An encroachment of 150 square meters certainly sounds like it was done intentionally. It is unlikely that any of the people involved in the construction of the property would not have picked up on the fact that the building is not within the confines of the erf. There may be other things that point to an intentional encroachment as well that I am not currently aware of.
I would assume that the HOA would have people tasked to make sure that the owner and the builder comply and ensure that the building is in the correct position. IF there is something to that effect in the constitution or other documentation of the HOA, then you would hold a claim against the HOA as well.
If the person or entity you bought from knew of the encroachment but failed to inform you, then you might have a claim against the previous owner as well.
Your remedies at the moment is as follows:
(a) You can either strike a deal with the owner of the encroaching erf whereby he will buy the 150 square meter from you and it can be formally added to your erf.
(b) You can take the owner of the encroaching erf to the High Court and apply that he demolish the part of the building that is encroaching. If, on a preponderance of probabilities, it is established that he did this on purpose, the court will most certainly grant your application/action.
(c) You can accept the situation and sue the previous owner or the HOA or both for your damages that is caused by the fact that you property is in fact 150 square meters smaller than what you thought you bought.
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