This is one of my favorite shows to watch on HGTV. The host of the show Scott McGillivray has been turning spaces (mostly basements) into extra income for home owners by creating apartments for them to rent out.
From what I understand Scott has been doing this kind of work for almost 10 years, it all started out as a University project and has grown into a great business for him.
The show is called Income Property if your interested in seeing exactly what he does and it is on the HGTV channel.