I have a terraced house with stairs running across the middle in an open plan kitchen/dining room so it isn't possible to have a downstairs loo under the stairs. The only possible place would be to move the lounge door along in the hallway and take a small rectangle out of the lounge to put a downstairs loo in, with a door onto the hallway. This would take a bit of space out of a decent sized lounge, but would also make it an irregular shape.
I might have to sell one day, so am interested to know what would put you off more - no downstairs loo or a slightly smaller, irregular shaped lounge?