The most important thing here is to set clear expectations - particularly how much time will eat up out of your relationship and for how long. I made this mistake when I tried to build my first startup and caused my relationship at the time to fail.<p>I never informed my partner that my side projects would take up days of the week, and because I didn't set up how much time it would take and when it would happen, I would tend to overwork on side projects and cut out more and more time until we spent less than 1/2 the time we used to together, and that, in part, caused irreparable damage to the relationship.<p>Now when I work on side projects or freelancing, I explicitly set my hour limits and when they happen so they don't infringe on my relationship, nor reduce the quality of my work. That expectation from myself has kept my personal life intact as well as my professional life.