If a company such as Mobil Oil Corp. decides to relocate major offices from Manhattan to metropolitan Washington, should the firm be forced to build its new facilities in the District rather than Fairfax County?

That's a hypothetical question being raised by the mortgage banking firm of Walker & Dunlop in discussing the federal role in local planning.

It is hypothetical because Mobil already has made a decision to move gradually to Fairfax County and, according to local leaders, never even considered a site inside the District boundaries.

But Walker & Dunlop, in its quarterly newsletter "portfolio," points to the Mobil case as an example to how firms make relocation decisions today and how they could be forced to make such decisions in the future.

According to drafts of President Carter's Cabinet-level task force on urban policy, the federal government is about to seek increased power to impose national goals on local land use decision-making.

The key question is how far the government will go. If the administration decides to apply sanctions against communities that choose not to accept a federal planning formula, "then it will have crossed the line into a new, disturbing era of federal-local relations," Walker & Dunlop concludes.

If a federal agency has the power to certify local and regional zoning and land decisions, Uncle Sam would have the power to tell a community what can be developed where, when and how, the mortgage banking company adds.

And, in the case of Mobil, federal land-use requirements favoring central cities over suburbs might block construction of major new offices outside the District. In such an event, Mobil may well have decided to stay in New York, and the Washington area would have lost an economic plum that will benefit city and suburbs over the long run.

The Walker & Dunlop warning is an example of the controversy that Carter's urban policy will generate, whatever is finally proposed in January. Among the major ideas included in policy drafts are the following:

The United States should start a new program of federal review and certification of regional land use plans; metropolitan areas that meet national goals of non-discrimination, farmland preservation and controls on pollution or urban sprawl would qualify for bonus allocations of federal money for sewage treatment, transportation and housing.

After a period of time, under one proposal, all U.S. funding would be conditioned on local adoption of federal strategies.

Priority in distribution of funds would be to "established urban areas and central cities . . . who already bear an inordinate tax and poverty burden."

Regional agencies, such as the Metropolitan Washington Council of Government (COG), would be strengthened with implementing powers "to shape and direct" area growth.

The performance of such regional agencies would be monitored by state and federal governments; local authorities that don't live up to their promises would be denied funds.

Reviewing these suggestions to the President, Walker & Dunlop concludes that federally-mandated regional land use planning "is virtually certain" to undergo a drastic transformation in the next two years.

The evolving policy would seem to have the beneficial impact of aiding an institution such as COG, which already had developed its own area growth policies that encourage area jurisdictions to limit extension of new public services and encourage more compact development, to redirect commercial and residential development toward the urban center of the District and close-in suburban communities to preserve older neighborhoods and rural areas and to develop regional water resources.

COG relies on cooperation and voluntary action by this region's governments. Often, that is a slow process that discourages persons who want to move quickly, for example, to shore up the District's weak economy.

The impatience with efforts to date by private enterprise and local governments to solve the problems of deteriorated urban centers is behind the drive to impose federal controls. But there is great impatience with the federal government, too, and a record in one aspect of urban life - public housing - that has not been distinguished.

MORE ASSOCIATION - Three trade associations recently made decisions to locate offices in Washington. The Chicago-based National Home Furnishings Association will move here in mid-1979 and employ 30 to 40 persons.

The Industrial Gas Cleaning Institute will move here in the spring from Connecticut and the New York-based Single Service Institute (manufacturers of throw-away food packaging) will open a Washington office in the spring in preparation for moving the entire association here in 1970.

SMALL BUSINESS - The U.S. Department of Agriculture's graduate school is planning a new evening course on establishing and operating a small business. Open to all interested adults, the course costs $57 and will be conducted from 6 to 9 p.m. on Thursdays, starting Jan. 2.

Designed for would-be and current entrepreneurs, the course covers financial planning, organization, forming a corporation, selecting a location, facilities, marketing, legal and government issues.

The course will be a repeat of a similar offering in September that filled up rapidly. For registration information, Washington area residents may telephone the USDA graduate school at 447-4419.