Advisory Neighborhood Commission 3B has agreed to oppose the application before the zoning commission by the Christian Embassy to convert the Hillandale Mansion into a convention center and private school.

An ANC spokeswoman said the commissioners are primarily concerned about the convention center's traffic but also oppose the private school because they consider it a commercial use of residential property.

The ANC has voted to support the extension of Hillandale Corp.'s permit to continue building the Hillandale housing development but will recommend that the permit be extended only five years.

An ANC spokeswoman said the corporation is asking the zoning commission for a 10-year extension, but the ANC would like to review the plan after five years.

The ANC also has decided to support the application for the Lab School of Washington, 1809 Phelps Place NW, to move into the vacant Florence Crittenton Home, 4759 Reservoir Rd., providing enrollment is limited to no more than 250 pupils and 47 teachers by 1987.

The school would like to increase enrollment to 311 pupils with 65 faculty members by that time, but the ANC wants to appraise the impact of the school on the neighborhood before it becomes that large. The ANC is asking that all 44 parking spaces needed for the projected 1987 staff be provided now.