The sims 3 PS3 - Building foundation.?

Basically, ive built this house. I built the walls around. Then i raise the terrain so it was the same height. It was the same of how a basement is made but higher up. But i have to build a garage coming off onto the terrain that i raised. I knew that i must of place foundation around the walls and terrain to make it all one. But every time i try to place foundation near the walls, it comes up 'can't level terrain'. I lowered the area around so it was the way i started, walls on flat surface. But it still comes up 'can't level terrain'. This is really starting to do my head in, as i've been trying to sort it out for a few hours now. Please help, it's bugging me !!!! :/