Ok, still sounds good. How do you prove when the house was built? And which company did this for you please?
Bruixot

We didn't go through the town hall so you could have a good chance if you have never been stopped.

I can confirm that this procedure works. It is fairly painless especially if the Architect is familiar with the process.

I heard about an amnesty and have modified my campo building and sold my mobile home.  Now I have been told by the adjuntament that there is no amnesty.  I am getting desperate, as we could not afford to buy in a village or town now as the chances of selling it are very limited.   
I have found information on different parts of the country issuing amnesties, one area on almost half a million  houses.  But cant find anything for the Tarragona area.  The nearest was that the local government was discussing the amnesty.  I also read it was a Spanish government national amnesty and being left to individual areas to carry it out. 
I have a fifty square on half an hectare, and my fault I suppose but was told we could re form it as a house by the estate agent.  And after six years it could not be demolished. Why did I think they would be any different in Spain?  When we bought we were given a padron, but as we were not here full time we missed a renewal date, and a new mayor had been elected so we were refused to renew it.  Many who live in illegal house who were able to renew their padrons are still able to keep renewing them. 
We have a cedula and were trying to do the right thing and register with cadastral so we paid our fair share of rates.  We now officially live in someone elses house in the village, and as I am applying for cadastral asked if we could change the address.  The current mayor said if we want a padron on our own address she would be forced to check the legality of the house, but if we stayed at the address we have been using she would turn a blind eye.  I know she is covering her own back, but blackmail springs to mind.
Does anyone know what is going on or not with the amnesty.  If it is extended to our area do we need to retrospectively apply for planning permission as they do in France, or do we need to employ a solicitor to request the amnesty.  Pulling my hair out.  so if anyone knows please let me know.  It would be useful if those who have done it could let us know what route they took, whether they built from new as we did, and on what grounds they were allowed to legalise.  Also what adjuntament they come under.  We unfortunately have a new mayor who is determined to be a new broom, despite the odd shutting of the eyes to the rules.

It sounds like you live in the Ginestar. I would advise you to go and discuss your problem with David Pique in Miami Playa.
I have not heard of any amnesty being given to in this area. However  " after six years it could not be demolished" I believe this is the case so long you are not in an area controlled by the Forrestals i.e forest area. If you are within forest area there is no mercy. My friend was ordered to demolish his beautiful house last week despite having permission after the forest people redrew the perimeter of their area. I am not a lawyer so better speak to Pique.

The best advice I was given when we first came here was "would you try and do what you want to do her as if it was in England".

The answer is of course ....... yes because we are all led to believe that everything here is a bed of roses and no-one would ever try and con you.

Sadly the reverse is true and I have heard so many stories of people having their finger well and truly burnt and having no recourse because they didn't consult an architect etc. However .... there are as many so called respectable businesses out there as there are dodgy ones so who the hell do you trust?

Get a quote for work, not just from one or two firms but from 3 or 4. And ALWAYS ask your friends if they know this or that company.

Stop being so naive .. strap on a pair and ask for written quotations, stick to a budget but before anything has been agreed go to the town hall with your builder and get approval. If the builder doesn't want to go to the town hall with  you then it is a sure sign that they have something to hide.

In my experience the town halls (ajuntaments) are on YOUR side.

Good luck