Carpet area $\text{= Carpet area/flat1} = 1200 sqft \\
\text{= Carpet area /floor} = 1200*3 \\
= 3600m²$

Total carpet are $= 3600* 3 \\
= 1080$

Total built up area $= 10800+\dfrac{10}{100} * 10800 + \dfrac{15}{100} * 10800 \\
= 13500$

Cost of superstructure $= 850/ sqft \\
= 13500*850 \\
= 11475000$

Cost of foundation $= \dfrac{20}{100} * 11475000 \\
= 2295000$

B.C. = 13770000

Cost of WC $= \dfrac{5}{100} * 13770000 \\
= 688500$

Cost of sanitation $=\dfrac{5}{100} * 13770000 \\
= 688500$

Cost of electrification $= \dfrac{7}{100} * 13770000 \\
= 963900$

Overall cost = 16110900

Contingency $= \dfrac{5}{100} * 16110900 \\
= 805545$

Work charged establishment $= \dfrac{2.5}{100} * 16110900 \\
= 402772.5$

Final cost = 17319217.5