The District of Columbia Wage-Hour Board yesterday ordered a $1.00 increase on Sept. 20 in the minimum wage for retail employes in the District to $3.50--which is 15 cents more than the federal minimum.

In its unanimous decision, the board also set two exceptions to the new minimum wage. It permitted retailers to pay $3.35 to youths under 18, and said that employers who take on extra help during this fall-Christmas shopping season will be allowed to pay those workers $3.35 until Jan. 3. Next year, those workers must get $3.50 an hour.

About 20 percent of the 29,000 retail employes who work in the District will be affected by the minimum wage increase, officials at the District's Employment Services Department estimated.

The three-member Wage-Hour Board, which sets minimum wages for District workers in nine different work categories, last established a retail minimum in 1976 at $2.50 an hour. Retailers with annual sales of less than $362,500 must pay their workers the D.C. minimum; those with greater sales must pay the higher of the D.C. minimum wage or federal minimum wage.

Representatives from both labor and retail management were pleased with the board's decision to set the new minimum at $3.50 but expressed disappointment over particulars of the wage order.

James Lowthers, special assistant to the president at Local 400 of the United Food and Commercial Workers, said the board's decision to set up a special youth minimum at $3.35 "is not one that we greet with particular enthusiasm." Lowthers said he was also unhappy with the holiday shopping exemption, saying "we oppose any special rates under the minimum."

The manager of the Greater Washington Board of Trade's Retail Bureau, Leonard Kolodny, said he wanted the board to put off the wage increase until the first of next year to provide retailers relief from "very, very soft" sales this year.

He said that, because the board's special holiday season minimum applies only to new employes, it will not afford the relief that local retailers need.