You don’t give much information, I’ll try to guess and give you some ideas, but a full knowledge of the situation would allow a more sounded answer
You say your property is due to complete this month so I assume you didn’t sign the deeds yet but you will be told to do so soon
I assume you signed a private contract and even paid a deposit but not the full price… I assume also that that private contract clearly specifies the square meters your property would have (or did you sign a contract saying something like buying a property described in a project with ‘X’ rough or approximate meters…???) Does this contract stipulate any precise date or deadline to sign the tittle deed before a Notario?
Charlie is right, you should know how many meters are exactly missing to value the ‘depreciation’ of your property
also we should know if that missing area is significant (so you can get the contract to be declared void and ask for a full refund plus compensations) or you may be still interested on the purchase (you will ask for a percentage refund plus compensations) I don’t know how is the bank involved in this operation and if the information given to you about the missing Square meters is based on a technical report, if not It would be wise to hire an appraiser (he would tell you the exact area missing and also it’s value)
In the other hand I don’t think a Catastral certificate would help as I understand this is an unfinished property so I’m afraid it won’t be described in the Catastro yet… (the Catastro is informed after the building is finished)
I don’t understand if the ‘missing’ meters are in the private area (your house) or in the common area, that could be crucial too.
This is a delicate issue, follow your independent lawyer’s advice, but I could tell you that if the missing is significant and it is in your private area don’t sign the tittle deeds if you can afford it, threaten them with a court action demanding the full refund, interests and a further compensation that could include the difference you paid/had to pay for this property and the full price of a similar property (same square meters and similar services) within the same area at todays price, any expenses made related to the failed purchase, bank charges related to the operation and even a moral damages compensation
If you are really interested in this particular property and the missing area, though important is acceptable if they compensate de decreased value you could sign the deed (if you really, really have to) but try to delay it as much as possible making clear you are not giving up your rights, and that you have required compensations
You do not automatically loose your rights by signing something different from your private contract but it makes it harder to complain later so anyway and no matter which case of the described matches yours, you have to make a formal requirement (once you know what is exactly missing) by a ‘Requerimiento Notarial’ or even by ‘burofax’ (cheaper but less efficent) I told you this was a delicate issue so let your lawyer to handle this, but do it ASAP
I’m afraid the nearest English speaking lawyer I really know in the area is in Sevilla, I guess this web site has a list of professionals but I don’t know them (I don’t know if they are ‘recommended’ by the site or the list is just advertising)