The very minimum headroom required (measuring from the 1st-floor plasterboard to the underside of your ridge) needs to be at least 2.4 metres for a Velux conversion, or 2.3 metres for a Dormer. If you have less headroom and your property is detached you can have a roof lift.
This option needs both a planning application and as always, Building Regulations Approval.
The thought of a roof lift may sound expensive and scary. The facts are, yes, this option is more expensive than the straightforward Velux or dormer, but only by 20%. Its more involved than any option we do, but you do not need to move out, the disturbance is no more than that of the Dormer option. Construction takes 2 weeks longer on average but weighing this against the benefits a huge roof space is now available to be used as a Master bedroom, possibly an en-suite and a dressing room. Or perhaps two bedrooms ? Or even a home office ?
The rooms use is only restricted by your imagination !