Sims builds not saving

Hey! So, I'm trying to build a house for my Sim's family, but every time I save and leave the game, and then come back to the same lot, I find the lot has reverted to its original build. Weirdly enough, it saves any and all lot name changes and lot traits I add, just not the build itself. What I'm doing is placing lots from the gallery to use as bases, so I was wondering if that's the issue. Is it not saving because they're gallery builds, or is it something to do with mods and/or cc?