Buildings you do not need to give a demolition notice for
- buildings under 50m³ in external volume; or
- a greenhouse, shed, conservatory or prefabricated garage that's part of a larger building; or
- an agricultural building - unless it's touching another building that's not an agricultural building; or
- an internal part of a building, where it's intended to continue occupying the building during the demolition, such as a house being refurbished while the family still live there.
For everything else, or if you aren't sure, ask for permission.