You may not need Planning if you are using Velux windows, depending on their position, but you will need Bulding Regulation Consent. Depending on what you have there already you will need to upgrade the existing ceiling to foor joists and depending on how many stories it will become, you will need fire doors on each of the habitable rooms and one for the top storey depending on how many rooms you are having up in the loft conversion. Concider a product such as Tri Iso super 10, most Local Authority BC dept accept this with an additional 25mm insulation for the roof along with eaves and ridge ventilation. You may need steelwork for both rafters and floor depending on what you have existing and spans etc. Expect o pay £650 per square metre as a costing. But loft onversions can be done very cheaply some for way less than £10,000 especially if your doing it yourself....