The formula to calculate the carpentry skill needed to build a house is the number of tiles + the number of outer walls - 5. For example: a 4 by 4 wall has 16 tiles and 16 outer walls so it would take 27 carpentry on the ground/first floor. For multi-story houses the formula is the same but there's an additional requirement based on the number of floors.
===Doughnut Houses and Other Non-standard Houses===
Some styles of housing, like doughnut houses (a house shaped like a ring, with a hollow center) or other non-square shapes, may require a higher carpentry skill that these numbers give, because of interior walls required. Make sure you account for interior as well as exterior walls in your calculations. It may be easier to plan these kinds of houses row by row instead of all at once, if your carpentry skill is not high enough. Planning all outside walls, then inside, may work for a doughnut house as well.
== Adding a Roof ==