what to build a second floor on my single extension

appoint a architectural technician or Architect (depends on your budget) to prepare plans and apply for planning permission ( get a price for this.This will 0vary a lot from £1000 to £3000) Plus council fees. With planning consent then apply for building regulation approval find a good builder and get a fixed price for job (subject to provisonal sums for unforseen items etc) also your architect schould be able to give a price for His own work. Get a final costing before starting and allow in your financing a buffer value of say 5%-10% for difficult alteration work or items that could not have been forseen.