Before it can be finished, your attic must pass some important tests for structural adequacy and comfort. Here's what to look for.
Grab a flashlight and take a peek up there. If you see a network of W-shape trusses supporting your roof, forget about it. You could probably build an addition more cost-effectively than remodel this type of attic. If, however, you find rafters that form an A shape to support the roof -- with open space underneath the rafters -- you've got a potential remodeling candidate.
Figure out how you're going to get up and down from the finished attic space. Enlarging an existing stairway or adding a new one will affect spaces below. A straight-run stair will chew up 10 to 14 feet of floor space; a stair designed with an intermediate landing and two runs needs about 8 feet on the lower level. Spiral staircases take up only about 5 feet.
Most building codes require that a living space be at least 7-1/2 feet high over 50 percent of the floor area. (Consult your local codes for specifics.) Keep in mind the thickness of finishing materials when you calculate headroom.
Consult a heating and air-conditioning contractor to evaluate the cost of routing (or rerouting) ducts. You'll also need to insulate.
If you have 2x4s, you'll need to reinforce them to carry the new load of people and furnishings.
If your attic plans include a bathroom, try to locate it above a bathroom on the floor below to reduce costs and avoid the need to cut a hole in the room for a new vent stack.