When a Florida businessman called Holiday Inn's national reservation system and asked for a room in the Washington area this week, he wound up with a reservation in Emporia, Va.-more than 150 miles south of the District of Columbia, near the North Carolina border.

After turning down that room and a $150-a-night penthouse suite at the Madison Hotel, he spent his first night in Washington sleeping in the spare room of a business associate.

Maybe $150 a day or 150 miles away isn't the rule in Washington hotels these days, but on the busiest nights of the week there's no room at the inn in the nation's capital.

April and May are the best months for Washington hotels. From Monday through Thursday nights most major hotels are uncomfortably close to full.

Last year was the busiest year ever for the Washington lodging business and this year is "about equal," said Leonard Hickman, executive vice president of the Hotel Association of Washington.

Occupancy rates are of slightly from a year earlier, Hickman explained, but there are more hotel rooms available because of construction of new facilities.

Hickman estimates hotel rates have increased 10 to 12 percent in the past year, attributing the costs to new federal minimum wage legislation, a new contract negotiated with the Hotel and Restaurant Workers Union last fall and the same inflation that boosts other bills.

As of March, the Hotel Association said room rates averaged $40.26 a night in small hotels, $45.97 in medium sized ones and $47.48 in large hotels.

Rates tend to be lower during the summer and dead of winter, Hickman said, and most major hotels have special weekend rates to boost occupancy then. The association this week launched a new weekend tourist promotion with theme "Why go to The Big Apple when you can go straight to the core."

Hickman said local hotel operators have reported some increase in "non-shows" as a result of the United Airlines strike and fears of a gasoline shortage.

But with more than 80 percent of the hotel business here coming from business and convention travelers, occupency is expected to keep booming unless the country runs out of gas.

The 73 hotels and motels in the District have about 16,300 rooms, according to a study by the District's office of Planning and Development.

By 1986, the study projects, that total will increase by 38 percent to more than 22,000 rooms as the District recaptures lodging business that has been lost to the suburbs in recent years.

At least eight new hotels or major additions are planned, city officals found and twice that many more could be built if the downtown convention center is constructed.

"We've seen interest in this city on the part of anybody who ever thought of being in the hotel business," said Austin Kinney, head of the Convention and Visitors Bureau of the Metropolitan Washington Board of Trade.

New construction could do much to relieve the high occupancy rate that guarantees both profits and problems for local hostelries.

As head of the hotel association, Hickman's job frequently involves convincing people that it isn't as hard as they think to get a room in Washington.

On Friday, Saturday and Sunday nights the hotel association's members ran about 60 percent full last year, he said. But Monday night's occupancy rate jumps to 80 percent and on Tuesday and Wednesday night 85 percent of the rooms are occupied. Thursday is off to 75 percent.

That figures out to a 72 percent occupancy rate for the year, when the hotel industry nationwide usually runs in the 60 percent range.

During April-the busiest month-the area average climbs to 86 percent, which means the major hotels are full much of the time. May is nearly as good a month, as are September, October and November.

"We've really been full for the last three weeks," said Peggy Cummings, public relations manager for the Sheraton Park, the District's biggest hotel.

"Next Monday and Tuesday are full and we're approaching it on Wednesday," she added, estimating the hotel averages about 72 percent occupancy.

The Sheraton Park has 1250 rooms now, about 200 less than it had before work started on a new wing. When construction is completed the hotel will have 1505 rooms.

"Business is fantastic all over the country," agreed Thomas Burke, Marriott Corp. vic president.

Marriot is the only hotel chain in the nation that consistently runs above 80 percent occupancy, he said, and this year the rate is the highest ever, "close to the mid 80."

"The Washington properties are slightly above Marriott's average," added Burke. Marriott has five hotels in the area, including a new 300-room facility at Pooks Hill in Bethesda that opened recently.

A 189-room addition to the 325-room Key Bridge Marriott is under construction along with the company's first District of Columbia hotel, a partnership with restauranteur Ulysses "Blackie" Auger. That 350-room hotel, adjacent to Blackie's House of Beef at 22nd and M Streets, NW, is to open in January of 1981.

An 830-room Pennsylvania Avenue Marriot adjacent to the National Press Building an National Theatre is to be started next year. CAPTION: Pictures 1, 2, and 3, New hotel construction includes the Sheraton Park, left, Marriott Key Bridge, top right, and the Four Seasons. By Ken Feil-The Washington Post