There are two types of stairlift. Straight and curved. The first is designed for straight stairs. The footrest of lift will normally stop level with top stair. It may be possible to use a straight stairlift on some configurations of curved stairs.The second, and generally far more expensive type, is a 'made to measure' or 'curved track stairlift' which can travel around bends on stairs and can be made to run on to landing. Prices vary according to length of stairs, number of bends and complexity of bends.
It is worth noting that although far more expensive, resale value is negligible, as rail is made to measure and can only be re-installed on an identical staircase.
Curved track stairlifts can be fitted on either side of stairs, layout of your staircase will determine which is most suitable. If you need a curved track stairlift you will need to pay a deposit to company that you are dealing with. This will normally be about 30% of cost of lift. The deposit is required because company are committing themselves to building a stairlift rail that will only fit into your house. You should not have to pay a large deposit if you order a standard straight lift as these can usually be adapted to suit another staircase.