Douglas County commissioners plan to increase the county’s share of property taxes by 9.12 percent for next year, now that they’ve closed the books on discussions for their 2011 budget.

The increase would be 2.993 mills — enough to add $51.63 to the property tax bill for the owner of a $150,000 home. One mill equals $1 in property tax for every $1,000 of a property’s assessed valuation.

Commissioners had started their initial budget negotiations with a proposed 15.9 percent increase, which would have added $90.22 to such homeowners’ property tax bills.

Nancy Thellman, commission chairwoman, said that the majority of e-mail communications she’d received during the past few weeks argued that “no new taxes” should be approved by the county.

“But each one of us knew, and stated in the beginning, that this was not possible,” Thellman said. “Everyone at this table has looked for ways to save, but also to invest.”

Commissioners plan to review their decisions during coming weeks, as they prepare for a formal public hearing Aug. 18. That hearing is set for 6:35 p.m. at the Douglas County Courthouse, 1100 Mass.

To help get to the lower increase, the budget endorsed Tuesday would:

• Cut proposed investments in economic development and open space efforts. Thellman and Commissioner Mike Gaughan agreed to pump $350,000 into each effort, down from the $500,000 for each that had been included in County Administrator Craig Weinaug’s draft budget. Commissioner Jim Flory opposed those decisions, favoring instead to provide $100,000 for economic development, and nothing for open space and heritage efforts.

• Give county employees a chance to share in merit pay increases, ones that would average 1 percent.

• Use $200,000 in money generated by a one-cent countywide sales tax — approved by voters in 1994 — to help offset the increase in property taxes.

“As difficult as this economy is, I think we, as a community, need to keep an eye on the future,” Gaughan said.

Flory said that the decision to finance open space, heritage efforts and economic development came at the expense of core services, such as law enforcement, roads and bridges. So he plans to vote against the entire budget for next year.

“You broke it,” he told fellow commissioners. “You own it.”

But approval of the $69 million spending plan is assured, with support from Thellman and Gaughan. Thellman noted that various cuts would allow the county to finance hiring:

• A new psychiatrist for Bert Nash Community Health Center.

• A new prosecutor to focus on driving-under-the-influence cases.

• Case managers at Douglas County Jail, to help released inmates keep themselves from returning.

“Those are all core services,” Thellman said.