Both as a residence and as a real-estate investment, condominiums have truly come into their own.

Condo prices, in the 1980 recession, held up much better than they did in the 1974-75 decline. Dollar-for-dollar, their average rise in value has been pretty close to that of single-family houses. A spokesman for Advance Mortgage Corp. says that, in some places, condos are doing even better than houses.

Realtors around the country credit the new stability of the condominium market to several converging trends.

First among them is that condominiums are now about the cheapest homes you can buy. In some markets, they're the only attractive housing under $50,000.

Just a few years ago, condominiums were sold mainly as second homes or vacation-spot investments. Now, they are a growing portion of the aminstream housing market. Young couples and singles are increasingly apt to buy a condominium as their first home. Condos also suit divorced people, who would rather own a unit than rent an apartment.

In the first four months of 1980, says Washington realtor G. V. Brenneman Jr., condominiums made up 40 percent of sales. Edward Havlik, of Home Data Corp. in Chicago, told my associate Billie Lopez that last year condos made up 50 percent of all residential sales in Chicago last year.

Builders have also gotten smarter about condominium development. What brought down condo prices in the last recession was a huge oversupply of units, especially in large, newly built projects. Today, more developers are making concominiums out of apartment houses, where units can be rented if they don't sell right away. This helps keep condo prices up.

Many condos today are small ones -- eight- or ten-unit buildings nestled in nooks and crannies of cities and appealing to people who already live in the neighborhood. Town house condominiums are sprouting in the suburbs.

Condominium financing is also coming into its own. Just as buyers have come to accept condos as home, so have banks and savings and loan associations. Mortagage money may be tight -- but, in general, it's no longer measurably harder to get financing for a condominium than for a single-family house.

There are, however, some exceptions. Advance Mortgage Co. named Philadelphia as one city where lenders haven't yet full accepted condos.

But for all its convenience and price advantage, buying a condo is different from buying a house. Some things to consider:

Is the building sound? Some old apartment houses are given little more than a coat of paint and a pretty lobby before being sold as a condominium. You and the other owners may soon find that you have to pay for a new roof or a new boiler. Many states now require complete disclosure of an older building's condition before sale; alternatively, prespective buyers should chip in for an engineering report of their own.

Can the building govern itself? After a break-in period, the builder or converter usually turns the building over to its tenants, who then become responsible for maintaining the property, making the insurance payments and seeing that owners keep their monthly assessments up to date. These unpaid volunteers may manage a budget of hundreds of thousands of dollars -- and the future value of your unit may depend on how well they do it. Some developers give management courses to the condominium's first owners' association. The Community Associations Institute also gives training programs through its 26 local chapters. For information, write to CAI at 1832 MSt. NW, Washington, D.C. 20036. CAI publishes two useful books on condo matters -- one for prospective buyers (70 cents), and one for condo owners (85 cents).

Are you protected against construction defects in a newly built conodominium? This is one of the biggest areas of condo lawsuits.

Finally, compare projected monthly assessments on a new building with that of similar buildings already in operation. Sometimes developers promise low monthly assessments but when the buyers move in they find that true expenses are much higher than they thought. If you can't afford to keep the building in good condition, you and the other owners will be the losers.