You've heard of single-family rental homes. You know about converted condominium units purchased by small-scale investors. But you may not be up to speed on the real-estate rage of 1985 that's sweeping the Big Apple.

The hottest deals in town, pushed aggressively on TV and in full-page newspaper ads, are condominium and cooperative units in prime Manhattan buildings that you buy with people already living in them. You read it right: Purchasers are lining up to plunk down hard cash for one- and two-bedroom units that come complete with young couples, elderly widows, retired teachers and other varieties of tenants.

The units range in price from the low $60,000s to $2 million. They are as modest as 450-square-foot, midtown efficiencies or as plush as four-bedroom, five-bath co-ops overlooking Central Park.

What they share, however, are three characteristics:

* They are for sale only if you take the rent-controlled tenants living in them as part of the package.

* They come with 40 to 50 percent discounts off the market values they'd command as vacant units, because the occupied units inevitably produce rents 40 to 60 percent below what they'd earn in the absence of rent control. Some of the apartments also feature guaranteed buy-back plans allowing the purchasers to recover every dollar they invested during the first five years of ownership.

* They are all units in rental buildings that have experienced so-called "non-eviction conversions." Protected by law from eviction when their apartment buildings went condo or co-op, the tenants have stayed on without seeking to purchase their units. So long as they pay the rent, they can live in the unit indefinitely -- subject only to the annual increases mandated under New York City's rent control or rent-stabilization program. Once they move out, however, the owner is free to charge full-market rent.

Buying and selling occupied condos and co-ops has been a phenomenon here the past 18 months. But until early 1985 the units tended to be sold quietly to deep-pocket, large-scale investors, often in bulk sales involving multiple units. Now, however, they're being sold like hotcakes to individual buyers who want a piece of Manhattan at a discount before Congress changes federal tax laws.

The highest-profile developer and marketer of occupied conversions -- Ahrens Barrell Inc. -- says it has sold $200 million worth and is besieged by would-be investors. Gerald Guterman, chairman of the firm, calls the Big Apple's 1985 boom in conversions "simply a recognition that buying an apartment that happens to have somebody in it can be a surprisingly good place to put your money."

Guterman offers the case of an average-sized condo purchase. As a vacant midtown Manhattan apartment, the condo unit would sell this month for slightly over $100,000. Because it's occupied by a long-time tenant protected by the city's rent-stabilization rules, however, Guterman will sell it for a 40 percent discount, or $60,000.

Most buyers qualify for 80 percent financing, which in this case means a 20 percent down payment of $12,000 and a $48,000, long-term mortgage that Guterman can help arrange at 12 percent. Monthly principal and interest charges come to $480. Condo maintenance fees, taxes and management costs come to another $480 per month. The rent-controlled tenant pays only $480 per month, leaving the unit owner with an apparent pretax $480 negative cash flow.

After taxes, though, the picture looks different. With depreciation write-offs of $333.33 a month, plus deductions for interest and maintenance expenses, Guterman calculates that the true, after-tax monthly cost of owning the unit to his typical buyer in the 40 to 50 percent tax bracket comes to $20 to $60 a month.

Assuming the tenant moves in the coming five years and the unit is freed from coverage under the rent-stabilization program, the owner can sell it, rent it at free-market rates or move into it.