Property law

Dear sir,

I wanted to ask we have a property in karachi, pakistan. It's under our parents name. Although property was purchased by the family members (4xsiblings). The property was 400 yard ground floor at the time of purchase.

Now one of the sibling is investing in the construction of first floor. All the design and construction cost was paid by him.

The plan is to sell the property by floor separately. Ground floor and first floor.

Argument is that the if the house was sold as purchased, it would have gone for higher price. Since now there is first floor it will be sold for much less.
Example: 400 yard ground floor selling price= 3 crore
After construction of 1st floor, ground floor = 1.5 crore and first floor 1 crore etc


Question is:
1. If the first floor is sold separately, will there be a share on other siblings on that?
2. Since the selling price of ground floor has gone down due frist floor, can sibling claim that cost?

Kindly advise.

Thanks