Condominium and cooperative sales now account for nearly a third of all home sales here -- especially in the under-$75,000 class -- and the percentage is rising.

Buyer acceptance of the condominium/co-op concept was a major housing trend in the 1970s that is expected to continue to strengthen in the 1980s. In the Washington area, about 13,000 units are converted to condominium ownership each year. Condo/co-op ownership now exceeds 70,000 units, or 7 percent of the housing here.

Many people who actually can afford single-family houses choose to buy apartments in luxury buildings because of the convenience of that style of living.

But for many young, moderate-income, first-time buyers who cannot get into the over-$50,000 market, the small condominium or co-op units -- mostly converted from rental status in an aging complex -- have become the only affordable choice. Typically, these purchasers can make small down payments, and have annual incomes that cannot qualify them for monthly mortgage payments much over $350 or $400.

A recent survey by the National Association of Realtors showed that 69 percent of Washington area condos are purchased by single men and women, 27 percent by married couples and 4 percent by unmarried couples. One-third of all condo sales are made to households with incomes of less than $20,000 annually.

With housing prices showing no signs of decreasing in this area, condominium conversions -- and the controversy over them -- are likely to continue.

Since the conversion concept took hold, tenants have been fighting back, often for their right to rent.

Backed by new legislation in the District and Montgomery County, some tenant groups there have organized to buy and redevelop their own buildings or to strike better deals with redevelopers. The District and Montgomery governments have responded to tenant pressures by imposing off-again, on-again moratoria on conversions.

Rep. Benjamin S. Rosenthal (D-N.Y.) last year introduced a bill in Congress that would impose a three-year nationwide moratorium on conversions of existing apartment buildings to condo or co-op ownership.

The House has not shown any inclination to deal with this legislative proposal. But Rosenthal refuses to let it drop, although he indicated recently at a seminar sponsored by the Community Associations Institute that he might cut back the proposed moratorium to one year.

Developer Giuseppe Cecchi, president of International Developers Inc., recently described a moratorium as "no answer" to the question of conversion.

Cecchi, whose firm has handled construction of more than 2,000 new condo units and conversion of 3,000 units in the Washington area, said that the nation needs more rental housing, which the "federal government could encourage with favorable tax treatment and special financing."

Cecchi said that he and other developers have established their own programs to help rental tenants become owners in converted apartment buildings. He said they also have made provisions to keep some rental units for needy or elderly tenants. "Now we need help to maintain such programs," he added.

Construction of rental housing in the Washington area has declined markedly in the past five years because of rising construction and financing costs and fear of actual or potential rent controls.

The proposal put before Congress by Rosenthal calls for a presidential commission to study the conversion question and come up with a plan to aid those displaced by conversions while at the same time stimulating construction of new rental apartments.

Rosenthal, chairman of a House subcommittee on consumer and monetary affairs, also plans to hold hearings concerning the current conversion of the 1,057-unit, luxury Promenade buildings in Bethesda.

American Invsco of Chicago purchased the building last summer for $51 million from Washington developer Nathan Landow. To avoid restrictions on condominium conversion, Invsco is converting the large, expensively priced Promenade units to co-operative ownership.

In his speech to the community associations' group, which includes homeowner association members, developers, managers and lenders, Rosenthal charged that Invsco is conducting a "blitzkrieg" sales program at the Promenade. He pointed out that the giant Invsco organization, which converts and manages apartments in 38 other cities in addition to Chicago and Washington, publishes a book "with pictures of every building they raped [converted] in the country."

Rosenthal said he had challenged the provision of long-term financing for the co-operative purchasers at the Promenade by Washington Federal Savings & Loan Association on the grounds that Washington Federal had acted with Invsco to circumvent the local conversion law. However, he said that his charge was denied by the Federal Home Loan Bank Board, which oversees S&Ls.

Actually, a three-page response, signed by FHLBB Chairman Jay Janis, reviewed the growth of condominium conversions, saying: "The answer to this problem is not to ban conversions of rental dwellings. The answer is to increase the supply of rental buildings."

In regard to the Rosenthal charge, the Janis letter stated: "In any event, it is questionable that Washington Federal could be considered to have 'joined' in any circumvention of local law simply by extending a commitment to find end loans on the project. This is a legal issue which must be left to the courts if this matter is to be further pursued."

Rosenthal was rebutted in a subsequent seminar by G. V. (Mike) Brenneman, a specialist in condominium conversions and sales. Brenneman said that a recent HUD report on condo conversions showed that the number is small in relation to existing housing. Most displaced tenants find alternative housing within 60 days -- often within six blocks of where they lived, he noted.

Brenneman also pointed out that studies of actual conversions show that more than 70 percent of purchasers are those who were formerly tenants in the same building or tenants in nearby buildings. It is also recognized that about 15 percent of condo conversion units are purchased by investors who keep the units as rentals, albeit at slightly higher rates.

Nick Mason, project director for Invsco at the Promenade and the man who headed that firm's recent condo conversion of the nearby Grosvenor, said he would not respond to the Rosenthal criticism of Invsco and its role at the Promenade.

Mason did acknowledge that tenant association picketing on weekends had "impacted on sales." He also noted that the co-op ownership plan represented a "conceptual change" for prospective buyers.

In a co-operative, buyers do not own an individual apartment but only the right to occupy it. They instead own a percentage share of the total building. In a condo, the owner holds title to an apartment and owns a share in all the common elements of the buildings and property. In both cases, the tax benefits are about equal.

There are relatively few co-op apartments in Montgomery County but some of the District's best-known high-rise apartment buildings are co-ops. They include the Watergate, Harbor Square, the Westchester, Tilden Gardens and Van Ness North.

Mason also said that the existing permanent mortgage on the Promenade, reported to be approximately $24 million, would be 14 percent to the new ownership. The original mortgage rate was about 9 percent.

This will obviously be more expensive for co-op buyers, but Mason said that the same buyers will have minimum settlement costs as compared to a condo settlement, and also will benefit by avoidance of the county's new 4 percent transfer tax on converted condo buildings. "The total price benefit to a co-op buyer will be somewhere between 5 and 7 percent," he added.

The Promenade apartments are priced from about $83 to $95 per square foot, depending on size and location of the unit. The average price is slightly over $90, and the bulk of the extremely large one-bedroom Promenade apartments are priced in the $100,000 range.

Tenants are eligible for discounts if they make an early decision, and older tenants are permitted rental occupancy for 24 months without buying. All other tenants were given six-month notices. Some vacated apartments are available for contracts, Mason added. He declined to say how many signed contracts are actually in hand after a 60-day selling effort.

In Reston, where the Donatelli-Klein partnership recently purchased several rental properties, the high-rise Heron House and several adjacent garden apartments -- a total of 107 units -- are being converted to condo ownership. Partner Louis Donatelli said that 41 tenants exercised their initial right to purchase and that 17 units were sold to outsiders within 10 days. The price range is $50,000 to $108,000.

In the District, four efficiency apartments are unsold and currently available to moderate income purchasers at the 21-unit Covington at 1848 Columbia Rd. NW. It recently was converted to a limited-equity cooperative ownership building by a tenant association purchase made with federal financing aid. Resident-owner William Black said that 14 of the units were sold to tenants. Down payments for the remaining units are $2,950 and monthly payments $245, he added.