You should expect your builder to include the following points in his estimate to you. If you are not sure what is included in your estimate, make sure you ask for a full breakdown. You need to know what you are paying for and its helps to work out a payment scheme.
Open up access to loft space.
New staircase.
Up-rate joists, to take the extra weight.
Remove trusses where needed and up-rate existing trusses
Fit windows, dormer or velux.
Construct internal stud work.
1st fix of electrics and plumbing.
Lay flooring.
Insulate within new stud work.
Fix plasterboard.
Skim ceilings and walls.
2nd fix of electrics/plumbing.
Building inspector visits.
This will leave the conversion ready for decoration.