A state grant is being sought to help senior citizens in Mattapoisett, Marion, and Rochester to make their homes handicap-accessible, said Mattapoisett’s town administrator, Michael J. Gagne, who proposed the idea at a meeting of selectmen from the three towns. The grant can be used to help seniors build wheelchair ramps, upgrade electrical and plumbing systems, fix roofs, and undertake many other projects that will help them stay in their homes. Gagne is former town administrator in Dartmouth, and said projects there cost on average $15,000 to $20,000, which can be prohibitive for seniors on fixed incomes. Gagne said he plans to make the application on behalf of the three towns by summer. If the money is awarded, he said, it would probably be administered through an existing agency, so that the towns wouldn’t have to incur administrative costs to run it.