If you own two houses in France and one sells at a loss and the other sells at a gain, can the loss on one be set against the gain on the other?
no, as far as I'm aware you can't off-set one against another - you'd need to be a company SCI style to do that and even then I'm not sure it's possible. I've paid plus value on one property I sold and paid it there and then at the notaire's when we completed (even though I didn't receive a penny from the buyers - long story...!). I wasn't living in a house I sold but was registered there for everything financial so we got round it that way and no-one asked. If you're an auto-entrepreneur then change your address to the place in question and if you can wait and pay there at least once - actively get into their system - then, from experience, that should be enough. As I say, it rather depends on the notaire in question and you need to pull the wool over his/her eyes or get them to turn a blind eye. Typically French, I know, but the only way to get ahead is to work the system in your favour and when you see day-in-day-out all the cassos I see each day and what they get out of the system whilst you're constantly paying in with very little out, your scrupples soon vanish into thin air!
now that's why I said 'if the situation isn't too complicated' ;-) bit of a bumber. I managed to work it the right way because I knew I'd be sellling both and had all my financial stuff registered at one whilst I actually lived at the other (aveyron and hérault so not exactly next door to each other!) Try the 'that's where I work from but I live in the other one...! Some notaires won't ask too many questions but you'll sign to say that's the situation.
simply declare your main residence as the one you made a plus value on, provided the situation isn't too complicated...!