What form must I file with SARS, an ITR12 or an IRP6 ?

If you are earning taxable income or in your case profit of less than R20 000 per annum, then you do not need to register as a provisional taxpayer and can submit based on your ITR12.

So it means that I will declare on my ITR12 my salary of /- R200K AND my rental income (under other incomes) of /- R5K.
When should I register asa provisionnal taxpayer ?

You would only need to register as a provsional taxpayer when or if you think that the taxable income or profit from the rental property exceeds R20 000 per annum. Once that happens you will need to register which you can do on your eFiling profile.

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The registration should be done ahead of the taxable year that I think I will reach the R20,000 income ?
I believe it would be for the 2014 season. So should I register in Aug 2013 or Feb 2014 ?
What happen if I register as a provisonal taxpayer, but at the end of the taxyear I haven`t reached the R20,000. Will I pay penalities for over-estimate my income ?

You should register now and then submit a provisional tax return IRP6 in August 2013 for the 2014 year. You won't suffer penalties for not meeting that R20 000 threshold.

One last question: my husband and I are married in community of property and consequently are co-owner of the mentioned apartment.
We will each submit the "other income" full amount on our respective ITR12.

In August 2013, should we both register in IRP6, and submit a rental income of R20,000? Or should we divide by two the threshold amount?

The rental income from the property must be divided between the two of you, so perhaps you won't even reach the threshold for provisional tax.