Trek permits of some form are necessary for most treks in Nepal. Check www.timsnepal.com for the latest requirements, since they do change. They are issued at the main office in the Bhrikuti Mandap Tourist Service Center, as well as, at the time of writing, a new TIMS office in the Annapurna market in Thamel. At the time of our visit, this office was completely empty of tourists, so the process should be Quicker here.
There are two types of TIMS. The Blue one is issued to trekkers organizing the trek through a Nepalese trekking agency and costs us $ 10. The green one TIMS is issued to independent trekkers and costs US $30 per person.
In addition, some restricted areas require special permits that must be obtained by a registered trekking agency from the immigration office. Such areas include Upper Mustang, Nar-Phu, Upper Dolpo, Kanchenjunga, Manaslu& Tshum Valley, Rara Lake, and Lapche Valley. Check this at Bhrikuti Mandap or online as they vary from time to time.