Preferably do not sign a reservation contract or purchase agreement (opción de compra, contrato privado), but first sign a letter of intent with the help of a Spanish/English speaking notary or lawyer located in your own country, whereby you and the seller agree to complete the transaction. No risks and no major payments before you know whether the property is in order.
In the letter of intent, agreements are recorded and the independent IGP inspection is included as a resolute condition to protect you and the seller from problems afterwards. But also, for example, that the seller does not offer the house to third parties during the agreed period and that you do not unnecessarily incur thousands of euros in lawyers’ fees for a house that appears to have unsolvable problems.
After signing the letter of intent, you will deposit an amount in deposit into an independent third-party account (not the real estate agent or lawyer), an amount agreed with the seller. This deposit is deducted from the purchase price if the purchase takes place, and is refunded if the IGP inspection turns out to be negative and you cancel the purchase.
Pay attention! A seller who does not allow an IGP inspection is very likely to face insoluble problems!