While the pace of solar energy construction here may not be as rapid as it is in New Mexico, Colorado and parts of New England, it is steadily increasing. Three years ago there were three solar heating companies listed in the local yellow pages, this year there are more than 30.

Some recent developments:

Architect William Graff, who has designed solar heated buildings in Kuwait, Italy and Switzerland, moved into a solar-heated house on Upton Street NW in the District and is working ons the renovation of another. Graff designed his residence and Stephen Molivades, president of Business and Technology, Inc. designed and installed the unique solar system that circulates freon rather than water or air through the rooftop solar collectors.

Ward Bucher, an architect in the Dupont Circle area, has completed designs for the remodeling of two inner-city Victorian town houses that will have solar additions. The additions will provide part of the heat for the old sections of the houses.

The VVKR Partnership in Alexandria has completed a solar heating system for the old Scottish Rite Temple on 16th Street NW.

Architect Winthrop Faulkner is using solar systems for heating domestic water in three large town houses he is building in Cleveland Park - as well as in the New Great Ape House he is designing for the National Zoo.

In Falls Church, architect Kim Kristoff and engineer Clyde Hurst III of Hurst Associates have set up a firm called KH Associates, which consults about alternate energy sources with architects, builders and individuals.

Hurst says that when he designed two solar-heating systems for a client in 1976, it was considered "eccentric." This year, he expects to complete between 40 and 50 solar systems for industrial, commercial and residential buildings. The use of solar energy has become "legitimized," he maintains.

Tom Rust of the Rust Construction Co. has built four solar-heated houses in Old Town Alexandria, all of which had to meet the exacting criteria of the Old Town Architectural Board of Review.

In Columbia, J. D. Evans, Inc. wanted to show that solar heating systems could be gracefully adapted to conventional houses so he asked Clarence Jackson of Dharma Designers to design attached garages incorporating solar collectors for four two-story colonial houses.

Evans has not yet started to build additional solar houses because he is still evaluating the long-range performance of the first four. The first system, in operation for a year and a half, has performed well but still requires occasional adjustment, he said.

Evans says his systems are well-engineered and expensive, costing from $10,000 to $12,000. He said he could cut costs by using less expensive components but is not willing to chance customer complaints.

Like others in the field, he believes solar systems will get better and cheaper in the near future. Tract builders are reluctant to consider solar heating systems because it means scrapping most of their architectural designs, he commented.

Another problem is that house lots are getting smaller all the time, which means that in many instances there is only one way that a house can be sited, not always in a way that is suitable for solar collection, Evans said.

Most builders, architects and homeowners are waiting for the government to provide financial incentive for investing in solar systems and to approve standards for solar equipment. The Department of Energy is starting a testing program through independent laboratories. Results will be available late this year.

For the do-it-yourselfer there are many solar energy workshops and seminars offered by such institutions as George Washington University, Northern Virginia Community College and Glen Echo Park. The Institute for Local Self Reliance, at 1717 18th St. NW, a nonprofit organization which explores the potential for high-density population areas to become self-reliant, also conducts workshops.

The people who organized "Earth Day" are bringing us "Sun Day" on Wednesday, when there will be solar demonstration, exhibitions, workshops, slides and films all day long on the Mall here and in cities all across the nation.

Studies by Mitre Corp. as well as the Department of Energy have shown that solar-powered water and space heating is economically competitive with heating systems using electricity or fuel oil in Washington. Designers of solar systems also expect to see their business increase if an energy bill that includes tax credits for investments in solar heating systems is enacted.