Your question is a little confusing. A loft is usually an area open on one side to a lower floor. I would recommend send a question that is more descriptive.
The desk could go either way. I would think building a desk can run into a couple hundred dollars and lots of time topped with a mess to clean up. You should shop around in thrift stores and see if there is something you can make work or go through the local home improvement stores for ideas. Generally office style desks in related stores are over priced yet easy to assemble.

Far cheaper to construct your own. They are getting a fair bit of money for the loft beds such as the ones at PB carrying price tags of well over 1K.

There are many ways to make a loft on your own. This is preferable as you can set your height according to your bed height and persoal height and cieling. One of the easiest ways to construct a loft is using the Ikea cabinets and securing a board over the top to hold your mattress. Utilizing modular peices you can create an excellent space with no sawing. http://dornob.c… or http://www.fine… More simplistically you can use 2 (or more) fence sections and tie them together with 2 x 4 cross pieces and add a shelf under it for your desk. http://www.prow…

As far as an "U" shaped desk that's another animal. Used office supply and thrift shops can sometimes find you a great deal. A fast DIY Would be to support a top peice of plywood or MDF and laminate or paint the surface using file cabinets as a base. Don't overlook used kitchen cabinets as an option to create what you want. http://www.goat… http://winnipeg…

In the end you have to choose the look you want and see how you can most economically achieve it.

Take a look at http://www.coll… they have loft bed kits and plans.