I bought this 3 bedroom duplex with I sitting room since 2019 ,but the upper part is opened which means they didn’t deck the upper part . And we are a family of 5 (3 children) . So I am thinking of covering the upper part (deck it ) and make it a sitting room to create more space for the family . The present design of the interior is a mordern style which allows the upper part to be a total waste of space and I am thinking of turning the space to a parlor probably for the children . I want to know how much it will cost me to deck base on the present cost of building materials and is it advisable to convert the empty space to a sitting room or I should just leave it the way it is .

ordovicianonly:
I bought this 3 bedroom duplex with I sitting room since 2019 ,but the upper part is opened which means they didn’t deck the upper part . And we are a family of 5 (3 children) . So I am thinking of covering the upper part (deck it ) and make it a sitting room to create more space for the family . The present design of the interior is a mordern style which allows the upper part to be a total waste of space and I am thinking of turning the space to a parlor probably for the children . I want to know how much it will cost me to deck base on the present cost of building materials and is it advisable to convert the empty space to a sitting room or I should just leave it the way it is .

kindly check this thread and chat the architect on WhatsApp, he will guide you on what to do, the guy is good https://www.nairaland.com/6672464/building-designs-construction-services

Nice choice, close the void

ordovicianonly:
I bought this 3 bedroom duplex with I sitting room since 2019 ,but the upper part is opened which means they didn’t deck the upper part . And we are a family of 5 (3 children) . So I am thinking of covering the upper part (deck it ) and make it a sitting room to create more space for the family . The present design of the interior is a mordern style which allows the upper part to be a total waste of space and I am thinking of turning the space to a parlor probably for the children . I want to know how much it will cost me to deck base on the present cost of building materials and is it advisable to convert the empty space to a sitting room or I should just leave it the way it is .

It is very possible for you to modify as you choose but you must get a Registered Structural Engineer to do a redesign based on an Architect modified plan (that will ensure that ventilation is not compromised) .
Based on the Architects modified plan you should get an cost estimate for the works.
Also you must be prepared to have a Registered Civil Engineer on site (everyday) to supervise the Artisans.