If you want to make the most of your loft with a master suite, second living room or home office, you can generally do it without needing planning permission, as long as the conversion is less than 40m³ for terraced houses, and 50m³ for semi- and fully-detached houses. There is also provision for dormer windows, but they must not sit higher than the highest part of the existing roof, or extend forward from the roof plane on the principal elevation.