A qué se debe el motivo de su visita?

A qué se debe el motivo de su visita?
 

Is there a fixed sentence for that in English?

If that is a formulaic question asking somebody politely why they are vsiting, then:

To what do I owe the pleasure (of your visit)?

Otherwise

Why has he/she come?

More context is needed

To what do I owe the pleasure (of your visit)?
That´s the right answer.
Thanks

Y con un poquito de sorna, (y plural mayestático)

And what do we owe, if we might ask, the pleasure of your visit?

Almost!

And to what do we owe, if we might ask, the pleasure of your visit? :)
And to what do we owe, if we might ask, the honour of your visit?

Oh, yes, thank you for the correction.