I can confirm that if a contractor RENOVATING your property provides BOTH materials and services, you need only pay IVA of 10%. I have just spent a very large amount of money renovating a house in Spain and all works were billed at 10%.
It was a contractor who first told me about the 10% rule but I then had to educate all the other contractors involved in the works. I checked the web and a local lawyer before starting the works, and everything tallied.
Unfortunately, I can not quote you a source as the relevant papers are in my house in Spain and I am outside the country. Good luck.