Basement Design Tips

Natural light and ventilation are very important in a living space and is often inadequate in most older basements. Surprisingly It's relatively simple & inexpensive to add, enlarge, or relocate basement windows - and the improvement can be dramatic.
If you have space in your yard you may even be able to add a door and stairwell for extra access. This is especially important if building a bedroom in the basement since it is required by law to have two exits.
Recessed lighting & perimeter light valances can provide artificial light without encroaching on limited headroom, while light colored surfaces help reflect this light, making the rooms appear more spacious and inviting.
Existing stairs can often be moved to a more central location in the basement to provide more convenient access. If the upstairs floor plan doesn't facilitate this, a small addition can provide a place for the new stairwell.
Consider opening up the stairwell with railings or a half-wall, and adding windows or skylights to make the stairs more inviting, and help integrate the basement into the rest of the house.
Sewer lines can generally be moved fairly easily by simple cutting and patching the basement floor. If your basement doesn't have sewer lines because the mains aren't deep enough, you can still have a bathroom or laundry in the basement with the addition of a "lift station" using a sewage ejector pump.
Use quality finish materials in the basement-at least match the materials in the rest of the house. This will avoid the stigma of being a second-class space.
If you wish you had a basement to finish, but have only a crawlspace, don't despair. It is often possible to dig-out the crawlspace under the existing house. Although this can be expensive, it is sometimes the best overall solution. In the same vein, if your basement ceilings are too low, try lowering the basement floors!
While beams, ductwork, and support columns can be problematic, they can usually be carefully concealed in soffits, walls, or built-in cabinets. If the existing location just won't work, they can probably be moved. Beams can sometimes be raised by "flush framing" to improve headroom. Likewise, replacing beams with larger ones, or reinforcing the existing one may allow support columns to be relocated. Ductwork, plumbing, or even furnaces can also be relocated if necessary.
Angled walls will add architectural interest and keep the basement from feeling like a box.
Give special attention to the heating and ventilation system. Proper ventilation will keep basements from feeling damp and stagnant.
If your home was built on expansive soil, special precautions should be taken when finishing basements over a concrete slab. Walls must be built in a manner to prevent movement of the slab from causing structural damage. Also, slip-joint details will disguise any movement, and minimize cosmetic effect.
Building standard 2 x 4 walls inside the concrete exterior wall allows crooked walls to be straightened, while providing enough space for insulation, wiring, and plumbing. It is often worth the small added expense and loss of space