Dareloth's Basement is the downstairs area of Dareloth's house. The guild headquarters will be open as soon as the Hero receive the Gray Cowl of Nocturnal after "The Ultimate Heist" has been completed.
Most of the Thieves Guild members can be found inside such as Methredhel and Amusei. There are plenty of bedrolls and rooms. It is considered the barracks of the Thieves Guild. Downstairs is also a training room and a small room where a few lockpicks can be found.
Upstairs in Dareloth's house, which is barred at the front door, are the Guildmasters quarters and a living area. Here one can find some of the items stolen earlier during the quests. Standing on the bookcase is the Savilla's Stone.
The guild hall becomes your house after you complete "The Ultimate Heist."