I always write original material but this is the first time that I found such a valuable writeup that I am simply providing here a link to an article written by someone else. This article, posted by the IRA Financial Group, describes the UBTI rules for flipping in a self-directed 401(k), also called a Solo (k). It is difficult to find well-written material on this particular topic that is crucial for people who intend to or already started flipping in their SD 401(k). I recommend that you read every word of this article, then read every single word again. Pay special attention to the phrase "regularly carried on."