Piglet John, I need your expertise

[Slavetominidictator]

Have put in an offer an a house, been looking for four years: literally the only suitable house we've seen in those four years.
It's a Victorian cottage which has been extended and done up by a property developer. Has a stand alone double garage with a flat roof and the kitchen extension also has a flat roof. All of it is rendered (presumably to marry the two parts cosmetically).
I noticed the rendering had some bubbling and white patches on some of the rendering and we've just received the surveyor's report and it confirms there are problems with the rendering, including damp in the new kitchen extension and a significant leak in the garage.
I'm asking two renderers to inspect and provide quotes for the repair work and I'm also asking a roofer to do the same.
I do know the house was taken off the market in the summer to re do the rendering (presume just patching up). It went back on the market in September. If it's only been done for 3 months, does that mean the rendering was probably badly done the first time and needs completely taking off and re doing? Am getting conflicting advice and don't feel any of it is impartial. I don't know if I'm being unfair in thinking a brand new part of the house shouldn't have so many problems. The seller is keen to fix it himself rather than lower the price to allow us to do so. I'm not sure.

Apologies for the length of the op.

I would expect a property developer to maximise his profits ==> do a cheap bodge job.

Old loose render can trap water and cause damp, it is likely to be on random patches on the wall, not particularly near the floor or the ceiling. New render should not trap water at all.

Wet patches starting near the ceiling are probably gutter or roof.

If the extension is fairly modern (50 years or less) it is likely to have cavity walls so wet should not normally track to the inside surface, unless the cavity is running with water and it shows at the bottom of the wall.

If the extension is new it should not have any wet patches at all.

If the vendor was capable of doing it properly, he would have done. He has failed so don't let him have another go. Anyway, you would need to wait three months to see if the walls dried out after repair.

Go round on a rainy day and see if water is running off the roof or the gutters or round the windows. A flat roof should not leak for 20 years or so, but if badly built it might be ponding. Have a look from an upstairs window.

Look up the plans on the council's website and see if it has all been certificated as completed. There is a chance the BCO walked past during the build and leant against it to see if it fell over. BCOs tend to pay more attention to jobs that they are not happy with.

The extension is brand new. There is one damp patch and it is above the window in the kitchen extension. The surveyor said either it was from the roof or the render. It sounds like you think something that high is more likely to be from the roof. The survey report said all guttering was sound (and is brand new).
Is requesting that he deduct the price of removing old render and applying new and roof repairs from the price reasonable? My main concern now is that we are buying a can of worms and will just continually find ways in which he's cut corners to increase profit. Also, if we have the old rendering taken off once it's ours, if we find further problems underneath, we have no redress.
Thanks so much for answering by the way.

Oh and I looked on the council website but couldn't find the relevant section. Interestingly he put 'to be forwarded' in the legal documents relating to the sale about the planning documents.