Buying out partner in property in Belgium

I'm trying to work out the possible costs of buying out my partners portion of our house here in Belgium. I own 60% of the house, he owns 40%.
Does anyone know whether I would have to pay the 12% property tax when buying the portion of the house that is currently owned by my partner ? Would I have to pay 12% on 40% of house value, or 12% on 100% of house value, or nothing as it is my primary residence ?