I don't think there's much you can do. If your game needs 3.5 GB on the process monitor, that's pretty much all the game can get (being a 32 bit application).
Are you building this mansion in a existing game ? If so, you can try saving it to the bin and then start a new, fresh game that does not have any bloating and place it in there to finish building.
Also if you have memories turned on, try turning them off. There's also ways to make the save games smaller but I can't talk about those here but you can google Deleting SNAP resources FAQ if you're interested in that solution.
How large is your savegame ?
It's about 30.5MB. Is that a lot?
