Commonest location of Spigelian hernia is at:

Spigelian hernias can occur anywhere along the length of the Spigelian line or zone--an aponeurotic band of variable width at the lateral border of the rectus abdominis. However, the most frequent location of these rare hernias is at or slightly above the level of the arcuate line. These are not always clinically evident as a bulge and may come to medical attention because of pain or incarceration. Ref: Schwaz's principle of surgery 9th edition, chapter 35.