I think the materials required depend on the shape of the plot & height of the perimeter wall. Secondly, we need we to cater for Site Clearance, Levelling Works and Foundation Excavations. We also have to carry-out Anti-termite Treatment on the foundation bottoms & sides (With Termidol), we need more Kalittunsi for Setting-out and Kirundu for the Scaffoldings & Formwork. We also need Steel Bars, Stirrups and Binding Wire for the Concrete Columns. Lastly, we need to water-proof the plinth wall, both internally & externally. So, your materials & labour charges are highly underestimated!