Top-down approaches
A "top-down" approach involves designing for the least functionally capable users (those with the most severe impairments), and then trying to make the resultant products more mainstream-friendly.
Design approaches such as rehabilitation design and design for disability correspond to top-down approaches and the products developed are typically classified as assistive technology that is they provide remedial assistance to restore or replace a functionality capability. The exact definition of what is 'assistive' is also a topic of hot debate. The traditional perception is that assistive products are effectively medical devices, for example crutches or wheelchairs. Others would argue that anything that enhances the user's capabilities is an assistive technology, for example a microphone when used to address a large audience could be classified as an assistive product.
Companies who have explicitly recognised the need to develop more 'inclusive' products most commonly adopt top-down approaches, because they are conceptually the easiest to understand and appreciate. They also come as close as possible to guaranteeing that the least able users will be able to use the product. If done well, the products and services developed using this approach should be so fundamentally accessible that other people outside of the initial user group considered should be able to use them.
The most commonly cited example of transferability of benefit is that of kerb-cuts. However, as with the double-decker bus example such designs do not benefit all users, and may actually present an increased inconvenience to those users not addressed explicitly by the designers.