For all those seeking to extend their Iran visa in Tehran, note that the place to do so has moved. The address mentioned in LP Iran is now the ominous sounding "Foreign Intelligence Office". The new address is at the intersection of Soroush Street and Hamid Alley. Soroush is parallel to the main thoroughfare Shariati Street.
آدرس : تهران - خیابان شریعتی - نرسیده به معلم - خیابان سروش - پلیس اطلاعات و امنیت عمومی - اداره اتباع امور خارجه
Google Map location here http://maps.google.com/maps?f=q&;source=s_q&hl=en&geocode=&q=tehran&ie=UTF8&hq=&hnear=Tehran,+Iran&ll=35.725141,51.445745&spn=0.003954,0.010986&z=17
1. On entry you may or may not be searched and your mobile phone may or may not be taken from you.
2. Enter building on the left, there's a man at the front desk handing out relevant forms. You'll be handed two visa extension application forms and a pink folder in which they are stapled, also a small invoice to take to Bank Melli. You may or may not be asked to pay a small amount for these.
3. Passport size photos and photocopying can be done on site. You'll need two photos and a copy of passport front page and current Iran visa page.
4. The closest Bank Melli is less than 5 minutes away - walk back up Hamid Alley and turn left. It's on the opposite side of the road. You'll need to deposit 200,000 rial.
5. Complete two copies of visa application form and hand to the guy who gave them to you at front desk - along with photos, photocopies, pink folder and Bank Melli receipt. He'll check that they're complete and staple them all together.
6. Walk to the end of the hall (you'll recognise it when you see it) and complete security check - very serious guy wearing very fancy gloves, sitting alone in office. He'll look you up and down and hopefully sign off at the bottom of the visa application forms.
7. Submit forms at relevant desk in the hall. On the day I was there, only one was open and there was a long queue. If you ask nicely (perhaps showing a ticket for when you'll be leaving Tehran) the visa extension can be issued quickly - mine was done in <45 minutes.
8. While you wait, there's a good juice place opposite Bank Melli.