How to know if you should hire a professional.
Do you have time?
If a repair needs to be made immediately to protect people or property, such as fixing a broken step or unclogging a backed-up toilet, you should call a professional if you can't get to it right away. When timing is less critical, consider the size and scope of the work. A project that might take a professional a day or two could take you several weeks of working in your spare time (if you can find it). Weigh the inconvenience of living with a repair in progress against the cost of having the work finished quickly.
Tools and equipment.
Compare the cost of buying or renting any special tools required for a job, particularly if you don't expect to use them again. A professional has already invested in special tools and includes those expenses in his fee.
Be realistic about your knowledge and abilities. If you aren't sure how to make a particular repair after reading about, or if you feel unsafe tackling the job, it probably makes sense to have it done professionally. When appearance counts, such as with finishes, may be beneficial to call in an expert if you have no woodworking or masonry experience.
Selecting a Professional
How to choose a professional for your job.
For smaller jobs, hiring a handyman or someone on an hourly basis is fine; but for a substantial job—especially one that includes wiring, plumbing, structural work, or roofing— it's best to get a written contract. The contact should be very specific about what will be done and which materials will be used. Whoever you hire should have liability insurance and an air-tight contract that protects both you and the contractor.