A sophisticated new breed of mortgage-financing vehicle is spreading rapidly across the country--one with highly attractive potential benefits for families struggling to buy affordable houses, and for upper-bracket investors with plenty of cash.

The vehicles are virtually all limited partnerships that co-purchase or lend money to purchase single-family homes, town houses and condos.

Some of them are aimed at producing tax shelters for individual investors, such as doctors and dentists. Others are designed to appeal to tax-exempt investors who need steady income, such as pension funds.

Each is a product of residential real estate's biggest challenge of the 1980s: how to make homes affordable to the thousands of Americans frozen out of the market by high mortgage rates and prices, without using direct government subsidies.

Typical of the new breed of problem-solvers is House Investments Inc., an Indiana firm headed by Timothy McGinley, 43, a Harvard-trained financier. McGinley raises large chunks of capital from local investors and pumps it into home-buying markets in different parts of the country. The funds are used to help make down payments and monthly payments for modest-income families who want to buy a condominium apartment or town house, but can't qualify for regular financing.

In exchange for that help, the home buyers give up a portion of their ownership stake in the property, and agree to share all future growth in market value with the outside investors.

One of McGinley's recent deals worked like this: Just under $1 million was "exported" from Indianapolis to Dallas. Offered to modest-income, first-time buyers by Dallas-area home builders, the funds went to purchasers of 60 condominium town houses. The individual buyers received half of their down payments from the outside investors, and currently are splitting with them their monthly mortgage payments of principal, interest, taxes and condo fees.

(Buyers who would normally have to come up with $800 a month for their regular mortgage payments, for example, only need send $400 to the bank. They also pay "rent" to the distant co-owners of their homes, raising their share of the total monthly payment to 70 percent, or $560 in this example.)

The Indianapolis investors, meanwhile, are getting what McGinley says is a two-to-one tax-shelter write-off on their invested cash per year. They also stand to take half of all future capital gains when the condos resell later.

An investor who puts in $10,000, in other words, writes off $20,000 the first year in the House Investments plan. A 50-percent-bracket investor gets his capital back in tax benefits from depreciation and other deductions during the first year. The investors' long-term cash-on-cash return could be "substantial," McGinley's says.

Here's another example of the new breed of "affordability" financing vehicles--one that's raising its capital from a markedly different set of investors than McGinley, and aiming its help at a very different set of consumers.

Home Partners Venture Associates is a limited partnership based in McLean that plans to finance $30 million in homes during the next several years. Unlike McGinley's firm, Home Partners expects to raise its equity dollars from tax-exempt entities, particularly small- to medium-sized employe pension funds that need high income, not high-ratio tax shelters.

Lois Vitt, head of the partnership, also plans to target her assistance to families and individuals seeking to buy homes of $100,000 and up. "There is an incredibly large group of renters out there," she says, "who have incomes that seem large enough to finance a house in the $100,000 range.

"What they don't have," says Vitt, "is the down payment. They are recent divorcees, or people who have suffered business reverses, or are simply short of capital for one reason or another. They lack those dollars--and we hope to become an important national source."

The financial structure of Vitt's new partnership is intricate. Investors in the initial venture-capital offering of $500,000 will have financial interests in a series of eight limited partnerships, each financing the down payments of consumers in different sections of the country.

Unlike McGinley's plan, the partnerships will hold only a 0.1 percent stake in the houses they help finance. The home buyers will own the other 99.9 percent, but will have to pay off a graduated-interest second mortgage to investors that also allows them a 50 percent share of net resale profits. Vitt projects high annual returns for her pension fund and institutional investors--14 percent and up--even if homes appreciate only 4 percent a year for the next 15 years.