How do I find out the exact address where my iPhone is? I know the area as I have the “Find my iPhone” app on iCloud.
As far as I know, there's no technical means of doing it. Sometimes, find my iPhone will be able to give you the position from the GPS coordinates (around 30-50 metres accuracy), and at other times it may be able to give you a more accurate location based on a WiFi hotspot, but there's nothing in the technology that would be able to track it down to a single household (in a crowded area). You may get lucky and get the precise address, or if you can take pictures remotely (e.g. if you've set it up with Dropbox to do that) then you may be able to tell from the pictures where it is, but again, it's not built into the technology - just a matter of luck.
Find my iPhone (as well as most other services) does not give you an exact address because it simply can't.
Locations are estimates and depending on the method used (GPRS, WiFi location data, triangulation, ...) aswell as 'the place' you are currently at it's accuracy can vary from 5m/15ft up to 1000m/over 3000ft.
On top of that they usually use cartographic material from third parties and therefor won't guarantee for their accuracy.
For Find my iPhone the situation simply is "the smaller the circle, the better the accuracy".