Am I responsible for any illegal building done by a former owner? Buying property in Mallorca

If you buy a property and any part of it is illegal, the penalty will be incurred by the one who carried out the building works without a license. So the penalty will be incurred by the former owner, not you.  UPDATE: Law 12/2017, entered in force on January 1st 2018 Had change the responsibility. 
But if the right of the administration to order the demolition has not expired, or if the proeperty is in a protected area where the right of order the demolition will never expire then YOU MAY have to knock it down/demolish it and also pay for the costs. UPDATE Law 12/2017, entered in force on January 1st 2018 allow Consell de Mallorca (Government of Mallorca) to order suppliers, like electricity, water, telephone, etc, to cut the supplies in illegal constructions based on rural areas.
But also, if the right of the administration to order the demolition has expired, then if in the future you ask for any building permits for your property, the administration may ask you to demolish any illegal parts as a previous condition of your being granted a permit. Although the administration may not be able to ask you to knock down the illegal parts of your property, but you will never be able to make any improvements or carry out any building works on it.


-The vendor told me 100% of the property had been registered with the Property Registry. I read the entry and the description matches reality.

Be careful. Just because your property is registered with the Property Registry and with the Cadaster does not mean that it is 100% legal. Only by checking the original building license at the Town Hall and aerial pictures (of very old properties) is it possible to be sure about the legality of a property.

NO. When the owner asks for this certificate, the Town Hall simply checks whether there are any files in their computer records referring to that property. This means they never notice any illegalities, although of course there may be some. Also, if they discover any illegalities in the future they will open a file.
That is why it is strongly recommended not to buy any property without hiring an expert Real Estate Solicitor.

-Infractions are only in rural areas?

NO. Infractions are also found in urban areas.