Fire & general safety
To ensure adequate fire safety for the dwelling a new stair serving the new room(s) will be needed. Where there is not enough room for a full traditional stair, it may be possible to use a "space saving" stair. Retractable ladders or stairs are not normally acceptable.
For general safety reasons, there are specific criteria that a stair should be designed to.
Opening for new stairs
This would normally be formed by cutting away some of the existing ceiling joists between the existing habitable areas of the home and the loft-space. As these joists support the existing ceiling and restrain the pitched roof from spreading, replacement support should be provided.
This would normally take the form of timber "trimmers" around the opening, most likely to be at least two timbers fixed together (double trimmer) to ensure the load is transferred to remaining timbers.