Make a list of needs that are unmet by your space. This can include storage, utility, aesthetic, or space qualities.
Make a second list, but list the endearing qualities of the space. What do you love, or feel an attachment to, that you do not want to lose, such as a view, a favorite room, or an original fireplace.
Establish how far you are willing to go. Are you willing to part with items, are you prepared to endure the work and expense required to build an addition?
Establish a budget for the project, and where the money is coming from. Are you working with a set cash budget? Is there equity in your home? are you banking on increasing the value of your property to create equity?
Establish what your personal style and design vision for the space is. Websites like Houzz.com and Pinterest are great resources for inspiration, and let you sort images in an easy to view and organize way.
Put together a PLAN.
This means layouts, drawings, finish selections, fixtures, lighting, paint colors, furniture.... and get ready for the next step!
MAKE IT HAPPEN!
Hire the contractor, put down the deposits and pack everything away. The hard part is done... now to just sit back and make it happen!