I have no affiliation with them, just a user for the last 3 years- yes, you give Marc Sevitz full access to your data and power of attorney. (He is one of the founders: a CA and Practioner on Record as required by the Tax Act).
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You still have full ownership and control of your profile. They merely comb over the data and ensure it's optimal. They are incredible IMO, my only annoyance point being weak answers to questions sometimes when you send them in or them asking you to pay for a basic answer
It's the percentage of the office in relation to the entire building. So in my instance, the office is an outbuilding approximately 7m x 1.5m attached to a main house of 69sqm. Since my office is 10.5sqm / 79.5sqm (total building area) - I get a percentage of 13%. Claiming for it, for the first time has almost tripled my return. The proof will be in the pudding but I have everything ready for them. From the HR letter/proof to the invoices paid -I want it all. My partner's been retrenched since 31 July 2020 so it's quite needed
Check out TaxTim's calculator if you wish.