When you sign the purchase agreement, you and the other party are giving your formal consent to the deal. The seller hands over the keys to your new house, and the buyer (or the bank that is financing the transaction with a mortgage loan) hands over the money. So, this is the point of no return. That is why it is best to observe all the formalities you can, and bear in mind that:
The agreement can be concluded in a private document, without a notary, just between the seller and the buyer. You can only do this when the sale is not being financed with a mortgage loan, as a mortgage loan will always require a notarial deed and registration in the Property Registry. Nevertheless, when buying an as-yet unbuilt house, private contracts are generally used to formalise the delivery of sums paid on account and the obligation to build and deliver the home. It is important for you to know that when a private document has been made, either of the parties can demand to have the document notarised. If the other party refuses, the first party can call for the courts to step in.
Remember that if you want to register your purchase in the Property Registry, you will always have to have a notarial deed.