U.S. Home Corp., the nation's largest home building firm, announced that it has opened North Creek, a subdivision of California-styled town houses in Germantown. Ronald Garshag, vice president for marketing for the U.S. Home division in this area, said that the new houses, priced from about $40,000, have a "thermo-proof" system for energy conservation.

Karras Realty in Columbia, Md., announced this week that it now is charging a straight fee instead of a commission for services in selling houses. George Karris, a broker in Burtonsville for more than 10 years, said that he has joined the National Association of Real Estate Service Agents, which helps owners sell their houses. Karras said his firm will charge of fee of $1,195 for every residential transaction, regardless of sales price, with an advance fee of $95 to cover expenses and the balance after closing. Real estate commissions in this area normally range from 6 to 7 per cent of a sale price.

James Z. Rouse & Co. announced that State Farm Mutual Life Insurance Co., Bloomington, Ill, has agreed to make a first mortgage loan of $3.2 million for the Great Falls Shopping Center that Western Development Corp., plans to start later this year n Great Falls. Safeway and People Drug stores will be anchors of the 84,000-square-foot shopping facility.

KRB, Inc., a Seabrook-based real estate development firm headed by Michael Rose, has announced the start of a 700-unit development of single-family houses and town houses called Woodstream in the Seabrook-Green-belt area of Prince George's County. Rose said that KRB will develop an 18-acre park on the 165-acre site and donate it to the county. Model houses will be shown next year.

The Prince George's County chapter of the Women's Council of Realtors will meet for lunch Wednesday at the Fireside restaurant in Greenbelt. Offiicers will be installed by Kenneth Woodring, new president of the Prince George's Board of Realtors.

A day-long seminar on how to make rental apartments make money will be sponsored Oct. 27 at the Ramada Inn in Rosslyn by the area chapter of the Institute of Real Estate Management. Elton M. Young, chairman of the program, said that registration will be limited to 125 persons. For informations, call 474-6207.