Rooflight Velux windows and easy to install and normally only require one of the existing rafters to be trimmed out per window. The Velux windows are installed to fit flush with the line of the roof and leave the existing roof structure untouched. As they do not require extensive alterations to the roof this option helps keep the cost of the conversion down. As the loft is not extended beyond the original roof line planning permission is not normally required (you should still check with your local planning department before proceeding with any works)
A velux type loft conversion works very well for lofts where there is a good amount of headroom or if there are constraints on planning, for example if you live in a conservation area. If headroom is going to be limited then either a dormer or mansard conversion may be a better option. Velux rooflights can be fitted quickly and easily meaning there is minimal disruption or delays due to bad weather.
As Velux windows are installed at the angle of the roof rather than vertically like a normal window they can let in a surprising amount of light. While this is great during the day and will give you a light and airy room it can become a problem at night or in the summer. Window blinds are available from various sources tailor made for Velux windows that perfectly fit into the window frame. You can even get blinds that have a thermal silver backing, great for retaining heat in the winter and keeping it out in the summer.