read
http://www.lvvta.org.nz/stdWheels&Tyres.pdf
in particular sections 2.5(1); 2.5(2); 2.5(3) 2.5(7)
also sections impacted by spacers 2.2(8)
In short: Spacers or adaptors must be certified, no matter how thick. They must also be secured to the hub or wheel face, no matter how thick
Sure people have got away with all sorts of things in all sorts of places (i've even seen people get away with certing slip on spacers). But if you want to know what the current official documents on the topic say, then read the above link.