Modifying a Car in Spain , some clarification from experts required

I understand from a lawyer that in Spain if there is not a law which permits one to do something, then they cannot.

 In UK it is the reverse,  if there is no law to say you cannot then you can.

The reflection on modifying a car is that one would  not expect there to be any laws which specifically say a modification can be made, so it would seem  that none can be made.

Well I´m sure its possible, from a bit more searching it seems minor modifications are straight forward  (lights, seat belts, tow bars, sun roofs)  are quite easy - just take the car to an ITV station with an approved installer certificate for the ITV man to see.  

Further modifications look possible but must also be passed by an engineer.  I have found a company called HGT-R I have contacted them and am awaiting a reply.  If we followed all the laws in Spain/Europe to the letter nothing would get done! 

** EDITED -Inciting- Against forum rules **  

 For anyone else who needs this in the future - homologastur seem to be popular on the Spanish Forums.  

Read thw whole thread and totally confused. I have an eagle SS kit car on a 1966 VW beetle chassis with a 1995 reg number 'Q'. I have been getting so many different opinions and advice I don't know what to do. The refurbishment is nearing completion and it seems I have to pay a small fortune to have it taken from Marbella to madrid for an expensive version of the SVA with no hope of passing as it is a 1995 car and was OK at the time.