Q1: As per Jno, who has given you a reasonable size indication. But if the roof space is pitched, the triangular section of that roof has to be calculated - it isn't the gross height of the pitch that counts as the height.
Q2: As per Jno.
Q3: Building Regulations are entirely different from Planning Permission and you will always need BR approval for this sort of extension. BR is to make sure that you construct the Works in a strong, safe, hygenic and insulated manner. It involves either submitting detailed drawings of the way the construction will be done, or applying for a Building Notice, when the inspector will physically come around at key stages to see what has been done. The cost is around �300. There is no need to waste money on agents - unless you mean to do drawings - builders you use will be used to working with Building Control.