The Basics

 The Standards states the following about recessed doors:

In a nutshell, what this is saying is that the 18" minimum clearance required at the pull side of the door, or the 12" on the push side of a door with a closer and a latch is not required if the door is recessed no more than 8".  You can use the face of the wall as part of the maneuvering clearance.

If on the other hand, the recess is larger than 8", then you have to provide the additional 18" on the pull side (for example) 

The US Access Board has come up with some animated guidelines that are pretty good.  One of the is all about door maneuvering (including recessed doors).  Here is the link.  I recommend them. 

Sometimes in small spaces, doors might have objects within their floor space.  The recess door section allows for objects, such as grab bars, to be located within the maneuvering clearance at the door as long as it is not more than 8" deep.  The section's advisory explains that these objects could be considered a "recess" and are allowed to be within the maneuvering clearance.

Advisory 404.2.4.3 Recessed Doors and Gates. A door can be recessed due to wall thickness or because of the placement of casework and other fixed elements adjacent to the doorway. This provision must be applied wherever doors are recessed. 

This figure shows an object that is less than 8" deep next to the latch side of the door.  This would be considered a recessed door due to the object and the face of the object would be allowed to be used as the maneuvering clearance

The grab bar in the photo above is in the way of the maneuvering clearance of the door at the latch side.  But because it is less than 8" deep, it will allowed per the recess door section.

This trash receptacle is less than 8" deep and therefore is creating a recessed door condition.  The 18" on the pull side can be taken from the face of the trash can.*

*Even though the advisory states that an object within the maneuvering clearance can be considered a recess, there are some jurisdictions that don't allow any object within the maneuvering clearance of the door and do not consider it a recessed door.  Check with your regulating agency for their ruling.