A condominium auction - the kind of sale that has been used to reduce building inventories in such areas as Florida and Hawaii - will take place here July 16 when the 16 remaining units in the Alexandria Knolls West project near Landmark Shopping Center will be sold to the highest bidders.

The auctioneers are Robert Rouse and Associates, a Los Angeles company that has been selling off units to buyers this way for nearly three years - at prices up to 50 per cent off list price, although they say the average auction sale is at 75 per cent of the original price.

Three-bedroom penthouse apartments, as well as two-bedroom and one-bedroom units are to be sold at Alexandria Knolls, which is located in an area of Alexandria at 6101 Edsall Rd. that has come to be known as "Condo Canvon." When the building was completed in January, 1975, its units were generally priced from $35,000 to $62,000.

"We won't be selling $5 apartments," said developer Arpad Domyan, who acknowledge that his company is liquidating the remaining units at a loss because the Union Bank of California has called his construction loan. A slash in prices failed to move the last apartments, so Domyan hopes the novelty of an auction will turn the trick in the condo-choked canyons near Landmark.

Some current owners at Alexandria Knolls say they are concerned over the move, saying it would serve to lower the value of their own units. One owner, who asked not to be identified, said the collapse of a 40-foot-long, concrete balcony railing at the 18-story building recently had also hindered his attempts to sell his unit. (Housing inspectors ordered all of the balconies to be shored up.)

Somewhere between a fifth and a quarter of the condos sold at a Reston auction customarily go to investors or speculators - who have been known to resell them at a profit before settlement. To discourage this practice, Rouse requires purchasers who do not intend to occupy the premises to make a minimum deposit of 20 per cent. The interest rate on the mortgage is also slightly higher, in this case 9 3/8 per cent plus one point.

Owner occupants can put down as little as 10 per cent and assume a 9.25 per cent, 30-year mortgage. With a 20 per cent down payment, the rate is 9 per cent plus one point and private mortgage insurance.

Nancy L. Ross