When building extensions, professionals differ in how the new build is or is not going to be fixed to the original structure. Nowadays, some designers require wall starters which are mechanically and / or chemically fixed to the original blockwork and the installation of a galvanised steel or UPVC movement bead at the junction between both buildings. Others just require the bead. Wall starters are quick and easy to use and allow for very minor settlement or shrinkage of the new blockwork. Of course, if the finish is in brickwork, constructing a seamless joint between both buildings takes a lot more time and effort.
Traditionally, some builders would tooth into the existing blockwork by cutting or breaking out alternate blocks and installing new blockwork to create a continuous course. However, this is very labour intensive and difficult to create a new bond with the original building.
If ever the extension was to fail, and of course the risk should be negligible, it would normally fail at this location. However, in this property, it appears the toothing in on the external leaf was completed so well that when the extension began to move it pulled beyond the toothing in and into the original structure. However, this extension is so poorly constructed, that demolition is the most economical approach.