Demolition of first floor of two storey rear part of building and erection of a replacement first floor and an additional two floors above to create new second and third floors at the rear, erection of a rear extension at main roof level to house an internal staircase, change of use of the replacement rear first floor, second floor and third floor level building from office (Class B1a) to residential (Class C3) in connection with the creation of 1 x 2 bedroom, 2 x 1-bed flats and 1 x studio self-contained flats; associated external alterations consisting of the removal of a window and the addition of 4 windows to the first and second floors and the addition of a rooflight to the third floor of the western elevation of the front part of the building; the removal of an external security grill to an existing first-floor rear window.