Building Regulations approval from your local council may be required for any of the following (among others):
Structural works in general, removal of walls, loft conversions, extensions, installation of electrical works, installation of gas-fed appliances, installation of mains-fed water systems, glazing replacement, & staircases. You must get the required tradesmen certificates & 'signing off' from the council if you wish to avoid difficulty selling your house in the future.