Is it legal to tile the flower beds

The flat below has encroached the flower beds and balcony inside their flat, now they are having leakage issues as they did not make proper arrangements of drainage from flower beds above. They want to treat our flower beds, which are in original state with waterproofing and tile them with tiles and marble chips. We are okay with the waterproofing but area averse to any tiling or flooring of the flower beds as: 1. It will create drainage issues. Last 2 time we permitted them, the choked our drains. If the water retains, it will dengue and other unhygienic conditions along with pigeon droppings. 2. The tiling work creates dust and dirt in the flat. My son has bronchitis problem. 3. Every time they work, they mess up our AC units and packing to access the area. Last time their workers wiped hands on my raw silk curtains (dry cleaning costs me upward of 7000/-) The society chairman has called asking us to "cooperate"! Further asking us to share the cost of the same Isn't the tiling of flower beds constituted as encroachment? What is the legal view on the above.