"And under here, Hedwig - [...] - is where I used to sleep! You never knew me then - blimey, it's small, I'd forgotten..."
-- Harry Potter (DH4)
On the ground floor of 4 Privet Drive, there is a cupboard under the stairs; this was Harry Potter’s bedroom for about 10 years. The cupboard is very small, containing only a bed. It is always full of spiders (PS2). The Dursleys often punished Harry by locking him in his cupboard for days. After Harry’s first Hogwarts letter arrived, Vernon burned it, but he allowed Harry to move from the cupboard into Dudley’s second bedroom — not out of compassion but out of fear, since the letter had been addressed to Harry in “the cupboard under the stairs.” Vernon and Petunia were terrified to learn that their home was under wizarding surveillance:
“Vernon,” Aunt Petunia was saying in a quivering voice, “look at the address — how could they possibly know where he sleeps? You don’t think they’re watching the house?” (PS3)
The Dursleys locked Harry’s school things in the cupboard after Harry’s first year at Hogwarts, so Harry couldn’t access them during the summer (CS3, PA2). The lock was no match for the lock picking skills of Fred and George, however (CS3), nor for the magical force of Harry when he was angry about Aunt Marge:
Harry tore from the dining room before anyone could stop him, heading for the cupboard under the stairs. The cupboard door burst magically open as he reached it. In seconds, he had heaved his trunk to the front door (PS2).
When Harry visited his cupboard for the last time in 1997, he saw that it was then used by the Dursleys to store shoes and umbrellas (DH4).