Hi StamperCat and welcome to the Forum!
It's sort of a debate with legacy challenges and their rules. The point of doing any kind of challenge is that you make yourself obey someone else's rules and not your own. That's why it's a challenge. While some people just change the rules to suit themselves, not everyone wants to do that. All of the suggestions here are good if you don't mind bending Pinstar's rules. But here's my advice if you're being strict.
The rules of
Pinstar's Legacy Challenge specifically state that your legacy family has to live on the same lot for the duration of the challenge. I've seen some houses on the Exchange described as being "legacy compliant" and I take that to mean that the house meets all of the restrictions of the challenge. I just looked at Pinstar's rules again and I don't see that it's forbidden to place a pre-made house on the lot. The problem I see with it, though, is how to technically do it. Your family is already living on the lot, right? So how do you place a new house there? The only thing I can think of would be to use Edit Town, put the family on the clipboard, place the house, then put the family back on the lot. I really think that would be a good way to do it and stay within the restrictions of the challenge. I thought that there were rules about certain items being allowed on your lot, but I don't see that in the rules now. As long as there's nothing that would give you a significant advantage over other players, it should be fine.
I hope this has helped you. We also have some very nice challenges of our own here. We have a
2010 Tournament going on that will have a world ranking for everyone who participates. A couple of the challenges have already closed, but there's still a few that are still active and one that just started yesterday. They're lots of fun and I hope maybe you'll join in.