I agreed a price on a house in early August 2012.
When the pre contract documents were sent to me, I noticed that the property was specified as having less than half of the actual number of square metres. I stated bluntly that, under no circumstances, would I purchase a property that was not already 100 % legal.
It turns out that substantial extensions had been made several years earlier, with all of the appropriate planning permissions, but the architect had failed to make the final declaration of end of work to the local authority.
I was given the same sort of advice that you have received, namely that things are different in Spain. However, I stuck to my guns and told the agent, make it legal and I will buy. Otherwise I walk.
Everything was sorted out, and we still met the planned completion date of September 20.
Why should you take the risk of making a property legal ?
Why would you agree to a deal where you have to pay 800 € that should have already been paid by the current owner ?
One thing to remember is that many people failed to properly register extensions, so they would not have to pay the inevitable increase in local IBI taxes. If you accept and register in the future, it is highly likely that you could be held liable for unpaid IBI at least up to the point of prescription, which is usually 4 – 5 years.
My advice, and I am nowhere near being a specialist in this area, is to ask for your deposit to be returned, as the property is not wholly legal.