For three years, the Reagan administration has campaigned to increase oil and gas drilling in the nation's open spaces and coastal areas. Now the effort literally has hit home.

A California businessman has applied to the Interior Department for drilling rights to oil and gas under President Reagan's ranch.

Robert Lynn, of Sacramento, said yesterday that he did not know until last week that his application for a lease on 7,085.46 acres of mineral rights overlaps with 480 acres--more than half--of Reagan's 688-acre Rancho del Cielo.

"We really didn't do it on purpose," said Lynn, 59, president of Lynn Title Services, which provides lease titles to oil companies and acquires some of its own. "It's just one of those things. There has been some drilling activity just to the south of this land that looked good, and we thought we'd just like to have a lease on it."

Lynn apparently has a good chance of getting the lease. The president owns only the surface of his ranch, according to Interior officials. When the land was released from the public domain years ago, the government retained all rights to minerals under it.

Such lands, known as "split estates," are common in the West, and are open to oil and gas exploration as long as the developer does not damage the surface owner's land or improvements. The government could deny the lease, however, if granting it would imperil national security or environmental values.

"This is a great test of the president's impartiality," said Rep. Morris K. Udall (D-Ariz.), chairman of the House Interior Committee and a persistent critic of the administration's mineral leasing policies. "Now we'll see whether he is willing to sacrifice his own land as well as the public's to the cause of energy development."

Former interior secretary James G. Watt "and the president have told us for three years that it's good for the country to drill every place we possibly have federal oil and gas," Udall said. "I think, with a $200 billion deficit, the taxpayers need some help, and if there's oil and gas out there, we ought to go get it so the government can get the royalties."

A spokesman for the Bureau of Land Management in Sacramento, which oversees federal mineral leasing in California, said Lynn's application "will be duly considered as all applications are, and it will probably be a number of months before the decision is made. It normally takes this long."

Lynn said a BLM official called yesterday to inform him that his lease request overlapped with Reagan's ranch, and that it would be processed "in the usual fashion. He said that came down from the White House."

White House spokesman Anson Franklin said the White House learned of Lynn's application yesterday, and that White House counsel Fred F. Fielding has discussed it with Interior.

"It is our understanding that Interior is going to proceed in the normal fashion, and that's the way it should be done," Franklin said.

Lynn said he expects the government to place national security restrictions on the lease, possibly requiring him to drill at an angle from outside Reagan's property.

Lynn said he likes Reagan's energy policies, adding: "I'm a Goldwater Republican, so I think I'm a little bit to the right of the president. He's safe on that score."