1. Whenever the board of supervisors shall determine it necessary to acquire additional lands or interests in lands or to acquire or to construct (1) water rights, wells, reservoirs or basins in order to maintain an adequate source of water supply, (2) water quality treatment units or devices and related apparatus and equipment, (3) additional trunk, interceptor and outfall sewers, pumping stations, sewage treatment and disposal works and appurtenances, street lateral sewers, or other facilities, (4) additional drains, pumping stations, or other improvements or to perform other work of a permanent nature such as dredging, widening or straightening of streams and water courses, or, (5) refuse disposal and incinerator plants, including all necessary facilities and equipment appurtenant thereto; or whenever the board of supervisors shall determine it necessary for the proper maintenance and service of such facilities to increase, improve or reconstruct the facilities thereof, including the acquisition of additional lands, or interests in land therefor, the board of supervisors shall cause a map and plan of the proposed improvement together with an estimate of the cost to be prepared by the county engineer or an engineer duly licensed by the state of New York. When the map and plan and estimate of cost have been completed, the board of supervisors shall call a public hearing thereon and cause a notice thereof to be published and posted in the manner prescribed in section two hundred fifty-four. Such notice shall describe in general terms the proposed improvement or the location of the lands to be acquired, shall specify the estimated expense thereof, and, if zones of assessment have been established in such county district, shall specify the proposed allocation of the cost thereof as between such zones. Such notice shall further state the time when and the place where the board will meet to hear all persons interested in the subject matter thereof. After such hearing and upon the evidence given thereat, the board of supervisors shall determine (a) whether it is in the public interest to acquire or construct the proposed improvement; and (b) if zones of assessment have been established in such county district, the allocation of the cost thereof as between such zones. If the board of supervisors shall decide that it is in the public interest to acquire or construct the proposed improvement, the board of supervisors shall direct the administrative head or body to proceed with the improvement in the manner provided by section two hundred sixty-two. In case the purchase of lands only is involved, the board of supervisors may cause such lands to be purchased for the district. In Suffolk county, notwithstanding any other general or special law to the contrary, a sewer district may construct excess capacity for the treatment and disposal of sewage and scavenger waste from outside the district if the district has one or more executory contracts with other districts, municipalities or corporations, public or private, that wish to avail themselves of such excess capacity.