In the course of our kitchen renovation in 2011, we closed the interior staircase to gain more space upstairs. That left us with a void space downstairs that we have now turned into two new closets. We used the space to add a utility closet for our rental suite, and we also added a locked storage room for our own use, access from a new door from the common room. Rich and I did the work entirely ourselves, and it only took about a week (including an entire long weekend devoted to the project!).
Part 2 - we needed to close the existing coat closet in order to install a door to create a proper common room for the shared laundry with our rental suite downstairs.