If you are not intending to alter the plane of the roof nor add dormers, PP is not usually required. Depending upon the status of your bungalow, detached, semi, terraced, etc. you will probably be able to carry out the works under 'Permitted Development Rights'.
If you are in a conservation area, the nature of any roof-lights will have to be agreed with the Planning Department.
You will need Building Regulations Approval (this will also help when you come to sell the property as it will confirm to any prospective purchaser that the works have been carried out properly).
Get yourself an Architectural Technician (we are usually cheaper than Architects) to discuss your plans. The building ought to be of robust enough construction to support any additional load...............however, the arrangement of the internal supporting walls may determine how difficult/expensive the project will be.
Also be prepared to lose space downstairs to accommodate the stair..............