I read a lot of the Nepal forums and I get asked all the time how to find long term accommodation in Kathmandu. It is difficult being a foreigner and looking for an apartment because there are basically no real estate agents in Kathmandu. The ones online will generally exploit the fact you have no contacts here and try to charge too much in areas no one wants to live in.
It took me a month and I was beginning to think I would have to live in Hotel Potala forever. In Nepal rooms are generally rented through word of mouth. It is hard to believe but I found my apartment through facebook.
I changed my status to ‘is sick of looking for an apartment in Kathmandu’ and 24hours later I was standing in my new home. This will only work if you are a member of a number of Kathmandu facebook groups or you have friends living in Kathmandu. I recommend joining Internations, a network of expats living in different cities and providing information to each other. There is a very helpful Kathmandu group.
Also try to post on travel forums, here on Nepal Travel Blogs, or the KTMKTM Google group is also really effective. Travel forums will help you connect with people wanting to rent their rooms or looking to share an apartment.
Ask everyone you meet, waiters, trekking guides, the laundry lady, EVERYONE! This is Nepal, everybody knows a guy, who knows a guy, who want to rent a room (This rule also applies to finding a trekking guide, buying a motor bike, selling a scooter).
There are plenty of options available, one bedroom, two bedrooms, fully furnished, non-furnished, inverter, no inverter, no electricity, security guard, water, no water, shared, single, executive. I would look for furnished because finding a bed was a huge hassle for me and pretty costly (check out Leknath Marg or towards the Sorakhutte Taxi Stand. If you are really stuck you can get a really nice wooden bed for around 10 000NR, but you need to bargain).
Temple Tiger Apartments have a nice complex with a cute restaurant, on the edge of Thamel. Rooms are $US900 a month. If you want convenient, modern and expensive this is the place.
Thamel Apartments are pretty similar, nice furniture, good location but maybe a bit less expensive.
12000NR – 10000NR in Lazimpath or Thamel will get you a one or two bedroom apartment with kitchen, usually furnished. For this price you should expect a very reasonable apartment. I was offered a one bedroom apartment, fully furnished, with generator, Wi-Fi, security, and in was on a quieter street in Thamel, for 12000 NR.
10 000NR will easily get a two bedroom apartment unfurnished in most areas in Kathmandu.
I went for a 7 000NR two bedroom, unfurnished apartment with kitchen and bathroom in Pucknajol, behind Thamel. There is no inverter. Water comes out of the taps but it is not suitable for drinking, so we get a jar delivered every few days for 45NR. There was no carpet or curtains, or stove but there is a big security gate with dogs.
For 2 000NR -4 000NR you should get a room sharing an apartment in a nice area.