Home building is pretty much of a gamble these days, so perhaps it is appropriate that the National Association of Home Builders will be in Las Vegas again in January for an annual convention/exposition.

Officially, however, home builders are cautiously optimistic for the first time in nearly a year. Housing starts registered a whopping 33 percent gain in June and a more modest 5 percent upturn in July.

"Although we're pleased with the recent upturn in the market," cautioned Merrill Butler, president of the 123,000-member trade association, "we've got to remember that July's annual rate of 1.266 million housing starts is still 23 percent below the July 1979 rate of 1.76 million and 40 percent below the July 1978 rate of 2.1 million.

"Mortgage rates, which had fallen below 12 percent in many areas of the country in June, are now 13 percent or more.

"Unless this trend is reversed soon, what recovery there has been so far could be nipped in the bud in the next couple of months," Butler said.It has been observed that as interest rates approach 13 percent, buyers start deserting the market.

Butler blamed housing's credit problems on heavy government borrowing. The Carter administration recently revised its budget deficit projection for fiscal 1980 upwards to $61 billion.

"The more money the government borrows," said Butler, "the less money we have available to finance houses, auto or business ventures."

But, Butler cautions prospective home buyers to buy now rather than wait for interest rates to come down. Housing economists don't see much reason to believe that interest rates will fall below 11.5 percent in the foreseeable future. And housing prices, like-everything else, are headed up.

"With home values increasing at about 10 percent a year, a home is still one of the best investments available to the average family," Butler said here recently.

"A 12 or 13 percent interest rate doesn't look quite so formidable when you consider the tax advantage of buying a home. Both the interest on the mortgage and property taxes are deductible on your federal income tax return."

A family of four with an income of $28,000 a year would pay a net interest of 7.8 percent on a 13 percent, $57,600 30-year mortgage when the tax advantage is considered, according to NAHB statistics.

"Waiting can be costly," Butler said. "Assuming that new home prices continue to increase about 12 percent a year, today's $64,000 home will cost $71,680 next year. Even if mortgage interest rates fell a full percentage point from 13 percent to 12 percent, you'd still be paying more. A 10 percent down payment would have jumped from $6,400 to $7,168, and monthly payments on a 30-year mortgage would increase from $637 to $664.

"We realize, of course, that many young families, can't buy a home at today's prices at any interest rate," said Butler. "The issue of housing affordability is going to be one of the major dilemmas facing us during the 1980s."

NAHB estimates that 2.3 million units will be needed each year to meet the demands of the so-called baby boom generation, as well as to replace dilapidated housing stock. By contrast, this year's forecast calls for 1.18 million units; next year's for 1.55 million units.

"The result can only be another rash of price hikes when the housing market recovers and releases this bottled-up demand," predicted Butler.