]

My partner and I are in the process of buying a new house (great timing I know with Brexit round the corner!).

The house is perfect, it is a period property with large rooms and a lot of downstairs living space - but it only has one bathroom and a downstairs toilet. At the moment this is fine as there is just the two of us, but with time we will have children and one bathroom seems a stretch for a 4 bedroom house.

How would you add another bath/shower room in this situation?

I agree if you can easily access it from outside.

Though you have to put up with teenage boys (and DH) wandering round the house in just a towel because they weren’t organised enough to bring clean clothes downstairs before they went out.