If you are middle-aged or older, you ought to know that your future real estate needs are the focus of a multibillion-dollar, rapidly growing new industry.

Retirement housing -- once a quiet field dominated by church-related and other nonprofit groups -- is heading for the big time, fast.

Corporate giants such as Marriott Corp. and U.S. Home Corp. are not the only ones plunging in with ambitious plans for new developments. So are huge private pension funds, Wall Street investment bankers, nursing home chains and large life insurance companies. Retirement housing "is almost certainly the biggest growth specialty in real estate today," according to M. Stroud Curran, a top retirement-shelter analyst with Laventhol & Horwath, a national accounting firm.

Estimates prepared for the firm suggest that more than $30 billion will be poured into new shelter for retirement-age buyers and renters before the end of this decade, spread among 2,000 projects nationwide. But according to experts such as Curran, the housing created by those billions will range through a "menu" of choices for senior citizens that no one would have imagined a few years ago.

In Curran's view, "What we're seeing is the emergence of retirement-shelter product lines that span the gap from complete housing, nursing and health-care insurance down to all-elderly condos" with little or no health or food services attached. Think of it as a highly varied, sophisticated "menu of shelter" that should be extremely helpful as the nation's elderly population swells in the l990s and beyond.

Among the new options popping up in the national retirement marketplace are: "Congregate condo" communities. These are designed for older homeowners who don't want to join a traditional "lifecare" retirement center, don't want to rent, don't want to feel they're in a specialized community, but who also want to enjoy two to three meals prepared for them each day and count on the availability of health care when they need it.

Aimed at owners in their 60s and 70s who have sold their family houses and have tax-free equity dollars to invest, these condos sell from $60,000 to $150,000, and beyond. Some congregate condos are aimed at younger, individual investors as well. They buy a condo and rent it to senior citizens with the help of the project management.

"Respite-care," "assisted-living" and "catered-living" rental units. These are being developed primarily by nursing homes as separate apartment communities on or near main facilities. They are designed for married couples and individuals who are more advanced in age and frailer in health than the condo buyers, but who don't want an institutional feel to their living quarters.

The apartment residents may need help dressing, eating, bathing and the like. The catered-living concept is intended to provide such services -- when and how the resident wants them -- in a home-like, noninstitutional atmosphere. "Retirement hotels." Increasingly popular in Sun Belt locations, these often are converted hotels or motels that have been repositioned to serve the retiree market. They function in most respects as hotels -- with meals in a restaurant and daily maid service -- but carry lease terms and health-care services tailored for the elderly.

Curran noted that, while the rapid diversification of the retirement-housing field into such specialized categories "is basically a healthy, exciting trend," would-be buyers, renters and their families should be on their guard.

Retirement shelter "is in such a growth binge that it's inevitable that some of the communities under development are going to have quality problems," he said.

To avoid signing up for the wrong retirement-care option, Curran suggests three rules:

*Look hard at the financial track record of the developer, or in the case of an existing community, the financial performance of the project itself. Is the developer a relative newcomer to the rental-housing or condo field? If so, look doubly hard. In the case of a new "lifecare" community -- especially one in the luxury category costing $100,000 to $150,000 in up-front fees -- ask to see the lender's feasibility study and a summary of the project's insurance reserves. Curran warns that some projects may not have the health-care insurance funds to pay for the services they promise consumers.

*Ask to see state-required registration or regulatory submissions from the sponsors of the project. Condo and lifecare communities, for instance, must put together extensive disclosures in some states. Others may have them available, but only give them out if asked.

*Don't necessarily choose the lowest-priced option in your area. Price discounts sometimes signify the lowest-capitalized, lowest-quality services. These projects are the first to develop financial strains, said Curran -- "precisely what retirees shouldn't have to cope with."