Translation:My apartment is small, however I have everything that I need.
"That" can introduce an explanatory dependent clause. "What" cannot. "That" indicates a specific quality of the object in question, or a subset of a larger set. "What" indicates the object in question itself, or the entire larger set. (you can write: "I have what I need", but not "everything what i need")
And it's not "float", it's "flat" :)