The Montgomery County planning board said last week it wanted to "rap the knuckles" of the school board for submitting plans for a new school building within a flood plain at the 11th hour.

"I don't see how the public sector feels it can go into a flood plain and break the law when no one else is allowed to," said planning board member Mable Granke after the board reviewed plans for the South Germantown Elementary School, off Cinnamon Drive, at a meeting last week.

Plans for the school, which is scheduled to open in the fall of 1986, show a corner of the building protruding into the flood plain of a creek known as Gunners Branch. Flood plains are protected from development by county law for environmental reasons.

However, the planning board has only an advisory capacity in the matter because the school board has final authority over the building of all new schools, said John O. Matthias of the planning staff.

He said the school board had already informed the planners that it was not going to redesign the building because it is from an architectural blueprint for elementary schools that the board uses over and over again to save money.

And because the lot is so small, the school board feels it cannot shift the building's position on the nine-acre site, he said.

"I want to write a letter to the school board and object to the attitude on their part," said planning board Chairman Norman L. Christeller when informed of the school board's intent at the planning meeting. "It should rap their knuckles for not adhering to the flood plain.

"I want to make a strong case to the school board that we ought never be put in the position of getting something at this late date with them telling us it's too late to change anything, even though it goes against the law," he said.

The board voted unanimously to send the letter.

Robert E. Shoenberg, chairman of the school board, said this week that he couldn't comment on the letter because he hadn't seen it. But he said the school board rarely gets involved with the technical details of a school-site plan.

"We rely on our staff to work those things out," he said. "But if a problem is brought to our attention, we will certainly see what we can do about it, even at this late date."

The flare-up comes at a critical time, since the two boards are currently working out a controversial policy dealing with new schools and residential development.

The County Council has asked the planning board to deny new development in areas of the county where schools are filled to capacity. The rule involved is the county's Adequate Public Facilities Ordinance, which allows the county to prevent development if there are not adequate public facilities -- such as roads, sewers or schools -- to accommodate new residents.

Several planning board members have been reluctant to deny development solely on the grounds that schools are filled to capacity because they say they don't think the decision would hold up in court if a developer sued.

For the past several months, the school and planning boards have been meeting to discuss creating a set, equitable policy that the county can put into writing.

Although the talks have been described by both sides as enlightening and amiable, the two boards still have deep-seated differences about the policy.

The planning board, concerned about possible legal actions over a decision they say is largely judicial, believe the policy should be detailed, with each proposed development denial backed up with extensive studies and school enrollment projections.

Meanwhile, school board members have said that such exhaustive studies would consume too much of their staff's time. They say the county has the authority under the Adequate Public Facilities Ordinance to deny new residential development if the school board decides that the system cannot handle any new students.

So far, the planning board has voted to defer only one application for a subdivision because of school capacity, and the developers of several other subdivisions have volunteered to defer their cases until a policy can be worked out.

Members of the two boards say they hope to have a policy finished by the end of the summer.