Cracks in k-rend - something serious or nothing to worry about?

Try to check the structure and fundation, if there s a Weak structure do some underpinning 🫡

A 12 year old building would have needed a structural warranty, and building control approval as well as a designed foundation. However the structural warranty may have run out (10 or 12yrs generally) and if this is the case the buyer is buying generally at their own risk. The surveyors job is to point out potential faults, and the worst case result of those potential fault, which they have done. The most logical course of action here is further investigations to assess the severity of the cracking, the cause or causes and the likeyhood of this getting worse. This case sometimes be referred to as a certificate of structural adequacy and is generally issued by a specialist structural engineering company with adequate insurance.

Suggest unless there is an insurance backed guarantee on the property guaranteeing it's structural integrity that you should move on. If there is a problem with the foundations, then underpinning is expensive, and sometimes if on clay soil you can cause more problems unless the whole property is dealt with, otherwise you'll likely find you'll just move the cracks to different areas. A lot of buildings suffered from clay shrinkage this year and suffered from various cracks where the building foundations hadn't been taken deep enough to avoid the ground shrinking and then expanding. Assuming the current owners put everything right, they would likely have to involve insurance companies and that would affect your insurance premiums in the future and also sale value (likely even if underpinning was carried out properly by a properly insured firm). You note this was a self build, self builds can go disastrously wrong unless the owners employed a suitably skilled and qualified project manager and site manager. You only have to watch television programs to see that. Having said all that, if you still wish to buy, speak to the building control inspector (hopefully the LABC, but if a private inspector, still in business), they will have knowledge of the building works and the site inspections carried out. They will also have knowledge of the local ground conditions which could be a starting point. You need to employ a qualified structural engineer before proceeding and may need to ask the seller to provide inspection pits to get an insight of foundations and/or open up some of the external and internal finishes to allow adequate inspection before they will issue any certificate of structural adequacy. There is a faint possibility that the K-rend may have been applied in very hot weather which caused the cracking - check the south facing elevation in relation to the North facing elevation. As with most things you buy solicitors would caveat buyers beware - any survey you receive may have those caveats and any insurance you buy may also have caveats. Any other purchaser will see the same problems and most will walk away. Even if the vendor offers to pay for surveys and put the work right himself I would be very careful, if there are problems with the self build already I wouldn't have confidence in any remedial work so carried out.

A possible clue is the lightweight materials - if aircrete block work was used in the outer leaf and was wet when the render was applied then the amount of shrinkage in the block work would be greater than that of the render thus causing cracking. This would tend to be fairly even width cracks rather than narrow at the bottom and widening as it goes up. The latter may indicate a more serious problem. If the former, then cutting back the render and inserting some helical ties and rerendering would likely sort this out. Usually there will be a couple of courses of brickwork at the bottom which is unrendered. If the cracks do not extend through these two courses then the problem is likely to be just shrinkage.