General Motors Corp. restored loan-rate discounts on two small-car lines yesterday, settling the domestic auto industry into a virtual price war.

GM's announced prices for the 1982 cars have been undercut by the auto giant's domestic competitors.

In other economic news yesterday:

* The Commerce Department reported that orders for U.S.-manufactured goods fell 1.9 percent in August, the first monthly decline since January.

* In another report, the Commerce Department said new construction put in place during August was estimated at a seasonally adjusted annual rate of $231 billion, 1.62 percent below July's level and the lowest rate since October 1980.

GM said it will offer car loans from its financing subsidiary through Nov. 11 at 13.8 percent compared with prevailing rates above 16 percent. The reduction could save buyers an average of $500 over the life of a loan contract, GM said.

The reductions apply to GM's X-car compacts and J-car subcompacts, which are among GM's highest-volume car lines and are under severe competitive pressure. Ford Motor Co., Chrysler Corp. and American Motors Corp. set base 1982 prices on many competing models below GM's.

GM offered financing rate discounts to 13.8 percent across its car lineup in August and September, matching a variety of rebates and cash sales incentives from its competitors. The programs were successful in reducing excessive inventories that had built up during an unexpectedly sluggish summer sales season.

Average initial 1982 car price increases by the four U.S.-based automakers were $617, or 6 percent, by GM; $473, or 5.6 percent, by AMC; $430, or 4.8 percent, by Ford; and $306, or 3.7 percent, by Chrysler. Volkswagen of America hasn't announced 1982 prices.

The August decline in factory orders was broad-based, including sub stantial decreases for categories ranging from home goods to primary metals in addition to the long-troubled auto-equipment and construction-materials industries.

In all, the seasonally adjusted value of total orders fell to $169.16 billion after rising a revised 0.8 percent in July, the report said.

Durable goods orders declined 3 1/2 percent to $86.32 billion, a bigger drop than the 2 1/2 percent decline the Commerce Department estimated earlier. Durable goods are heavy-duty items such as major appliances and cars, whose replacement often can be postponed, and which therefore are often the first victim of any cutback in consumer spending.

Orders for nondurables rose a bare 0.1 percent to $82.84 billion.

On the other hand, manufacturers' inventories rose only 0.3 percent to $272.6 billion, indicating that factory operators are not about to get caught with the big stockpiles that lead to big recessionary production cutbacks and worker layoffs.

Significant declines in automobile inventories occurred in August because of rebate promotions. Stockpiled oil supplies also decreased. By the end of August the value of unsold cars and unsold oil was the same, $9.1 billion.

Shipments of manufactured goods in August were down $1.4 billion, a 0.83 percent drop.

August's 1.6 percent decline in the value of new construction was the seventh monthly drop in the past eight months.

As usual, housing bore the brunt of the decline, which was measured at 1.6 percent for the month.

The new Commerce Department report said overall construction rose by 0.3 percent in July rather than declining 1 1/2 percent as first reported.

But that gain was quickly wiped out in August as the seasonally adjusted value of new construction dropped to an annual rate of $231 billion, the worst since last October's $226.1 billion.

A big loser was new private housing, which dropped 5 1/2 percent to a rate of $57.2 billion, the report said.