The D.C. Board of Zoning Adjustment, in an unusual action, yesterday hurried its approval of a 170-unit garden apartment project near Catholic University so the developer will not lose vital financing at a federal subsidized low rate.

Even with mortgage money at a 7 1/2 percent rate -- about half the level available on the private market -- developer Joseph F. Horning told the board that spiraling construction costs will force him to charge rents at least $100 a month higher than the $408 to $475 he collects from an adjoining identical project that is now nearing completion.

The project approval yesterday, immediately after a public approved yesterday, immediately after a public hearing, is the second section of a development bounced by Taylor Street, Second Street and Varnum Street NE called The Heights. The first section, containing 153 units, is one of only two new large rental projects to be built in the city in recent years for occupancy by the general public.

Ordinarily the board delays final action until its next regular monthly meeting after a hearing is held. Yesterday it heeded Horning's plea for quick action so he will not lose $8 million in financing for the 170 additional units. He won the money through a lottery recently conducted by the Government National Mortgage Association to allocate scare funds to projects around the nation.

Commission member Douglas J. Patton and chairwoman Connie Fortune noted that it was unusual for the panel to go beyond pure zonging issues to consider financing.

Horning said the second phase of the project originally was going to consist of 67 three-bedroom townhouses for sale, but rising construction and financing costs priced them out of the market. So he said he decided to ask the commission's approval to convert it into a project with 170 one- or two- bed- room rental units.