Some projects around the house may be a do-it-yourself way to improve the house while enjoying the use of knowledge and tools to perform maintenance or upgrades. Some people don't enjoy such chores and prefer to hire a contractor. For those people who do in fact enjoy such things, there are still times you need to hire a contractor. Sometimes the need is based on the fact a particular job requires a specialty tool which is cost preventive to buy for a single use, other times there may be a job you aren't familiar with or fully understand how to conduct, or it may be a matter of not having the time to do it.
Building a New House
Building a new house always requires the services of a contractor. Even if you've worked in construction and are capable of building it, the contractor knows about current codes and the licensing requirements, and is going to have an easier time doing the job right the first time. The contractor is also going to be versed in current building techniques and know which materials provide the best value. There is also the parallel issue of whether or not you can take time off work to build the house in a reasonable time frame or spend seven years working on your days off to build a house.
Refurbishing an Older House
Remodeling an older house has many of the same restrictions for the homeowner as building a new home, with a few exceptions. Most homeowners aren't going to be able to take time off from their regular jobs to completely gut and rebuild an old house, but you may be able to schedule a long weekend to repaint. Most older homes fall in between, and there's nothing wrong with hiring a contractor to conduct the bulk of the heavy work while leaving the minor details for you to finish.
Hiring a Contractor
Once you decide to hire a contractor, there are a few things to look for after interviewing at least three different companies. Obviously the lowest estimate is important, bu there's more to the final decision. Of course you expect their license, insurance, and worker's comp to be up to date. Other than that, also consider how they describe the work, whether or not they seem excited to do it, and determine who you think is going to do the best job.