From the company that runs the ITV stations, in Galicia, here is a link to a PDF that confirm the rules and regulations for the test. Although this is from a Galician website the rules are the same throughout Spain.
http://www.mityc.es/es-ES/Documentac...CION%20ITV.pdf
It is in Spanish and it is over 400 pages too.

I have also found the government document that gives details of how you go about getting modifications to your car tested and then documented.
http://www.mityc.es/es-ES/Documentac...05_09_Corr.pdf
Again in Spanish but only something like 150 pages long.

Hopefully someone will add that to the Free Beer thread!

Be careful not to hit the print button.

I found it because when your ITV is due they now send a SMS and/or email. I was on the site booking a test for one of the cars.
My Spanish is ok but some of the technical stuff is very complicated.

Now added to the Wiki as the FBT is closed to new entries.

Any info on trailers? I´m having one build, but I guess it has to go through the ITV as well.

I thought it depends on the carrying capacity. Some have a red matriculation plate. These will have to be tested.