We had the same problem when extending and decided to go for an internal window. The downstairs loo is placed behind the the units with the cooker+microwave; we used pocket doors to make the kitchen entrance manageable.
That's the only floor plan I've managed to find - we've done this some 7 years ago now, and I can't find the 'proper' ones atm.
Hope this helps, but let me know if you need more details.
Q
