The simple answer: Read your rental agreement and contact your property owners to get permission before any semi-permanent changes to your living space. Permission is better (and often cheaper) than forgiveness.
The long answer: Some things are more negotiable than others, and there are some creative solutions for most of the projects that you would want to create in your apartment.
For example, let’s say that you want to paint the interior of your bedroom walls. In most living situations, this is unlikely to happen. Painting even sections of the wall isn’t something property managers want to deal with, especially if you spill paint onto carpeted floors. One clever solution to change the color of your walls is to put up curtains instead. Screwing into studs isn’t nearly as permanent as painting, and a quality, colorful fabric is going to look a lot better than the hippy tapestries that cover the walls of freshman dorm rooms. Along the same lines, covering most of the space with individual framed paintings can achieve a new sense of color for the walls of your room. Painting large swaths of butcher paper is a cheaper and more “hip” way to achieve the same goal.
Light fixtures, door handles, appliances, and furniture are often free to alter or replace, or at least very simple to put back to how you found it. Changing the construction of the room itself, however, is obviously out of the question.
Or is it? There are some ways to change the dimensions of a room that can drastically affect your experience in the space. One simple, elegant way is with the use of a room divider, which can easily bisect a space. Suddenly, a desk in the middle of the living room almost becomes a room unto itself, and offers a sense of privacy. Fabric room dividers are light and adjustable, allowing for quick changes when company comes over.
Another way to change your apartment is with lighting. Many people overlook how powerfully lighting affects our mood and energy. Tinted lights, like sepia or blue lights, are an easy way to control how a shade of color is perceived, turning an uncomfortable green-yellow into a more soothing gold.