Micler, I owe you an apology and an extra point on your score. I've been having trouble with my computer crashing when I play Sims, so I moved to my daughter's gaming computer so that I could look at and play in all the contest houses without being booted out every couple of minutes. Anyway, I thought I had installed all the free stuff needed for the houses on her computer, but somehow I missed the set with the balloons. Because I didn't have them installed, my game replaced them with something else.. the rose stand which is wider at the base then the balloons and therefore blocked the stairs! I just went and installed the balloons and reinstalled your house and now there are balloons at the bottom of the stairs and my sims can easily walk up the stairs. If I had paid more attention to your photos I would have seen the balloons and picked up on the problem earlier, but I prefer to live in the houses and explore them that way rather than relying on a few photos. I'll go and adjust the overall scores now to reflect this finding.
Again, sorry for the mixup.