The old rule of thumb was to spend 25 percent of one's net income on housing. In other words, the monthly payment should have been no more than a week's take-home pay.
But times change and so do the rules. As housing prices have escalated, the percentage of income which most people spend on their shelter has climbed as well.
Annual payments on mortgage principal, taxes, and interest now run about one-third of most households' after-tax income. However, they can run much higher, from 35 to 45 percent, according to Warren Matthews, senior economist of the Mortgage Bankers Association. The higher figures often may include the cost of utilities.
As a result, many lenders have relaxed their underwriting standards for how much income potential homeowners may devote to their purchase. Mr. Matthews says that the old rule of about 25 percent may have been unnecessarily conservative.
''With high interest rates and prices, lenders have relaxed these guidelines, and with good results,'' Mr. Matthews says. ''They have found that they can allow people to pay a larger percentage of their income and continue to do well.''
The reasons, he suggests, are such factors as more two-income families, which can absorb higher costs, and decisions to delay having children, thereby keeping financial commitments low.
Robert Gough of Data Resources Inc. in Lexington, Mass., says the median mortgage percentage currently is about 38 percent - ''way above the rule of thumb which most financial institutions permit.'' Creative financing, in which a buyer might take on a mortgage as well as receive assistance from the seller, can push the percentage up.
Mortgage burdens vary by area of the country. For example, buyers are particularly hard hit in parts of California, where housing costs have skyrocketed, but incomes have remain moderate. Prices in the Northeast and the north-central states remain lower than in faster-growth areas of the United States.
In metropolitan Boston, the typical house with three bedrooms and a two-car garage costs about $95,000. In Chicago, by contrast, it costs only $80,000. But the same house in Houston will run $110,000, and in Los Angeles it will shoot up to $175,000, says Don Ursin, president of Coldwell Banker Thorson, a nationwide real estate brokerage.
Thus, buyers who move may face the choice of either scaling down their living style or carrying a much heavier mortgage in order to buy the kind of house they want.
Heavier mortgage burdens have contributed to the perception of some buyers that it takes a two-income household to buy the house of one's choice. According to a recent Louis Harris poll, 67 percent of those in the $18,000-50,000 income bracket felt that this was so. The figure for households earning under $18,000 was 71 percent.
Samuel J. McDonald, a Weston, Mass., realtor has a program to help interested spouses of transferees find jobs, because without the second income, purchasing the house of the couple's choice may be difficult, if not impossible. The majority of families he serves earn two paychecks.
At Provident Institution for Savings and Loans in Boston, assistant vice-president William Mullin says that the bank's usual income ratio is 28 percent, although it varies, depending on the individual's long-term debts as well as the size of the down payment, which thus adds to his investment in the building.
Mr. Mullin cautions against exceeding a reasonable mortgage-debt ratio for a buyer.
''There's nothing worse you can do than put someone in a house he can't afford,'' Mr. Mullin says. ''Even if the person keeps up the payments, then the maintenance of the place might slip and decrease its value.
''The lender's security goes down, and you're not doing anyone a favor.''