DIspute over work taken on undivided share of land

Dear Experts, I own a ground floor flat in a 8 flat apartment. The flat is around 15 years old. I need some expert advice in dealing with a situation currently i'm having with one of the flat owners. To give you a background on the construction, there are 4 flats adjacent to each other situated in the ground floor with a common entrance/exit to the flat i.e if i need to reach to the 4 flat in the ground floor i will need to pass through the first 3 flats in the stretch. Over the years the public road outside the flat had been raised and now the flat entrance is now 2-3 feet below the level of the road. We had issues during the rainy seasons where the water from the road will get into our houses, so we had made alterations to the house by raising our basement to 2 ft along with the common entrance outside our flat. Out the 4 flats in the ground 3 flats have raised the basement. The issue now is the 4th owner has not raised the floor which incidently is the first flat at the entrance, due to this the common passage outside his flat is still below the road height (2-3') and due to this other residents are having issues entering/leaving the flat if there is water logged because of rains. We do not have a society or association registered. Over the past few days the other residents took the initiative to raise the common entrance of the 4th owner to the same level as the road so that we can overcome the issue with the water logging issue during rains. After 3 days works the owner of the flat has raised a police complaint stating that the alteration outside his flat to the common passageway will affect his flat ( he has 1/8 of the undivided share), he wants to pledge in writing that he will not be affected (Is it ok to give it in writing?). We have suggested the owner to raise his flooring too but he is not willing to do so. Due to the water logging senior citizens residing in the other flats are having issues, fever etc are plaguing the residents. Is there a way we can handle this situation legally?? Any suggestions would help.