Comment by danybittel

I'd say these are essential complexities, they are features necessary for the client. Accidental complexity would be, if you for example, assigned the work on your new house to different "teams". Then the "guest room" team also build a drainage, or used a prebuilt drainage, not connected to the drainage the other team built.

To quote a quote from the article: "In my experience most of the complexities which are encountered in systems work are symptoms of organizational malfunctions."