IF YOU THINK local regulation of condominium conversions is difficult, imagine what federal regulation could involve. That prospect was raised at a Senate hearing Thursday on the implications of the condominium boom. The trend, which one witness called "condomania", is spreading far beyond New York, Chicago, Washington and the other superheated real-estate markets where it began. It is fueled by powerful forces: the unfavorable economics of rentals, the trend toward smaller households, the popularity in-town living, and consumers' anxieties about dwindling chances for home ownership and financial security.

Thus the problems associated with conversions, especially consumer protection and tenant displacement, are likely to grow as familiar and frustrating nationwide as they have become here. Does that justify federal intervention? The Carter administration's answer is a limited "yes". The administration supports a buyer-protection bill that, among other things, would require developers to make extensive disclosures and give prospective purchasers 15 days in which to change their minds.

Anyone familiar with the tangled course of HUD's regulation of large-scale land development is bound to be uncomfortable with the idea of imposing elaborate federal rules on another part of the real-estate business. A HUD spokesman testified Thursday that this is not the administration's intent. The national legislation, he said, is designed to encourage more states to adopt condominium regulations of their own. Yet the states and cities hit by the boom are already managing, however painfully, to work out rules that suit their own particular situations. And federal regulation now could be more complicating than helpful.

District Mayor Marion Barry has a better idea. He testified Thursday that the best thing HUD could do would be to provide more housing aid for the lower-income people being displaced by the condominium boom. No new programs are required. Of course the budget is tight. But Congress and HUD could probably make more resources available by easing the tight restrictions that now affect virtually every federal housing program. In short, the most useful federal response to the condominium "crisis" would be less regulation - not more.