The Velux conversion is the simplest way to add some extra space and light to your loft space, although not that common these days as most people will opt for the larger rear dormer or mansard extension. But It's simply a matter of our making some changes to your roof and installing the velux windows, which can be low enough to allow you to see out if, for example, you are planning an office with a desk.
There is no roof extension dormer to the rear and to flats and maisonettes you will not require planning permission unless you're in a conservation area.
They also tend to work better on the larger loft where the loft room already has a great deal of space and doesn't necessarily benefit from an extension.
There are some plans here for few velux conversion so you can see how they are usually converted.
More ideas on http://www.velux.co.uk