Hmm, I think the idea of having an outdoor staircase is a little far off and I'm pretty sure that the sims would just high tale it back into the main house during the worst part of the seasons of being too frightened. Have you tried to use the stair case tool and bending the stairs inward on themselves and placed them in the space where your current laundry room is to make it go into the basement from the inside so you could renovate it that way? That's what I would have done but if they still act up, why not just make a new addition to the main level?