Carpet area = $\text{Carpet area/flat} = 100m² \\
\text{Carpet area/floor} = 100*5 \\
= 500m²$

Total carpet area $= 500*5 \\
= 2500 m²$

Total built up area $= \text{C.A. + wall area + circulation area} \\
= 2500 + \dfrac{10}{100}* 2500 + \dfrac{20}{100} * 2500$

**Built up area = 3250 m²**

Cost of superstructure $= 12000/m² \\
= 3250 * 12000 \\
= 39,000,000$

Cost of foundation $= \dfrac{20}{100} * 39,000,000 \\
= 78,00,000$

Building cost $\text{= foundation + superstructure} \\
= 4,68,00,000$

Cost of WC $=\dfrac{5}{100} * 4,68,00,000 \\
= 23,40,000$

Electrification $= \dfrac{7}{100} * 46800000 \\
= 3276000$

Cost of sanitation $= \dfrac{5}{100} * 46800000 \\
= 2340000$

Overall cost = 54756000

Contingency $= \dfrac{5}{100} * 54756000 \\
= 2737800$

Work establishment $= \dfrac{2.5}{100} * 54756000 \\
= 1368900$

Final cost = 58862700.