Sims 2 PC ? please please help me!!?? double delux?????/?

ok i putt a staircase in and i putt in the interior foundation.......... inbetween the found dation and the staircase there is a gap where the sim cannot get up..... i dont know how to fix it.......... i see that you hafto put a "mini" staircase in but it wont let me PLEASE HELP!!!!!!!