Lowering springs are a direct replacement for the original springs. Over 30mm lowerings usually require dampers which are specially designed for lowering. Bilstein B8 is a good example of this, and is available from us.
Lowering amounts are always counted from the original height. Factory sports kits, like BMW M-sport or Audi S-line, reduce the actual lowering as the car is already lowered a bit.
The subcategories have more information about different brands of lowering springs. Our sales
is also happy to help!