I have just noticed your post so I apologise for the late response
There are 2 factors that effect the liability of the builder as far as who is responsibility of thge builder is concerned with your problem
The first is that yes, as with the UK, all properties have to be covered by a 10 year, insurance backed warranty which the builder, by law, must take out and the policy details should be with your deeds.
It si the responsibility of your lawyer to check that this is in place before completion and it is the law that it has to be in place for all new builds in Spain. It is an insurance policy and therefore any claims are taken to the covering insurance company as well as the builder.I know from customers that we have helped it can be a long and frustrating process, as it alos is the the UK as well.
The second is a statute under Spanish building law that states that anything that makes the propery non-habitable, is covered for the first 3 years and has to be rectified by the builder.
I would suggest that your problem fits in both areas
The first step is to make sure that the survey has been carried been out by a Spanish registered structural engineer and this must be registered and signed at a Notary
There will always be some settlement cracking in any property as it settles but it depends on the amount and severity
The key mressage is that it is the builders responsibility to put it right