The repairs are extensive. You assess your situation and determine that the fix is rather complicated. It's one thing to repair a leaking pipe or snake a clogged drain, and completely another to lay long sections of pipe or design the plumbing system for your new add-on recreational room. It's important to know your limit, even if you're confident in your skills.
Your house is older. If you've purchased an older home that has not been maintained for some time, any visible repairs that you fix now could lead to more complicated problems in the future. It's best to get your old house the professional treatment it deserves.
You get a reasonable quote. Given the difficult nature of some home plumbing projects, if you receive a low estimate from a local plumbing service, save yourself the hassle and cut them a check. You'll fix the problem and get to your kid's little league game this weekend.
You need gas line work. Any work on your gas lines is dangerous and complicated, requiring an array of special tools. This work is usually required by code to be done by either a certified gas technician or a licensed plumber.
You have water or sewer main problems. The mains are connected to your public water supply. While doing work on these lines might not require a licensed plumber by code, you might want to get professional help so that if anything does go wrong, your neighbors can blame the certified plumber and not you.
Keep in mind that all home plumbing projects, big and small, are very time-consuming and require a lot of patience. For all the jobs that are too much to handle, find professional plumbers in your area on Kudzu.com.