UPDATE inside an Update trigger

I have a trigger that performs an UPDATE query on the same table that
the Update trigger is defined. Of course, it loops infinitely, is there
a way to do this?

CREATE OR REPLACE FUNCTION "public"."clear_common_groups" () RETURNS
trigger AS'
DECLARE
  norows integer;
BEGIN
  RAISE NOTICE ''Looking for common areas in Building
%'',NEW.hud_building_id;
  IF NEW.common_area = ''t'' THEN
    UPDATE tblhudunits SET common_area = NULL WHERE unit_id <>
NEW.unit_id AND hud_building_id = NEW.hud_building_id;
  END IF;
  IF NEW.exterior_area = ''t'' THEN
    UPDATE tblhudunits SET exterior_area = NULL WHERE unit_id <>
NEW.unit_id AND hud_building_id = NEW.hud_building_id;
  END IF;
  GET DIAGNOSTICS norows = ROW_COUNT;
  IF NOT FOUND THEN
     RAISE NOTICE ''Nothing updated'';
  ELSE
     RAISE NOTICE ''% rows updated'',norows;
  END IF;
  RETURN NULL;
END;
'LANGUAGE 'plpgsql' IMMUTABLE CALLED ON NULL INPUT SECURITY INVOKER;

CREATE TRIGGER "insert_new_common_area" AFTER INSERT OR UPDATE
ON "public"."tblhudunits" FOR EACH ROW
EXECUTE PROCEDURE "public"."clear_common_groups"();

--
Robert