At what point in the purchase process must KYC begin — at the viewing, or only at the offer?

In law the KYC check must be complete at the latest before the business relationship is established or the transaction carried out (Art. 3, para. 2, and Art. 4.1 Ley 10/2010). Where in your process you start — when the client file is opened, or at the first serious negotiation — is a workable internal policy, not a fixed line drawn by the statute. A non-committal viewing without recording client data does not yet trigger the duty; once data is recorded or an offer is made, it should begin.

Ley 10/2010 requires diligencia debida at the establecimiento de la relación de negocio — when the relationship is entered into. For practical purposes that translates into four stages:

• First contact or enquiry: no KYC duty yet. A general enquiry by email or phone is not a business relationship.
• Viewing: merely recording data to arrange an appointment is not automatically an AML duty.
• The statutory end point: Art. 3, para. 2, and Art. 4.1 Ley 10/2010 require verification before the business relationship is established or the transaction carried out. Deferring to the reservation or the notary appointment is not a rule of law; Art. 12 permits only narrowly drawn exceptions — and never where there are signs of risk.
• Notarial deed: the notary carries out their own KYC checks; that does not release the agent from their own duty.

Since money often already moves at the reservation stage, an early start is advisable: by the time the transaction is carried out at the latest, verification must be complete under Art. 3, para. 2, and Art. 4.1.

Legal basis: Ley 10/2010, Art. 3

Practical tip: For viewings with international buyers, ask by email before the appointment for a copy of the passport and proof of address. That lets you run the PEP screening before you even meet.