If you are looking for rental housing in Northern Virginia and find something you like, you had better put down a deposit without taking time to comparison shop. The rental market is a landlords' market.

That's the advice coming from property managers and real estate agents involved in the leasing of apartments, town houses, condominiums and single-family homes.

Apartments are especially hard to come by in northern Fairfax, as are other so-called "affordable housing units."

It may be easier to find something a young family can afford in other sections of Fairfax or Arlington, one agent said. For example, Lynn Humphrey, a young professional single mother with two children, looked for six months before finding a town house she could rent in Springfield. After looking near Tysons Corner, where she works, Humphrey decided she wanted to keep her children in the same Springfield school they attended while she was sharing a house and searching to find something on her own.

"I lucked out," she said simply. A friend of a friend leased her his town house.

A friend of hers scoured the Northern Virginia market before finding an apartment in the $700 price range. At least once, she was looking at a unit only to have another apartment seeker put a deposit on it sight unseen.

Their stories are not unusual. According to a recent survey of the Northern Virginia rental housing market, rental occupancy is 99 percent. That survey of U.S. housing markets, prepared by Advance Mortgage Corp., a part of the Lomas & Nettleton Financial Group, found that many apartment owners in Northern Virginia have long waiting lists.

"If you find something you want, you had better put something down. This is no time to shop for an apartment in Northern Virginia," said John Adler, a founder of Quixsearch, an apartment search firm that is associated with Woodward & Lothrop.

"Even though the rental market has loosened up a bit since the first of October, it is still tight," Adler said. "In this crazy Virginia market, it is all a matter of luck," he said.

Finding single-family detached houses to rent in the $700 to $900 price range inside the Beltway is hard to do. "It is very difficult to find much of anything right now in the Fairfax and Vienna areas in the $1,000 range," according to one property management specialist.

"The inventory of houses for sale is 10 times the size of the rental inventory," according to a McLean real estate broker whose property management division handles houses all over Northern Virginia. Realtors agree "it is a landlords' market in both apartment and house rentals."

Ironically, the apartment market is softest inside the Beltway, Adler said. That is because prices are higher. "You are looking at a market geared to GS-15 salaries," he said.

"It is very tight outside the Beltway, where there are few apartment communities. Metro has made what developments there are outside the Beltway very attractive," Adler said.

According to Quixsearch, the average price of an efficiency in Virginia is $425 to $450 per month. The average rent for a one-bedroom apartment is $550. Two-bedroom apartments average $650, and three-bedroom units average $770.

The tightest part of the market is in three-bedroom apartments and town houses, because people are sharing the larger units, getting larger living quarters and paying less rent than they would for a one-bedroom unit, experts agreed.

That lack of available three-bedroom apartment units is also putting a squeeze on lower income Fairfax residents who must depend on subsidized vouchers to help pay for their rents. There is very little housing available for low-income residents in Fairfax anyway, county officials said. There is virtually no new construction of apartments, either subsidized or nonsubsidized.

The battle for those larger apartments by groups of singles hurts families, housing experts said. Not only is the inventory limited, but landlords also are able to increase rents because they know the singles will be able to pay. The increased rents are pushing poor families away, however, because the higher rents exceed the limits set by the federal government for housing that a low-income person can rent using federal subsidies to help make the payments.