In a special election on Saturday, voters in Ayer and Shirley will consider a measure to spend $56.5 million on renovations to the Ayer-Shirley Regional High School complex off Washington Street in Ayer. The state has agreed to cover 70.25 percent of the total cost, leaving $19.4 million to be funded by the two towns. If the proposal passes, the average Ayer tax bill would be increased by $205 a year for 20 years, according to Scott Houde, chairman of the town’s Finance Committee. The funding is contingent on both towns passing the proposal. If one town rejects it, then in all likelihood another election will be held, Houde said. An endorsement by both towns would result in work beginning on the high school building, constructed in 1964, next summer, Houde said. Polling hours in both Ayer and Shirley will be 8 a.m. to 4 p.m.