The Tasador may have valued the property as it states in the Land registry. It could be a case of let sleeping dogs …… When the change of new ownership will take place some one might wake up to either fine you ( remember they are desparate for cash ) or instruct you to demolish the extra 40M2.
The current seller will not do it as I expect the seller is Spanish and it is inbuilt for them to not respect the law, rules, regulation & who can blame them. They have to muddle through town hall etc & no one will give the correct information/advise and later have to pay taxes & the Notary fee.
Why, should they bother as it does not enhance the value or quality of their life. It stands to reason that the extension may have been built without proper foundation etc.