It will soon be even tougher to buy your first home. And tougher for middle-income families to trade up to a much more expensive house. D-Day is Oct. 15, for the most significant tightening up of the mortgage market that the younger generation has ever seen.

Starting then, one of the biggest investors in home mortgages -- the Federal National Mortgage Association (known as Fannie Mae) -- is going to make it harder for you to buy a house. The new rules will especially affect buyers with a down payment of less than 10 percent cash. Other major mortgage investors are expected to follow suit.

Why does it matter to you what investors do? The reason: because of a radical change in the mortgage market in recent years, which most home buyers don't know anything about.

Nowadays, when you borrow money to buy a house, the lender probably will not put that loan in his vault. Instead, he will sell it to an institution that invests in mortgages. You'll never learn that your mortgage now belongs to someone else. You just keep on making monthly payments to the original lender, who subtracts a handling fee and sends the rest of your payment to the investor.

Recently, major mortgage investors have started to worry about the soundness of their investments. Foreclosures are up, and because of flat or falling real estate prices in many parts of the country, investors can't always be sure of getting all their money back after a loan goes into default.

As a result, they are getting more cautious about the mortgages they'll buy. A bank or savings and loan still can make whatever mortgages it wants. But if it wants to resell them to an investor -- as most lenders now do -- it has to jump to the investor's tune.

Here are the new rules that will start affecting many home buyers in October:

* You will need a higher income to qualify for a mortgage, especially if you plan to pay less than 10 percent of the house price in cash. To buy a $76,500 house, with a 5 percent down payment and a mortgage at 12.2 percent, will require an income of $41,232, compared with only $36,814 today.

* You will have to cover at least 5 percent of the down payment with your own money, in addition to any help you get from family or friends.

* Sellers, builders and real estate agents will not be allowed to make big cash contributions to the deal to make the house seem more affordable. (These payments are often recouped by inflating the price of the house.)

One popular form of contribution is the "buydown," which finances a mortgage-rate discount in the early years of the loan. There's a new, lower limit on the buydowns allowed on fixed-rate mortgages, especially when you put less than 10 percent down. As a result, monthly payments will go up. No buydowns at all will be permitted on adjustable-rate loans, regardless of the size of your down payment.

If mortgage rates fall, home buyers may not feel these changes very much, says Leon T. Kendall, head of the Mortgage Guaranty Insurance Corp., because lower rates will offset the effect of tougher borrowing standards. But if interest rates rise, cash-shy buyers will be doubly frozen out: by the higher monthly payments and by the tighter income and down-payment rules.

There are still some strategies left for buyers without a lot of cash:

* If you're close to a home-buying decision, make it now. For the next few weeks, most S&Ls and other lenders will be granting mortgages on the current, more generous terms. "A majority of our S&Ls will make the push to lend as much as they can," predicts Dennis Jacobe of the United States League of Savings Institutions.

* Apply to a mortgage banker who specializes in FHA loans. Although lenders are looking harder at all low-cash applications today, FHA loans should still be generally available to borrowers with only 3 to 5 percent down. But there are limits on the size of the mortgage you can get, so FHA loans are useful only for middle-priced houses.

Veterans Administration loans, for qualifying veterans, can still be had for no down payment at all, if the lender permits.

* Talk to several lenders about how large a loan you can get. Some may plan to sell your loan to an investor who isn't as strict as Fannie Mae, in which case you may get more generous terms.

* Save money. "Ultimately, first-time home buyers will have to save up more money before leaping in order to purchase a home of their own," Jacobe says. That's the way it used to be, in the days before runaway real estate inflation, and that's the way it's going to be again.