May I suggest finishing the Thieves Guild quest line?
Spoiler You get access to their hideout, Dareloth's House, near Dareloth's Garden in the IC Waterfront. Since you become the TG guildmaster, the top room is yours with a decent amount of safe storage.
Comes fully furnished, even with a bed you can sleep in and it won't be trespassing. Did I mention safe storage too?
Be warned though, the TG quest line can be buggy. Don't fence more than you need to get the next TG quest started.