Most of the loft conversions you see in your local area are normally constructed in accordance with 'Permitted Development' regulations. All homeowners whose property is a house and is not located in a conservation area are entitled to construct their loft using this legislation. The main benefit of using 'Permitted Development' is that you require no formal permission from your local council planning department. You can simply start building without any notification or permission.
However, you can optionally apply to your Local Authority for a 'Certificate of Lawful' development if you wish. In this case, the plans for your loft will be submitted to the council, they will look over the plans and provide you with a written confirmation that the development is lawful and is in accordance with permitted development rights. As aforementioned this is not a requirement but can be beneficial if you want some extra peace of mind or if you are considering selling your property in the next 5 years, as sometimes solicitors can request a 'certificate of lawful development' and it will help speed the sale process up if you have it to hand from the outset.