ᐅ The floor slab is sagging by approximately 4 to 5 cm.

I am interested in purchasing a 30-year-old single-family house. The ceiling joist is sagging by about 4 - 5 cm (1.5 - 2 inches) over a length of 4 m (13 feet). On the underside, there is drywall, and on top, there are wooden floorboards installed.
What is the most cost-effective solution: removing the drywall from below to reinforce the joists, or opening the floorboards from above?

You should have a professional take a look at this. If the ceiling is sagging that much, I find it very concerning! What if you have heavy furniture or even a waterbed? In that case, you could be waiting for the ceiling to give way.