In Franz Kafka's The Metamorphosis, the fact that the entire story takes place in the Samsas' apartment has a strong influence on the themes of the story.
There are several themes in the story—among them are: father-son antagonism, betrayal, isolation and self-sacrifice, escaping, and seizing power.
Just as Gregor becomes a prisoner in his room, the rest of the Samsa family has also become imprisoned in their apartment. While Gregor is able to provide for them, they have all they need and are satisfied, rarely going out. They live quietly, entertain each other and need not worry about anything. When Gregor goes through his transformation, we can be sure the apartment (and what goes on there) has certainly contributed to this sad change of events in Gregor's life. Whether he actually turns into a bug, or simply feels like an outcast whose life is valuable only in terms of the money he earns, is not important. The change in the characters due to Gregor's transformation drive the major themes in the story.
For instance, once Gregor changes, there is no way to escape the apartment. Whereas we learn early on that Gregor would have greatly appreciated some leisure time for his physical and mental health, being confined to his room destroys him. Where going out everyday had taken its toll, his only escape now will come with his death.
Father-son antagonism and seizing power are themes that are blatantly seen in the story: because Gregor cannot work, his father becomes antagonistic. Though Gregor's "malady" forces Mr. Samsa to get a job and provides him with a feeling of increased self-worth, Gregor's situation is an excuse for Mr. Samsa to torment Gregor in any way he chooses. Mr. Samsa's newfound independence does not lead him to show any appreciation for all of Gregor's work over the years, but provides the father with a sense of superiority by which he need not worry about Gregor any longer because Gregor no longer brings them money. This apartment is now Mr. Samsa's "kingdom," and Gregor's part in providing for his family all these years does not now guarantee him a safe refuge from the world. Gregor's father sees himself now as the dominating force within the apartment. He even drives out the lodgers.
Gregor's situation also allows Mr. Samsa and Grete to seize power in the family. Mr. Samsa is violent and abusive toward Gregor. Grete, who starts out being Gregor's only visible and active supporter in the house, changes her attitude. She does not want to have to care for Gregor, but enjoys the power she has over him, and even over her mother. This sense of self will allow Grete to convince her father that they must get rid of Gregor. The power struggle erupts because Gregor cannot leave the apartment (connecting this theme to "escaping").
The theme of isolation and self-sacrifice pertains primarily to Gregor. In all the years he has been working, he has be isolated from people at work because he travels, and self-sacrificing and isolated at home because he works long hours and days, unable to connect in a meaningful way with his family. The apartment contributes to joining the other members of the family into a separate unit that Gregor cannot belong to.