Construction of a $20 million downtown Kings Park sewer system could start within two years, Suffolk County Executive Steve Bellone said Friday.

Bellone, speaking at the hamlet’s Long Island Rail Road station, also said he had directed Suffolk County Public Works Commissioner Gilbert Anderson to start planning for a system in downtown Smithtown by assembling an engineering and design team in the next six to eight weeks.

The projects hinge on $40 million in New York State funding Gov. Andrew M. Cuomo proposed for the town this week, part of a broader program for infrastructure investment across Long Island.

For Bellone, joined by Smithtown Supervisor Patrick Vecchio and local officials, the sewer projects are key to making “our region once more attractive to young people by building vibrant downtowns,” the hub for a “21st-century innovation economy.”

Sewers would allow for the construction of apartments and expansion of restaurants and office buildings in areas where a reliance on septic systems now constrains growth, civic leaders and businesspeople in Smithtown said this week.

“Without that, nothing can happen, there’s no economic redevelopment,” said Kings Park Civic Association vice president Linda Henninger.

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The association, along with the chamber of commerce, last year helped develop a plan calling for extending sewers and making other major changes to revitalize a downtown business district that has stagnated since the 1996 closure of the Kings Park Psychiatric Center, which once employed thousands.

Bellone also announced completion of a draft plan for Kings Park and downtown Smithtown by the Manhattan-based Regional Plan Association, handing a thick binder to Vecchio.

That document could influence land use decisions in Smithtown for years to come. Town planning chief David Flynn has said he hopes to analyze both before making any recommendations to the town board, drawing on a recent $200,000 planning grant from the county.