I don't think our characters will be so far back that they forget how to build a simple wooden two story house :P
Yes they will. For a "simple" two story house you need to known how to cut wood, make nails, make tools, known how to use all those. For a two story house you would also need to known how to make a pair of stairs to your second story and above that you need known how to build in properly without it collapsing duo to weight (especially the second floor!)
Let's put it this way. How many people on these forums would known how to cut wood, mine ore and refine it, then make tools and nails and use those to make a one story wooden building?
Seems to be more realistic though that people find pieces of wood, put then next to each other in a square and put over it a blanket, or find a tent someone left around.