An attic is a small space found directly under the pitched roof of a house or other building. Often times, the attic can only be accessed through a trap door that deploys a ladder. These are acceptable for a temporary base and are typically unreachable by the common undead and sometimes unnoticeable by the common bandit. Scavengers, however, will most likely find an attic if they search well enough.
Attics, if only accessible by ladder, are unreachable by zombies. If a window is included in an attic, it may be used as a lookout in a time of need. An attic can quickly become a temporary safe-haven against enemies. In larger houses, some attics may provide a fairly large and easily securable space for a survivor or two. You might be able to access the roof via an attic window, which is a second exit.
The small space may not be enough room to store supplies or shelter more than a couple of humans. Also, if trapped in an attic by a siege, it is impossible to open the trap door to see if the coast is clear without being spotted. An extended stay in an attic can become very messy, very quickly. Remember to bring the necessary equipment up there too. There may be zombies in the house, and they may wander into the house after you set up in the attic. Overall, an attic may be useful as a temporary safe house, but consider looking for a more suitable shelter as soon as possible.