Well i have noticed that in almost all two (or more) storey buildings the staircase is just made out of wood more or less. You are allready guessing it, aren't you? What if you just destroyed the staircase with an axe or a sledgehammer? Especially in the houses that the stairs are just supported by some wooden pillars it would work 100% and your second storey would be out of reach of the undead for ever. When you have a more sturdy staircase foundation it would work a little worse and a little worse as the foundation gets stronger from case to case. You just have to make a (preferably retractable) wooden ladder or hang your sheet rope from a window. That's it. Bonus: you don't have to go around building (totally unrealistic as they are designed until now) wooden fences and stuff. Simple feature to design, totally realistic, really practical. What do you think?