Re-rendering turns into cavity wall tie failure

[Static]

What kinda cracks are you getting, horizontal, vertical? etc.. photos may help..

Cavity tie failure cracks are pretty obvious (especially in render) and most of the time are horizontal cracks every 1-2 block courses and pretty much always the most weathered side of the building..

Wall removal and replace should only be a last resort.. go Helix route first (as another has said) normally alot cheaper

 

[theoldun]

Charlie, I would also be very sceptical. Tell you why. Has he physically opened up the wall to see that there are no ties? Has he run a metal detector over the wall?
Has he had a endoscope in side the wall? Though they are not a lot of good if blown cav wall insulation inside. What about the front of your house and your neighbours houses, are they the same? Is it definitely cavity wall or solid?
You say you have a rear extension. Was this built at the same time as house, or is it a later addition. What size is extension and what type of roof, and are you being told that the extension also has no ties?
If there are no ties, which I doubt, or the ties are corroded and failed, it is no big deal to insert new ties either screwed or resin bonded.
With regard bedroom window, if head of frame on cavity work, is level with soffit, you do not need a lintel. Roof will be pitched on back skin, with hopefully a lintel or decent bressummer over opening Give me more information and will give you answers. old un

 

[Thermo]

i wouldnt be sceptical, id be laughing my head off. Load of old bollax. cavity wall ties can be replaced if they really need doing, with minimal disruption. Mind you that was another great industry that started up in the late 80s, like damp proof injections.