While school district quality traditionally has dominated the top of the wish list for home buyers in Northern Virginia, the No. 1 request these days is for proximity to a Metro station.

Real estate agents say prices for homes along Metro's Orange Line extension, which opens today in the I-66 median from Ballston to the Vienna-Fairfax City area, are escalating and the inventory of available houses for sale is extremely low.

Prices are up at least 20 percent and as much as 35 percent over 18 months in many areas within walking distance of the four stations, according to an informal survey of local real estate agents.

Opening today on the Orange Line are the East Falls Church station at Washington Boulevard and Sycamore Street on the Arlington-Falls Church boundary; the West Falls Church station at I-66 and Rte. 7; the Dunn Loring station along Gallows Road between Merrifield and Tysons Corner, and the Vienna station at Nutley Street and I-66, just outside the town limits of Vienna.

Real estate brokers said homes near the stations are selling fast for prices that surprise agents. They said they have agreed to list houses at prices sellers insist on rather than at prices they normally would recommend.

Tom Stevens, head of Shannon and Luchs' operations in Northern Virginia, said an agent in his firm's Vienna office last week listed a town house in the Townes of Moorefield development near the Vienna station for $159,000 even though comparable past sales indicated the price should have been in the $130,000 range.

Stevens said the unit sold within 48 hours for $154,000.

Town houses in the Vienna-Oakton area close to the station and Oakton High School are selling well, Oakton agents said.

Connie Jenkins, an agent with Laughlin Realty, said people almost always ask her about school districts, but now they also want to talk about convenience to Metro.

According to sales records, an end unit in the Country Creek development near the Vienna station recently sold for $150,000. Last year, the same unit sold for about $126,000, a 19 percent increase.

Such increases follow similar patterns in many neighborhoods in suburban Maryland when Metro lines opened there, according to Stevens, a past president of the Northern Virginia Board of Realtors. "Those prices have never gone down," Stevens said.

Agent Libby Ross of Town & Country's North Arlington office said she believes that price increases as well as the strong demand for homes near stations caught agents off guard.

"We agents should have been out looking around a year ago, learning what neighborhoods are really within easy walking distance to the East Falls Church station," she said.

She said even the "little houses that would have sold for $100,000 a few months ago now sell for $115,000 or $124,000 near that station. There has easily been a 20 percent increase in values," and the demand is persistent.

She said she recently rushed to take buyers to see a house that "never in a million years would I have thought of as being close to Metro," only to find that the house, which is less than a mile from a station, sold before she could drive her clients there.

Homes close to the West Falls Church Station range in price from $100,000 to $300,000.

A four-bedroom, three-bath split-foyer house for sale off Idylwood Road in the McLean High School district has just been offered for sale by owners who no longer need a large house. Listing agent Linda Thompson of the Long & Foster realty company said its proximity to the West Falls Church station certainly plays a part in its $239,000 price tag.

Homes near the West Falls Church station were selling in the low $200,000 range 18 months ago, but officials of the Northern Virginia Board of Realtors said that today's higher prices reflect "very real increases in actual values" and not simply inflated prices.

Several Arlington agents agreed. One said she felt guilty recently when a house close to the East Falls Church station sold for what she thought was too much money. But she said she felt worse when the appraisal came back higher than the selling price.

Industry sources said such reports show that, at least in some cases, appraisers already have added a home's proximity to Metro as an indicator of its fair market value.

Detached homes generally dominate residential areas adjacent to the Dunn Loring station. Prices range from $100,000 to above $250,000. There are few town houses, in contrast with the Vienna station area.

Wally Adamson, a broker and sales manager in the Vienna area, said he sold a house on Cedar Lane in Dunn Loring in 1978 "in the $90,000 range. Now they could easily get $150,000 or $160,000 for it."

In recent months, new-home construction has been abundant in the Gallows Road corridor from Tysons Corner to Dunn Loring. Prevailing prices today are in the $180,000 range to more than $250,000 for customized homes.

Ross said, "It's not just the young professionals that you read about that want to live near Metro. It is families with children who see Metro offering them both a reduction in commuting and parking costs and a reduction in travel time, giving them more time with their families. At the same time, neighborhoods around each of these stations are close to good schools."