Many people add additions to the home to gain more space but this is often much more difficult than remodeling a garage. Unless the garage is not a viable option, it should be considered. (Some local communities or associations require some form of off-street parking ,so check with local building officials before starting the project.
Also, consider whether to use the entire garage for your new room or only part of it. If you have a two-car garage, you might decide to finish only half of it into living space. Think carefully about how to manage noise from the cars in that case.
One big reason why remodeling a garage should be considered is because it will be less expensive (walls, the roof, lighting, etc. are already built) and it saves your living area (the actual house) during a long renovation process. So, before you add to your house for a living room, gym or home office, take another look at your garage.
DIY or Contractor?
As with all of your home improvement projects, think about whether to do the work yourself or hire a professional contractor. As always, that decision has a lot to do with your experience, your desire to do the work and the amount of time you have available. Many decide to hire a contractor for the more complicated work like electrical (never do electrical work yourself unless you really know what you are doing), plumbing, etc. yet do the rest themselves. Either way, you’ll find that the cost of converting your garage to living space is far less than adding space to your actual home.
The beauty of remodeling a garage is that structurally all that might be required is to close off the garage door opening and perhaps open up the wall between the garage and the house. You will likely have a cement slab floor and an adequate roof. And since an attached garage is usually close to the kitchen, it's a good candidate for conversion to a family room.
If there are no windows, you will need to decide whether to install them. Also, if your garage has finished walls you will need to determine whether the walls are insulated and, if not, determine how to do so. Also make sure you plan for heating and cooling and adequate lighting and electrical outlets.