Are you the owner of a $100,000 house and happy as an oyster because you paid less than $40,000 for it in 1971?

Now make a guess about the total market value of all the privately owned single-family houses in the nation. It's an amazing $2 trillion, according to a report from the economics staff of the National Association of Realtors. That's right, $2 trillion!

It is impossible for most home owners to get a mental handle on a figure that large, but the NAR points out that it represents one-fourth of the entire wealth of the nation. Possibly even more surprising: One-third of all single-family houses are owned free and clear. More than 43 million U.S. households own the houses in which they live, and an additional 12 million houses are owned as second homes or investment properties.

Two old Capitol Hill town houses renovated into seven apartment units in the 1960s are now being redone for conversion to condominium ownership by Three R Inc., a restoration-renovation-rehab firm based at 1729 21st St. NW. Richard Naing is the redeveloper and seller. Prices of the units at 101-03 2d St. NE range from $75,000 to $134,000.

If you've driven or walked around the Spring Valley shopping area on supper Massachusetts Ave, you may have noticed that W.C. and A.N. Miller has started a group of 11 detached houses and 24 towns on a previously dormant 3-acre triangle. Already there are sales contracts on nine of the single houses-the higest for $247,000. The rest will be priced above $250,000 . . . Georgetown University recently sold a commercial property at 1722 U St. NW for $1.5 million to George and Pota Sembekos . . . Bernard Fagelson and others, as trustees, recently sold a small office building at 1025 Vermont Ave. NW to Leo Goodwin Jr. . . . A gasoline station has been razed at the corner of 30th and M streets in Georgetown to make way for the construction of a large hotel, according to realty street talk.

Mortgage insurance friends and others who knew him from his long career with former Sen. John Sparkman (D-Ala.) and later as chairman of the Federal Home Loan Bank Board recently honored John E. Horne. He has moved in semi-retirement (that's a euphemism for consulting when you're not vacationing). Horne had been serving on the board of the Mortgage Insurance Companies of America, the trade group for private mortgage insurers, which last year handled a higher mortgage volume than did FHAVA. . . . And Maurice P. Foley, former electrical contractor and now head of International Realty Consultants Inc., said he has broken ground in St. Petersburg on the first phase of a condominium complex called Hidden Bay Villas.

Murray Cohen, 37, and William Witte, 41, now are building Potomac-priced contemporaries and traditionals in Potomac. Both studied construction in college. Cohen got his start in building with Levitt & Sons in Chicago in 1969 with houses priced under $40,000. Witte joined Kaufman & Broad (another volume building firm) in Chicago. Later he, too, was with Levitt. Cohen and Witte have been building in this area since 1977 . . . Gladys Leming, recently installed for a second term as president of the Maryland chapter of the National Society of Professional Resident Managers, heads a staff of 80 to administer the more than 2,000 rental units in Montgomery Village, where he own home has a view of the lake.