The one I have is the Ferplast Sin Rabbit sized house - it is just one big space inside. It's the same dimensions as the Leif house but without the compartments - and it is quite a lot taller, so it can sit on the base of the cage, be surrounded by substrate outside the house (and I also put a couple of inches inside the house so there isn't just bare floor) so you can leave the house where it is, part buried in substrate, and just take the top off. The door is so big, that being part buried means it is still big enough (or you can scoop a bit of substrate out near the door. I got the Rabbit sized one because the potty fits inside. The guinea pig sized one is an ok house as well but not big enough for a potty too. The rodipet one Jen has is a better size than the guinea pig sized Sin house. The Ferplast one works out a bit cheaper (cheaper from Porton than Amazon). But rooms in the houses, like the Rodipet one, are a good idea too - but Charlie is a big hamster and I thought the height of the Rodipet ones would be a bit low for him.
Sin 4646 Wood House Rabbit