Does the building need building regulations approval?

Usually not below 15 square metres with no sleeping accommodation. Between 15 and 30 square metres it is generally still exempt provided there is no sleeping accommodation and the building sits either a metre or more from any boundary or is built substantially from non combustible materials. Above 30 square metres, approval is required. That said, we insulate and glaze every room to a proper standard whatever the floor area, because insulation is what makes the difference between a room you can trade from all year and one that empties out in January. Drainage and electrical work must meet standards regardless of size.