The two main forms of construction at present are building free form on semi-flat ground or on a foundation. Structures that can be placed directly on the ground usually decay with 1-2 weeks of not being maintained whereas structures placed on a foundation can last a month before they start decaying.
Basic Shack - Not much bigger than an outhouse, the absolute basics.
Small Shack - A little room to stretch but still quite small.
Shack - Largest freestanding structure
Deck Foundation - Basis for all foundation buildings. Basically a platform on stilts. No longer comes with stairs. Ramp or stairs must be crafted separately.
Shelter - Smaller building requiring a foundation.
Metal Shed - Small and secure but requires metal.
Large Shelter - The largest structure available that can be built.
Structure Stairs - Used for multi-level buildings.
Basic Shack Door - The only door that fits the basic shack.
Wooden Door - Fits most structures and can be destroyed.
Metal Door - Fits most structures and provides more security and cost of more expensive materials.