dcmd_house(playerid, params[])
{
	new
		command[128], owner[255];
	if(Account[playerid][pAdminLevel] >= 20)
	{
	  if (sscanf(params, "s", command)) SendClientMessage(playerid, 0xFF0000AA, "Usage: /house <add>");
	  else
	  {
	    if(strcmp(command,"add", true ) == 0)
	    {
				if (sscanf(command, "s", owner)) SendClientMessage(playerid, 0xFF0000AA, "Usage: /house add <owner>");
				else
				{
					new Float:x,Float:y,Float:z,Float:angle;
					GetPlayerPos(playerid,x,y,z);
					GetPlayerFacingAngle(playerid,angle);
					AddHouse(owner,x,y,z,GetPlayerVirtualWorld(playerid),GetPlayerInterior(playerid),angle,0/*exitx*/,0/*exity*/,0/*exitz*/,1337/*exitinterior*/,0/*exitangle*/,0/*owned*/,0/*rentable*/,0/*rentcost*/,0/*houseprice*/,0/*locked*/);
				}
			}
			else
			{
			  return SendClientMessage(playerid, 0xFF0000AA, "Invalid house command.");
			}
	  }
	}
	return 1;
}

sscanf(command, "s", owner)

sscanf(command,"ss",command,owner)

sscanf(params,"ss",command,owner)

// somewhere on the top of the script:
#define COLOR_RED 0xFF0000AA

dcmd_house(playerid, params[])
{
    new
      cmd2[64], // second command will be stored in this array
      string[128]; // for additional parameters in the second commands (see below)
    if (Account[playerid][pAdminLevel] >= 20) // admin level check
    {
        if (sscanf(params, "s ", cmd2) != 0) // cmd2 will become the first parameter, sscanf is tricked with the extra space to stop processing the string
        {
            // send proper command syntax for player, include only the first parameters (the second commands):
            SendClientMessage(playerid, COLOR_RED, "Usage: /house <add / whatever more params here>");
            return 1; // exit
        }

        if (strcmp("add", cmd2, true, 3) == 0) // check if the first 3 (!) characters of cmd2 are the same as 'add'
        {
            if (sscanf(params, "ss", cmd2, string) != 0) // read the additional parameters after the second command
            {
                // send proper syntax again, now with the needed parameters for the second command the player typed (in this case 'add'):
                SendClientMessage(playerid, COLOR_RED, "Usage: /house <add> <owner>"); // we are expecting only one extra parameter, the 'owner'
                return 1; // exit
            }
            /*
            // you may want to add this extra check for more security ^^
            if (strlen (string) > MAX_PLAYER_NAME) return SendClientMessage(playerid, COLOR_RED, "The given owner name is too long!");
            */
            // command code follows (yours), note that string is now the owner
            new
                Float:x,
                Float:y,
                Float:z,
                Float:angle;
            GetPlayerPos(playerid,x,y,z);
            GetPlayerFacingAngle(playerid,angle);
            AddHouse(owner,x,y,z,GetPlayerVirtualWorld(playerid),GetPlayerInterior(playerid),angle,0/*exitx*/,0/*exity*/,0/*exitz*/,1337/*exitinterior*/,0/*exitangle*/,0/*owned*/,0/*rentable*/,0/*rentcost*/,0/*houseprice*/,0/*locked*/);
            return 1; // successful, exit
        }
        /*
        // additional second commands may come here in this format:
        else if (strcmp("remove", cmd2, true, 6) == 0)
        {
          // check for other params if any
          // check validness of other params
          // code follows
        }
        else if (strcmp("info", cmd2, true, 4) == 0)
        {
          // check for other params if any
          // check validness of other params
          // code follows
        }
        // you may add as many as you want
        */
    }
    /*
    // you may want to send this warning if player if not a level 20 admin:
    else SendClientMessage(playerid, SOME_COLOR, "This is a level 20 admin command!");
    */
    return 1; // end of command
}