The list belows includes the necessary resources to build a basement.

• Read the equivalent "In the basement" magazine.
  
• A hammer and a shovel.
  
• 1 doorknob
  
• 2 door hinge
  
• "Small" - 20 planks, 30 nails, carpentry 3.
  
• "Medium" - 40 planks, 60 nails, carpentry 6.
  
• "Large" - 60 planks, 90 nails, carpentry 9.

You can build a basement in any enclosed area such as a house, building, etc.
Unfortunately, player made structures do not count.
There can only be 1 basement per enclosed area.

Once you have at least a hammer and a shovel, if you right click an available structure (See above) a new option called "Basement" will appear in the context menu.

It will allow you to create a hatch for whatever basement you want. You will also find a list with all the resources you need to build each hatch there. Place the hatch anywhere you'd like and just get in! It's that easy