Hi there and thank you for your question,
I will try to assist you with your legal question but please feel free to ask as many follow up questions as you like until you are 100% satisfied.
Depending on the wording of your sale agreement, and depending on whether the support structure is material or not, you may be able to cancel the sale agreement. You might however be bound by the sale agreement but will be able to recover the costs of rectifying the problem from the seller.
There are some excellent articles here which are exactly on point!
Building plans and voetstoots
The importance of obtaining approved building plans from the seller
I should also draw your attention to the fact that IF your sale agreement is subject to you obtaining a bond, and you want to get out of the sale agreement, then you should TELL your bank (whichever institution is going to provide you with a bond) that there are no plans for the house, and that there might be defects in the property. The bank will then most probably refuse to give you a bond, which would then mean that the conditions in the sale agreement won't be fulfilled, and the agreement would fail. You would then be excused from the sale agreement!
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Good luck and best regards,