discrepancy between nota simple and cadastro

Sorry if this seems obvious but you have decided to employ a lawyer so I assume you have confidence in him/her.     

The information you need is what you are paying them to provide for you

It's probably sortable. Well, in fact it is sortable. All you have to do is to spend some money to have it put right. Maybe pay an architect to draw plans, maybe pay some unpaid taxes. But why buy something  that's at all dodgy? There are hundreds of properties for sale.

It's usually the other way around of course. The Catastro usually has the property as smaller than reality.

thanks all for your answers. The fact is that the cadastro was modified in 1996 (from 118 to 159 m2 as vivienda), so there must have been a permit - however it seems that we cannot put our hand on it and therefore it might become challenging to modify retrospectively. The actual landlord did not modify it as such he just bought from a bank 2 years ago, refurbished it entirely and with style and then put it on sale for a good price. He is not interested in doing the step to align the nota simple with the cadastro. This is the only flat in this place that really appealed to us in that area of Alicante as it is completely fresh and properly renewed.
Most of the flats in playa san juan Alicante have this kind of terrasse modifications. I would not be surprised if most of them are in this situation (nota simple or cadastro not modified appropriately).
Well my opinion is also to walk away from it, however this is very hard to find such a nice appartment in that area. All we have seen would need the same treatment as the one we looked at.
Do Sapnish people bother about such issue when buying a flat for example? If yes, then sure we shall run away now.

I said speak to your lawyer as I suspect the registration is not ( cannot be)  legit.

In an apartment block in spain one pays community fees on a percentage basis     That is decided on the  ‘built area’ of your apartment, as a percentage of the whole of the ‘built area’ of the community

If one enclosed an open area (terrace, balcony, section of a communal corridor etc)  then the percentage of ‘built area’ of your apartment  would be more than it was.  Thus every other property would have a smaller percentage and would have to be re assessed.

For that reason almost certainly no community would approve such alteration, albeit that they might turn a blind eye

Not an unusual situation in Spain, and such situations have been  ignored/lived with ‘forever’ but if you want to be 100% sure walk away

I had the same problem when I was looking for property for sale in Alicante, Spanish law seems simple and understandable but you can interpret it in so many different ways    So i just decided to talk to the lawyer and he told me to be careful about moments like yours. So at the end we bought a new apartment which had all documents in order.